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FEATURES
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LTC3455 Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger DESCRIPTIO
The LTC(R)3455 is a complete power management solution for a variety of portable applications. The device contains two synchronous step-down DC/DC converters, a USB power controller, a full-featured Li-Ion battery charger, a Hot Swap output, a low-battery indicator, and numerous internal protection features. The LTC3455 provides a small, simple solution for obtaining power from three different power sources: a single-cell Li-Ion battery, a USB port, and a wall adapter. Current drawn from the USB bus is accurately limited under all conditions. Whenever a USB or a wall adapter is present, the battery charger is enabled and all internal power for the device is drawn from the appropriate external power source. All outputs are discharged to ground during shutdown to provide complete output disconnect. The device is available in a 4mm x 4mm 24-pin exposed-pad QFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. *U.S. Patent 6,522,118
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Seamless Transition between Input Power Sources: Li-Ion Battery, USB, and 5V Wall Adapter Accurate USB Current Limiting (500mA/100mA) Two High Efficiency DC/DC Converters: Up to 96% Thermal Regulation Maximizes Battery Charge Rate without Risk of Overheating* Full-Featured Li-Ion Battery Charger Hot SwapTM Output for SDIO and Memory Cards Pin-Selectable Burst Mode(R) Operation Output Disconnect: All Outputs Discharged to Ground During Shutdown Available in a 4mm x 4mm x 0.8mm 24-Pin QFN Package
APPLICATIO S
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Handheld Computers Digital Cameras MP3 Players
TYPICAL APPLICATIO
USB 5V 1 4.7F WALL 5V 1 1F 10F 3.32k 1k 4 11 0.1F 1.24k 2.49k 9 4.7F SINGLE CELL Li-ION 3.3V TO 4.2V VBAT 3 2 TIMER PROG CHRG WALLFB USB CONTROLLER 8 6 5 10 USB SUSPEND USBHP VMAX
MODE HSON ON2 PWRON RST PBSTAT LTC3455
21 15 19 22 20 23 1M 24 14 1M 1.8V ON/OFF 3.3V, HS 1F C
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60
1.8V 0.4A 10F 80.6k
SWITCHER 2 VOUT2 = 3.3V
ON HSO
HSI SW2 FB2
13 12 18 4.7H 10pF 249k 3.3V 0.5A 10F 80.6k
+
1.8V 1M LBO VBAT 2.49M 16 806k AI 17 AO
VBAT = 3.6V 1 10 100 LOAD CURRENT (mA) 1000
3455 TA01b
SW1 FB1 GND
3455 TA01a
7 1 25
4.7H 10pF 100k
U
U
U
Efficiency
SWITCHER 1 VOUT1 = 1.8V
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1
LTC3455
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PBSTAT PWRON MODE ON2 RST ON
VBAT, VMAX, USB Voltages ...........................- 0.3V to 6V SW1, SW2 Voltages ....................- 0.3V to (VMAX+0.3V) TIMER Voltage ............................. - 0.3V to (VMAX+0.3V) PWRON, ON, ON2, HSON Voltages .............- 0.3V to 6V PBSTAT, RST, CHRG, AO Voltages ..............- 0.3V to 6V HSI, HSO Voltages .......................................- 0.3V to 6V MODE, USBHP, SUSPEND Voltages ............- 0.3V to 6V WALLFB, AI, PROG Voltages .......................- 0.3V to 2V FB1, FB2 Voltages ........................................- 0.3V to 2V Junction Temperature ........................................... 125C Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................. - 65C to 125C
ORDER PART NUMBER
18 FB2 17 AO
24 23 22 21 20 19 FB1 1 PROG 2 TIMER 3 CHRG 4 USBHP 5 SUSPEND 6 7
SW1
LTC3455EUF UF PART MARKING 3455
25
16 AI 15 HSON 14 HSO 13 HSI
8
USB
9 10 11 12
WALLFB VMAX VBAT SW2
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN
TJMAX = 125C, JA = 36C/W, JC = 2.5C/W EXPOSED PAD (PIN 25) IS GND MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON2 = 0V, VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER Battery Undervoltage Lockout Voltage Battery Undervoltage Lockout Hysteresis VBAT Pin Quiescent Current (Note 3) Burst Mode, Battery Powered PWM Mode, Battery Powered USB Powered Wall Powered Shutdown ON Pin Threshold PWRON Pin Threshold ON2 Pin Threshold MODE Pin Threshold WALLFB Pin Threshold Voltage WALLFB Pin Hysteresis ON Pin Pullup Current PWRON Pin Pulldown Current ON2 Pin Pulldown Current MODE Pin Pullup Current WALLFB Pin Input Bias Current PBSTAT Pin Low Voltage RST Pin Low Voltage RST Pulse Duration VON = 1V VPWRON = 1V VON2 = 1V VMODE = 1V VWALLFB = 1.35V VON = 0V, IPBSTAT = 100A VON = 0V, IPBSTAT = 1mA IRST = 100A IRST = 1mA After FB1 and FB2 in Regulation
q
ELECTRICAL CHARACTERISTICS
CONDITIONS VBAT Rising
MIN 2.9
TYP 3.0 450
MAX 3.2
UNITS V mV
VON2 = VMODE = 1V, Not Switching VON2 = 1V, VMODE = 0V, Not Switching VUSB = 5V, Charger Off VWALL = 1.5V, VMAX = 4.5V, Charger Off VPWRON = 0V, VMAX = 0V
110 500 10 10 2 0.8 0.8 0.8 0.8
160 800 20 20 4 1.0 1.0 1.0 1.0 1.26
WALLFB Rising
q
1.20
1.23 60 2.5 2.5 2.5 2.5 1 0.02 0.20 0.02 0.20 200
30 0.10 0.35 0.10 0.35
2
U
A A A A A V V V V V mV A A A A nA V V V V ms
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LTC3455
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON2 = 0V, VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER Battery-VMAX PMOS VMAX PMOS Switch On-Resistance VMAX Switch Current Limit VMAX Switch Current Limit at Startup Gain Block AI Pin Threshold Voltage AI Pin/FB2 Pin Voltage Difference AI Pin Input Bias Current AO Pin Sink Current AO Pin Voltage Switching Regulators FB1, FB2 Voltage FB1, FB2 Voltage Line Regulation FB1, FB2 Voltage Burst Mode Hysteresis FB1, FB2 Pin Input Bias Current Switching Frequency PMOS Switch On-Resistance NMOS Switch On-Resistance PMOS Switch Current Limit USB Power Manager USB Undervoltage Lockout Voltage USB Undervoltage Lockout Hysteresis USB Minimum Voltage to Charge Battery USB PMOS Switch On-Resistance USB Current Limit USB Suspend Mode Bias Current SUSPEND Pin Threshold USBHP Pin Threshold SUSPEND Pin Pulldown Current USBHP Pin Pulldown Current Hot Swap Output Hot Swap PMOS Switch On-Resistance Hot Swap PMOS Switch Current Limit HSON Pin Threshold HSON Pin Pulldown Current VHSI = 3.3V VHSI = 3.3V, VHSO = 2.5V 120 0.9 160 0.8 2.5 1.1 mA V A VSUSPEND = 0.5V VUSBHP = 0.5V VUSB = 5V VUSB = 5V, VUSBHP = 2V VUSB = 5V, VUSBHP = 0V VUSB = 5V, VSUSPEND = 2V
q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP 0.15
MAX
UNITS A A
2.5 With VMAX Rising, VMAX = 3V, VBAT = 3.6V 0.4 0.784 -8
q
4.0 0.9 0.805 0 1 1.8 0.8 0.826 8 25 2.5 1.2 0.826
V mV nA mA V V %/V mV
VFB2 - VAI VAI = 0.85V VAI = 0.6V, VAO = 1.5V VAI = 0.6V, IAO = 1mA
1.0
0.784
0.805 0.01 8
VMAX = 3V to 5V VMODE = 2V VFB1 = VFB2 = 0.85V Both Switchers Both Switchers Both Switchers Switcher 1 Switcher 2 From Low to High 450 700 3.75
q
1 1.2 1.5 0.35 0.45 600 900 3.90 150 4.0 0.5 440 60 475 80 4 0.8 0.8 2.5 2.5
25 1.8
nA MHz
850 1200 4.10
mA mA V mV V
500 100 20 1.1 1.1
mA mA A V V A A
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LTC3455
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON = 0V, VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER Battery Charger Regulated Charger VBAT Voltage Charger Current Limit (USB Powered) Charger Current Limit (Wall Powered) Recharge Battery Voltage Threshold Trickle Charge Trip Threshold Trickle Charge Trip Hysteresis Trickle Charge Current PROG Pin Current PROG Pin Voltage CHRG Pin Output Low Voltage Timer Accuracy Junction Temperature in Constant Temperature Mode Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3455 is guaranteed to meet specified performance from 0C to 70C and is designed, characterized and expected to meet these extended temperature limits, but is not tested at -40C and 85C RPROG=2.49k, VBAT = 2V Internal Pull-Up Current, No RPROG RPROG =2.49k ICHRG = 5mA CTIMER = 0.1F 0C TA 85C RPROG=2.49k, VUSBHP = 2V, VUSB = 5V, 0C TA 85C RPROG=2.49k, VUSBHP = 0V, VUSB = 5V, 0C TA 85C RPROG=2.49k, VMAX = 4.5V, 0C TA 85C VBAT(REGULATED) - VRECHARGE Battery Voltage Rising 425 4.158 4.200 400 50 500 150 2.85 60 65 2 1.23 0.75 10 105 4.242 470 90 575 V mA mA mA mV V mV mA A V V % C CONDITIONS MIN TYP MAX UNITS
ELECTRICAL CHARACTERISTICS
Note 3: Quiescent current is pulled from the VBAT pin when neither USB or wall power is present, and from the VMAX pin when either USB or Wall power is present.
TYPICAL PERFOR A CE CHARACTERISTICS
Burst Mode Quiescent Current
120 100 BOTH SWITCHERS ENABLED 600 BOTH SWITCHERS ENABLED 500
QUIESCENT CURRENT (A)
QUIESCENT CURRENT (A)
ONLY SWITCHER 1 ENABLED 80 60 40 20 0 -50 -25 VBAT = 3.6V NOT SWITCHING 50 25 75 0 TEMPERATURE (C) 100 125
400 300 200 100 0 -50 -25
ONLY SWITCHER 1 ENABLED
QUIESCENT CURRENT (A)
4
UW
3455 G01
PWM Mode Quiescent Current
5
Shutdown Quiescent Current
VBAT = 3.6V
4
3
2
1
VBAT = 3.6V NOT SWITCHING 50 25 75 0 TEMPERATURE (C) 100 125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3455 G02
3455 G03
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LTC3455 TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pins (FB1, FB2) and AI Pin Voltage
815 2.0
SWITCHING FREQUENCY (MHz)
810 805 800 FB2 795 790 785 -50 -25 AI
VOLTAGE (mV)
FOR BOTH SWITCHERS 1.0
CURRENT LIMIT (mA)
FB1
50 25 75 0 TEMPERATURE (C)
USB Pin Current Limit
500 USBHP = 2V 5.0 4.5
VMAX CURRENT LIMIT (A)
400 USB PIN CURRENT (mA)
4.0 3.5 3.0 2.5 2.0 1.5 1.0 STARTUP
HSO PIN CURRENT LIMIT (mA)
300
200
100 USBHP = 0V 0 -50 -25 VUSB = 5V 50 25 75 0 TEMPERATURE (C) 100 125
Battery Undervoltage Lockout
4.00 3.75
BATTERY UVLO (V)
RISING
FALLING
WALLFB TRIP VOLTAGE (V)
USB UVLO (V)
3.50 3.25 RISING 3.00 2.75 FALLING 2.50 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125
UW
100
3455 G04 3455 G07 3455 G10
Switching Regulator Oscillator Frequency
1000
Switching Regulator Current Limit
SWITCHER 2
1.5
800
600
SWITCHER 1
400
0.5
200
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3455 G05
3455 G06
VMAX Pin Current Limit
200 NORMAL OPERATION
HSO Pin Current Limit
150
100
50 VHSI = 3.3V VHSO = 2.5V 50 25 75 0 TEMPERATURE (C) 100 125
0.5 0 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125
0 -50 -25
3455 G08
3455 G09
USB Undervoltage Lockout
4.00 3.75 3.50 3.25 3.00 2.75 2.50 -50 -25 1.26 1.24
WALLFB Trip Voltage
RISING 1.22 1.20 1.18 1.16 1.14 1.12 FALLING
50 25 75 0 TEMPERATURE (C)
100
125
1.10 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3455 G11
3455 G12
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LTC3455 TYPICAL PERFOR A CE CHARACTERISTICS
Battery Charger Regulation Voltage
4.30 4.25 4.20
VBAT (V)
4.25 4.20 4.15 4.10 4.05 4.00 -50 -25
TRICKLE CHARGE THRESHOLD (V)
VRECHARGE (V)
4.15 4.10 4.05 4.00 -50 -25
50 25 75 0 TEMPERATURE (C)
Charge Current When Wall-Powered
600
BATTERY CHARGE CURRENT (mA)
500 400 300 200 100 VBAT = 3.6V VMAX = 4.5V RPROG = 2.49k 0 50 -50 -25 25 75 0 TEMPERATURE (C)
BATTERY CHARGE CURRENT (mA)
THERMAL CONTROL LOOP IN OPERATION
400 300 200 100
BATTERY CURRENT (A)
PROG Pin Voltage vs Charge Current
1.50 VBAT = 3.6V VMAX = 4.5V 1.25 RPROG = 2.49 TA = 25C 1.00
VPROG (V)
RDS(ON)
PMOS
0.75 0.50 0.25 0
0.3 0.2 0.1 0 -50 -25
RDS(ON)
0
400 300 100 200 CHARGE CURRENT (mA)
6
UW
100
3455 G13
Battery Charger Recharge Threshold
4.30 3.0
Battery Charger Trickle-Charge Threshold
2.9 RISING 2.8 FALLING 2.7
2.6
125
50 25 75 0 TEMPERATURE (C)
100
125
2.5 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3455 G14
3455 G15
Charge Current When USB-Powered
600 VBAT = 3.6V VUSB = 5V 500 RPROG = 2.49k VUSBHP = 2V THERMAL CONTROL LOOP IN OPERATION 15.0 12.5 10.0 7.5 5.0 2.5
Battery Current When USB- or Wall-Powered
VBAT = 4.2V CHARGER OFF
VUSBHP = 0V
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3455 G16
3455 G17
3455 G18
RDS(ON) for Switching Regulator Power Switches
0.7 0.6 0.5 0.4 NMOS VBAT = 3.6V 1.4
RDS(ON) for VMAX, USB, and HSO PMOS Switches
VHSI = 3.3V VUSB = 5V 1.2 V BAT = 3.6V HSO 1.0 0.8 0.6 0.4 0.2 0 -50 -25 VMAX USB
500
3455 G19
50 25 75 0 TEMPERATURE (C)
100
125
50 25 75 0 TEMPERATURE (C)
100
125
3455 G20
3455 G21
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LTC3455
PI FU CTIO S
FB1 (Pin 1): Feedback Pin for Switcher 1. Set the output voltage by connecting feedback resistors to this pin. PROG (Pin 2): Charge Current Program and Charge Current Monitor Pin. Connect a resistor, RPROG, from this pin to ground to program battery charge current. IBAT = 1000 * 1.23V / RPROG In all modes the voltage on the PROG pin can be used to measure charge current. PROG has a weak pull-up current source to turn the charger off if the pin is left open. TIMER (Pin 3): Timer Capacitor Pin. Connect a capacitor, CTIMER, between this pin and ground to set the charge cycle termination time. The timer starts when USB or wall power is first present. The timer period is: TTIMER (hours) = CTIMER * (3 hours) / (0.1F) Tie TIMER to ground to disable just the internal timer function. Tie TIMER to VMAX to use the charger in a constant-current-only mode (which disables the timer, voltage amplifier and trickle charge function). CHRG (Pin 4): Open-Drain Charge Status Pin. This pin is pulled low with an internal N-channel MOSFET whenever the battery charger is enabled, and is forced into a high impedance state whenever it is disabled. USBHP (Pin 5): USB High Power Mode Pin. This pin is used to select the appropriate USB current limit (either 500mA or 100mA). Pull high to select 500mA (high power mode); low to select 100mA (low power mode). SUSPEND (Pin 6): USB Suspend Pin. When this pin is pulled high, the internal USB power controller is disabled and the USB pin current reduces to less than 20A. SW1 (Pin 7): Switch Pin for Switcher 1. Minimize the length of the metal trace connected to this pin. Place the inductor for Switcher 1 as close to this pin as possible. USB (Pin 8): USB Supply Pin. Input current into this pin is limited to either 100mA or 500mA based on the state of the USBHP pin. The charger and Switcher 1 will remain alive whenever USB power is present (when USB pin is above 3.9V and SUSPEND is low). VBAT (Pin 9): Battery Input Pin. Bypass this pin with a capacitor as close to the device as possible. VMAX (Pin 10): Max Voltage Pin. This pin is used to power the two internal step-down DC/DC converters and is provided externally to power other devices (i.e. LDOs or Switchers for LCD bias, white LED backlight drive, etc). When the LTC3455 is on and neither USB or wall power are available, an internal PMOS switch connects this pin to the VBAT pin. When either USB or wall power is present, they provide power to this pin, and the battery charger draws power from this pin. In shutdown, this pin is discharged to ground to provide output disconnect. WALLFB (Pin 11): Wall Power Detection Pin. This pin is the input to a comparator used to signal the presence of a 5V wall adapter. A resistor divider taken from the wall adapter input is connected to this pin to tell the LTC3455 when the adapter voltage is high enough to provide power to the LTC3455. When this pin is higher than 1.23V, the battery charger is enabled. The 5V wall adapter is connected to the VMAX pin through a Schottky diode. Tie WALLFB to ground if a wall adapter is not used. SW2 (Pin 12): Switch Pin for Switcher 2. Minimize the length of the metal trace connected to this pin. Place the inductor for Switcher 2 as close to this pin as possible. HSI (Pin 13): Hot Swap Input Pin. This pin is connected to the HSO pin through a current-limited PMOS switch. HSO (Pin 14): Hot Swap Output Pin. This output is used for memory cards or other devices that would appear as a short if they were hot-plugged directly to one of the outputs (typically the 3.3V output). The current out of this pin is limited to 160mA. HSON (Pin 15): Hot Swap Enable Pin. This pin turns on the PMOS that connects the HSI and HSO pins. AI (Pin 16): Gain Block Input Pin. This pin is the inverting input to an amplifier that can be used as a low-battery detector or as an LDO with the addition of an external PNP or PMOS. The non-inverting input of the gain block is connected to the 0.8V internal reference. AO (Pin 17): Gain Block Output Pin. This pin is an opendrain output, and is pulled low when the AI pin is less than 800mV. This output can be used as a low-battery detector, or as an LDO with the addition of an external PNP or PMOS. This pin can sink up to 1mA.
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LTC3455
PI FU CTIO S
FB2 (Pin 18): Feedback Pin for Switcher 2. Set the output voltage by connecting feedback resistors to this pin. ON2 (Pin 19): Enable Pin for Switcher 2. This pin turns on Switcher 2 only if ON is low or PWRON is high. Switcher 2 cannot be turned on by itself. RST (Pin 20): Reset Pin. This pin is an open-drain output that provides a 200ms reset signal during power-up to initialize a microcontroller. MODE (Pin 21): Burst Mode Enable Pin. Tie this pin high to allow Burst Mode operation for the LTC3455. Burst Mode operation will provide superior efficiency when both outputs are operating with very low output currents. Tie this pin to ground to force PWM operation under all load current conditions. Burst Mode is disabled initially at startup (for 200ms) and also whenever external power is available (even if MODE is pulled high). PWRON (Pin 22): Power-On Pin. Pull this pin high to turn on the LTC3455. This pin is typically used in conjunction with the ON and PTSTAT pins, and a momentary-on switch. Tie PWRON to ground if not used. PBSTAT (Pin 23): Push-Button Status Pin. This pin is an open drain output that indicates the state of the ON pin (which is usually connected to a momentary-on pushbutton) to the microcontroller. This pin follows the state of the ON pin (PBSTAT goes low when ON is pulled low). ON (Pin 24): ON Pin. Pull this pin to ground to turn on the LTC3455. This pin is typically used with a momentary-on push-button switch to turn on the LTC3455. This pin would be held low until the PWRON pin is pulled high by a microcontroller to keep the LTC3455 turned on. If a momentary-on switch is not used, this pin can be held to ground to keep on the LTC3455. Leave ON open if not used. This pin has a weak pull-up current source. GND (Pin 25 - Exposed Pad): Ground Pin. The exposed backside pad is the only ground pin for the LTC3455 and must be soldered to the PC board ground plane for the device to operate properly.
SI PLIFIED BLOCK DIAGRA
USB POWER 3.9V TO 5.3V
USB POWER MANAGER
Li-Ion BATTERY 3.3V TO 4.2V
BATTERY PMOS SWITCH
BATTERY CHARGER USE FOR LDO OR LOW BATTERY INDICATOR
GAIN BLOCK
8
W
U
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VMAX IS CONNECTED TO THE BEST AVAILABLE INPUT POWER SOURCE (WALL ADAPTER, USB OR BATTERY)
5V WALL ADAPTER
VMAX USE TO POWER OTHER DC/DCs AND LDOs SWITCHER 1 VOUT1 1.8V TYPICAL
SWITCHER 2
VOUT2 3.3V TYPICAL
HOT SWAP
HOT SWAP OUTPUT 3.3V TYPICAL
3455 SBD
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LTC3455
BLOCK DIAGRA
USB 5V
USB 1
8 1 1000
4.7F
SUSPEND USB CONTROLLER
6
USBHP
5
1k VMAX
CHRG TIMER
4 3 2
0.1F 2.49k
PROG
GND
25
VBAT 3.3V to 4.2V 2.43M 4.7F 806k
VBAT
9
AI
16
LBO 806k 1.8V ON/OFF
AO
17
ON
24 VBAT
PBSTAT 806k 1.8V
23 3.0V
C
PWRON ON2 RST MODE HSON
22 19 20 21 15 200ms RESET PULSE BURST MODE ENABLE
W
WALL 5V 3.9V USB POWER MANAGER
-
EXTPWR
3.32k
1
+
BATTERY CHARGER
+ -
1.23V
11
WALLFB 1.24k 1F
- +
REF
1
1000 BATTERY PMOS SWITCH 10
VMAX
R
4R
- + +
VMAX 10F
1.23V 2.41R
-
+
CHARGE CONTROL
R
SWITCHER 1 PWM DRIVER SW1 4.7H 1.8V
7
100k FB1 80.6k 10F
- + - +
0.8V SWITCHER 2 ENABLE 0.8V
1
+ -
UVLO
PWM DRIVER
12
SW2
4.7H 3.3V
249k EXTPWR FB2 80.6k 10F
- +
ENABLE 0.8V
18
HOT SWAP ENABLE
13
HSI
14
3455 BD01
HSO
3.3V, HS 1F
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LTC3455
OPERATIO
The LTC3455 is designed to be a complete power management solution for a wide variety of portable systems. The device incorporates two current mode step-down switching regulators, a full-featured battery charger, a USB power controller, a Hot Swap output, a low-battery comparator (which can also be configured as an LDO) and numerous protection features into a single package. When only battery power is available, the battery PMOS switch connects the VMAX pin to the VBAT pin to provide power to both switching regulators (and any other devices powered from VMAX). When external power is applied, the LTC3455
APPLICATIO S I FOR ATIO
Undervoltage Lockout (UVLO)
If no external power is present, the LTC3455 will start only if the battery voltage is above 3.0V. This prevents starting up with a battery that is too close to deep discharge. Once started, the battery must drop below 2.6V before the LTC3455 will shut off. This hysteresis is set intentionally large to prevent the LTC3455 from turning off at an inappropriate time, like during the read- or write-cycle of a hard-disk drive (which could potentially damage the drive). The internal UVLO is meant only as a last chance safety measure to prevent running the battery voltage too low and damaging it. An accurate, user-settable lowbattery threshold can be implemented using the gain block (see the "Gain Block" section for details) which gives the microcontroller complete control over the timing of a shutdown due to a low-battery condition. If external power is present and the battery voltage is less than 3.0V, the VMAX pin voltage must be greater than 3.9V for the LTC3455 to start, and once started, the VMAX pin must stay above 3.1V for the device to continue running. Selecting the Input Power Source The priority for supplying power to both DC/DC converters, all internal circuitry, and the VMAX pin is: Wall adapter, USB, battery.
10
U
W
U
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seamlessly transitions from battery power (a single-cell Li-Ion cell) to either the USB supply or a wall adapter. The battery PMOS switch is turned off, the charger is activated and all internal power for the device is drawn from the appropriate external power source. Maximum charge current and charge time are programmed using an external resistor and capacitor, respectively. The USB power manager provides accurate current limiting for the USB pin under all conditions. The Hot Swap output is ideal for powering memory cards and other devices that can be inserted while the system is fully powered. Whenever the WALLFB pin is above 1.23V, system power is drawn from the wall adapter via the VMAX pin, and the battery charger is active. The 5V wall adapter output is connected to the VMAX pin through a Schottky diode, and a resistor divider from the 5V wall input is connected to the WALLFB pin to signal the LTC3455 that wall power is present. A higher voltage adapter can also be used, but the 6V maximum rating on the VMAX pin requires the use of an additional regulator to step down the voltage. If USB power is present and above 3.9V (and wall power is not available), system power is drawn from the USB pin. The battery charger is active, but charge current will be held off until the USB pin increases above 4.0V to prevent the battery charger from further loading down an already low USB supply. As long as the USB pin stays above 3.9V, the USB port supplies all other system power. If the system needs more power than the USB bus can supply, the charger turns off completely, the USB power controller becomes a 500mA (or 100mA) current source and the VMAX voltage begins to decrease. If VMAX continues to decrease, eventually the battery will provide the additional current needed. This allows the LTC3455 to withstand load current transients that briefly require more power than the USB power supply can provide.
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Operation When No Battery Is Present
As long as USB or wall power is available, the LTC3455 will operate with no battery present, a crucial requirement for systems with a removable battery. Keep in mind, however, that if the LTC3455 is USB powered and the battery is not present, absence of the battery means that there is no reservoir if the system needs more power than the USB port can supply. Similarly, if external power is available, the LTC3455 will operate even if the battery is bad or in deep-discharge. The LTC3455 is also a good choice for systems that are always powered by a USB supply or wall adapter. The charger can then be used to charge a large capacitor or backup battery, which can briefly provide power to the system after the external power has been removed. This gives the microcontroller enough time to follow proper shutdown procedures even after the main power source is abruptly removed. If USB powered, the large capacitor or backup battery will also be used to provide additional current if the system briefly needs more power than the USB bus can provide. Concerns When Wall Adapter Powered Always choose a wall adapter that can provide power for all load and battery charging requirements. Choosing a wall adapter with a power rating that is too small will result in very long charge times and erratic system operation. If the total current needed (load and battery charging) exceeds what the adapter can provide, the voltage on the VMAX pin will begin to droop. If it droops close enough to the battery voltage (the VBAT pin), the charge current decreases and eventually reduces to zero. If the load current is still too much for the wall adapter to provide, the wall adapter will provide what it can and the battery will provide the rest. When wall powered, this operation is intended only for surviving fault conditions and should not be a normal mode of operation. Concerns When USB Powered The popularity of USB (Universal Serial Bus) makes it an attractive choice for transferring data in a variety of portable devices. Therefore, utilizing the USB port to power these portable devices while charging their battery is very
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desirable, but it is not necessarily an easy task. As the performance of digital cameras, handheld computers, and MP3 players increases, the power needed to operate them also increases. The power available from a single USB port (maximum 2.5W) is barely enough to support the peak power needed by many full-featured portable devices, even without the power needed to quickly charge their batteries. To further complicate matters, a USB port is not the ideal power source. Each device can draw a maximum of 500mA (in high power mode), but the voltage provided to the portable device can vary quite significantly. Although a USB power supply has a 5V nominal rating, when you include normal supply variations, cable losses, and transient conditions, the USB voltage showing up at the portable device is typically much lower--often falling to only 4V. Since the USB port has a strict current limit of 500mA, this means the amount of power available to the portable device can be as low as 2W. The reduced USB voltage also presents problems when trying to fully charge a single-cell Lithium-Ion battery (that has a 4.2V final charge voltage) when the USB voltage may itself be below or near 4.2V. The LTC3455 is specifically designed to alleviate these problems and make the most of the power the USB port has to offer. See the sections entitled "Large Transient Loads when USB powered" and "Special Charger Features when USB powered" for more detailed discussions of the LTC3455's special USB features. USB High Power/Low Power/Suspend Modes There are three basic modes for the USB power manager: high power, low power, and suspend. High power mode allows the LTC3455 to draw up to 500mA from the USB port, and is selected by pulling the USBHP pin high. Low power mode reduces the allowable current drawn to 100mA, and is selected by pulling the USBHP pin low. The USBHP pin has a weak internal pulldown current source to ensure that the LTC3455 always starts up in USB low power mode. The SUSPEND pin will disable the USB power manager completely, reducing the USB pin current to under 20A.
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Operation in USB Low Power Mode
Most applications that draw power from the USB bus should be in low power mode only for a brief amount of time. All devices must be in low power mode (draw no more than 100mA of current from the USB bus) upon power-up, and can transition to high power mode (draw up to 500mA from the USB bus) only after the device has been given permission to do so by the USB host controller. The change to high power mode is usually very quick, so the full 500mA of current is available shortly after connecting to the USB bus. While the LTC3455 will operate when in low power mode, the amount of power available is so small that it is difficult or impossible to charge a battery or even provide enough current to power the rest of the system. For this reason, USB high power operation should always be used with the LTC3455. Handling Large Transient Loads when USB Powered Many portable devices have nominal loads that can easily be supported by the USB supply, but they have brief transient loads that can exceed the maximum available USB power. The LTC3455 is designed to handle these overloads while drawing as much power as possible from the USB port. If the USB bus is providing power but the LTC3455 (or any other devices connected to the VMAX pin) needs more total power than the USB bus can supply, the battery charger turns off completely and the USB power controller becomes a 500mA (or 100mA) current source and the VMAX voltage begins to decrease. At this point, the capacitance connected to the VMAX pin provides the additional current needed by the system. As long as the USB pin stays above 3.9V, the USB bus will continue to provide as much current as possible. Once the VMAX pin drops just below the VBAT voltage, the battery will provide the additional current needed. This operation allows the LTC3455 to withstand load transients that briefly demand more power than can be provided by the USB bus.
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The oscilloscope photographs in Figure 1 show how the LTC3455 handles load transients when USB powered. The top photo shows a brief transient load that turns off the charger but does not dip the VMAX voltage. The bottom photo shows a prolonged transient condition that turns off the charger and completely dips the VMAX voltage to the point where the battery must provide current. For both cases, normal operation resumes as soon as the transient passes. Extra capacitance can be connected to the VMAX pin to act as a reservoir to help support large transient currents. For most systems this is not necessary, as the LTC3455 cleanly handles heavy transients. For some designs, however, it may be desirable to use a larger capacitor connected to VMAX to act as a larger reservoir. Up to 50F of
VMAX 2V/DIV IMAX 500mA/DIV IUSB 500mA/DIV IBAT 500mA/DIV 100s/DIV
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USB Maximum Current Condition
VMAX 2V/DIV IMAX 500mA/DIV IUSB 500mA/DIV IBAT 500mA/DIV
100s/DIV
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USB Heavy Over-Current Condition Figure 1. Handling Load Transients when USB Powered
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ceramic capacitance may be connected to the VMAX pin without difficulty. More than 50F requires using a capacitor with some ESR or adding some resistance in series with some of the ceramic capacitance. This is necessary to ensure loop stability in the battery charger loop when under USB power. Using the VMAX Pin to Power Other Devices The VMAX pin can be used to provide power for other devices within the system. This pin is connected to the battery when no external power is available, and it is connected to either the USB bus or the wall adapter when either are available. This ensures that all devices powered from VMAX will always draw power from the best available input power source. The internal PMOS connecting VMAX to the battery is current limited to 900mA at startup (to minimize in-rush current) and to 4A once VMAX has risen close to the battery voltage. Because of the reduced startup current limit, the turn-on of other devices powered from VMAX should always be delayed to minimize the currrent initially needed from the VMAX pin. The best choice is to enable these devices from either switcher output, since the turn-on of both switchers is always delayed until the VMAX pin has reached the VBAT pin voltage. The VMAX pin is discharged to ground when the LTC3455 is shut down, so that any device supplied by VMAX will have its input grounded during shutdown. This ensures output disconnect for all supply voltages within the system.
PBSTAT ON C PUSH BUTTON
PWRON ON2
Figure 2. Momentary Push-Button Operation
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Startup and Shutdown when Battery-Powered When only battery power is available, the LTC3455 turns on when either the ON pin is pulled low or the PWRON pin is pulled high. Either of these pins will keep the device running, but typically the ON and PWRON pins are used together to provide turn-on and turn-off using a single momentary-on push-button switch. Figure 2 shows the method for using a momentary-on pushbutton to turn the LTC3455 off and on. When the momentary-on switch is first pressed, shorting the ON pin to ground, PBSTAT goes low and the LTC3455 first brings up the VMAX pin, then enables Switcher 1 to power the microcontroller. Once up and running, the microcontroller provides the PWRON signal to keep the LTC3455 turned on after the push-button is released. When the push-button is pressed again to turn off the device, the PBSTAT pin is pulled low to notify the microcontroller that the push-button has been pressed. The microcontroller prepares for shutdown then pulls the PWRON signal low. When the push-button is released, the ON pin goes high and the LTC3455 turns off. The ON and PWRON pins enable Switcher 1 (along with all the internal circuits needed for normal operation), and the ON2 pin enables Switcher 2. Switcher 2 can only operate when Switcher 1 is also enabled. The turn-on of both switchers is always delayed until the VMAX pin has reached the VBAT pin voltage.
LTC3455 23 24 22 19 SWITCHER 1 ENABLED SWITCHER 2 ENABLED
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ON2 19 PBSTAT 23 ON 24 PWRON 22 VBAT 9
+
SWITCHER 1 ENABLED 3V
-
WALLFB 11
+ -
CHARGER ENABLED
1.23V
USB 8
+
3.9V
-
SUSPEND 6
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Figure 3. Turn-On Logic Diagram for LTC3455
Startup and Shutdown When USB or Wall Powered Whenever USB or wall power is present (as sensed by the USB and WALLFB pins), Switcher 1 and the battery charger will always be enabled. If the LTC3455 is off and external power is applied, both the charger and Switcher 1 will start independent of the state of the ON and PWRON pins. This provides maximum battery run-time by always allowing the battery to charge whenever external power is available, and ensures that the microcontroller is always alive when external power is available (this is important for designs that utilize coulomb-counting or other battery monitoring techniques). Switcher 2 starts only if ON2 is also pulled high. Figure 3 shows the turn-on logic diagram for the LTC3455. Starting Switcher 2/Power Supply Sequencing Switcher 2 can operate only when Switcher 1 is also enabled and in regulation. The ON2 pin can be driven by a logic signal for independent control of Switcher 2. If both outputs always operate together, tie the ON2 pin to the VMAX pin. This will enable Switcher 2 after the output of Switcher 1 has reached 90% of its final value. This powerup delay ensures proper supply sequencing and reduces the peak battery current at startup. Figure 4 shows the
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LTC3455 SWITCHER 2 ENABLED USB POWER CONTROLLER ENABLED
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output sequencing when both switchers are enabled at startup with the ON2 pin tied to VMAX. The turn-on of both switchers is always delayed until the VMAX pin has reached the VBAT pin voltage. Reset Signal (RST) A 200ms reset signal (the RST pin is pulled low) is provided for proper initialization of a microcontroller whenever the LTC3455 is first turned on, either by the ON or PWRON pins, or by the application of external power. The RST signal is also pulled low whenever the LTC3455 is in shutdown, ensuring no false starts for the microcontroller as the output voltages are rising or collapsing. In the event of a fault condition the RST pin will be pulled low.
PWRON/ON2 2V/DIV VMAX 2V/DIV VOUT1 (1.8V) 2V/DIV VOUT2 (3.3V) 2V/DIV
100s/DIV
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Figure 4. Sequencing for Switcher 1 and 2 Outputs
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Low or Bad Battery Protection (200ms Timeout) The 200ms reset timer is also used to prevent starting the LTC3455 when there is insufficient external power or insufficient battery voltage to regulate the outputs. When first turned on, the internal 200ms timer starts. If only Switcher 1 is enabled (ON2 is low) and its output does not reach 90% of its final value within 200ms, Switcher 1 is shut down even if the ON pin is held low or if the PWRON pin is held high (the VMAX pin will remain on as long as ON is low or PWRON is high). This automatic shutdown feature prevents possible damage to a defective or overdischarged Li-Ion battery. If ON2 is tied to VMAX so that Switcher 2 is also turned on at startup, then both outputs must reach 90% of their final values within 200ms. Once the output(s) are in regulation, the timer is reset for a full 200ms. Schottky Diode Selection/WALLFB Resistor Selection When a 5V wall adapter is used, power is provided to the VMAX pin through a Schottky diode. The most important specification in picking this diode is its reverse leakage current. When the LTC3455 is turned on but wall power is not present, the Schottky will leak current to ground through the WALLFB resistor divider (see Figure 5). This leakage current should be minimized (by picking an appropriate low-leakage Schottky diode) as it can dramatically reduce Burst Mode efficiency at light loads. In addition, a high leakage current can also false trip the WALLFB pin and turn on the LTC3455 even if wall power is not available. To help prevent this false turn-on, use the WALLFB resistor values shown in Figure 5. The diode forward voltage drop should be around 500mV or less at its maximum rated current to allow charging even when the wall adapter voltage is lower than normal. Some manufacturers have recently introduced Schottky diodes optimized for a very low forward drop, but their reverse leakage currents can be more than 100A at room temperature, and over 1mA at high temperatures. These diodes are not recommended for use with the LTC3455, but if they are used operation at high temperature should be checked thoroughly to avoid problems due to excessive diode leakage current.
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Three good diode choices are the MBRM110E (1A, 10V), MBR120ESF (1A, 20V), and the MBRA210E (2A, 10V). All are available in very small packages from ON Semiconductor (www.onsemi.com), have reverse leakage currents under 1A at room temperature, and have forward drops of around 500mV at their maximum rated current (1A or 2A).
VMAX VMAX 10 LTC3455 11 1.24K
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ILEAKAGE 3.32K
WALL 5V
WALLFB
Figure 5. Schottky Leakage Current Path
Switching Regulator General Information The LTC3455 contains two 1.5MHz constant-frequency current mode switching regulators that operate with efficiencies up to 96%. Switcher 1 provides up to 400mA at 1.5V/1.8V (to power a microcontroller core), and Switcher 2 provides up to 600mA at 3V/3.3V (to power microcontroller I/O, memory and other logic circuitry). Both converters support 100% duty cycle operation (low dropout mode) when the input voltage drops very close to the output voltage, and both are capable of operating in Burst Mode operation for highest efficiencies at light loads (Burst Mode operation is pin selectable). Switcher 2 has independent ON/OFF control, but operates only when Switcher 1 is also enabled and in regulation. If both are enabled at power-up, Switcher 2 is allowed to turn on only after Switcher 1 has reached 90% of its final value. This power-up delay ensures proper supply sequencing and reduces the peak battery current at startup. If the output of Switcher 1 drops to below 85% of its programmed output voltage, Switcher 2 will turn off. This ensures that any problems with the core supply will shut down the rest of the sytem.
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Switching Regulator Inductor Selection
Many different sizes and shapes of inductors are available from numerous manufacturers. Choosing the right inductor from such a large selection of devices can be overwhelming, but following a few basic guidelines will make the selection process much simpler. To maximize efficiency, choose an inductor with a low DC resistance. Keep in mind that most inductors that are very thin or have a very small volume typically have much higher core and DCR losses, and will not give the best efficiency. Choose an inductor with a DC current rating at least 1.5 times larger than the maximum load current to ensure that the inductor does not saturate during normal operation. Table 1 shows several inductors that work well with the LTC3455. These inductors offer a good compromise in current rating, DCR and physical size. Consult each manufacturer for detailed information on their entire selection of inductors.
Table 1. Recommended Inductors
Inductor Type DB318C L (H) 4.7 10 4.7 10 4.7 10 4.7 10 4.7 10 Max IDC (A) 0.86 0.58 0.75 0.5 0.9 0.55 1.29 0.82 1 0.68 Max DCR () 0.1 0.18 0.19 0.37 0.11 0.21 0.12 0.28 0.2 0.36 Height (mm) 1.8 1.8 1 1 1.8 1.8 1.2 1.2 1.2 1.2
Manufacturer Toko (847)297-0070 www.toko.com Sumida (847)956-0666 www.sumida.com Sumida Cooper (561)752-5000 www.cooperet.com Panasonic (408)945-5660 www.panasonic.com
CLS4D09
CDRH3D16 SD12
ELT5KT
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Switching Regulator Output Capacitor Selection Low ESR (equivalent series resistance) ceramic capacitors should be used at both switching regulator outputs. Only X5R or X7R ceramic capacitors should be used because they retain their capacitance over wider voltage and temperature ranges than other ceramic types. A 10F output capacitor is sufficient for most applications. Table 2 shows a list of several ceramic capacitor manufacturers. Consult each manufacturer for detailed information on their entire selection of ceramic capacitors. Many manufacturers now offer very thin (<1mm tall) ceramic capacitors ideal for use in height-restricted designs.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden AVX Murata TDK (408) 573-4150 (803) 448-9411 (714) 852-2001 (888) 835-6646 www.t-yuden.com www.avxcorp.com www.murata.com www.tdk.com
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VBAT Pin Capacitor Selection For the VBAT pin, a 4.7F to 10F ceramic capacitor is the best choice. Only X5R or X7R ceramic capacitors should be used. VMAX Pin Capacitor Selection For the VMAX pin, a 10F ceramic capacitor is the best choice. Only X5R or X7R ceramic capacitors should be used. Do not use less than 10F on this pin. For some designs it may be desirable to use a larger capacitor connected to VMAX to act as a reservoir when the LTC3455 is USB powered. Up to 50F of ceramic capacitance may be connected to the VMAX pin without difficulty. More than 50F requires using a capacitor with some ESR (like a Tantalum or OS-CON) or adding some resistance in series with some of the ceramic capacitance. This is necessary to ensure loop stability in the battery charger loop when under USB power.
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USB Pin and Wall Adapter Capacitor Selection Caution must be exercised when using ceramic capacitors to bypass the USB pin or the wall adapter input. High voltage transients can be generated when the USB or wall adapter is hot plugged. When power is supplied via the USB bus or wall adapter, the cable inductance along with the self resonant and high Q characteristics of ceramic capacitors can cause substantial ringing which can easily exceed the maximum voltage pin ratings and damage the LTC3455. Refer to Linear Technology Application Note 88, entitled "Ceramic Input Capacitors Can Cause Overvoltage Transients" for a detailed discussion of this problem. The long cable lengths of most wall adapters and USB cables makes them especially susceptible to this problem. Even if this ringing is not large enough to damage the part, it can couple to the VMAX pin (and to the switching regulator outputs) and be mistaken as loop instability. To bypass the USB pin and the wall adapter input, add a 1 resistor in series with a ceramic capacitor to lower the effective Q of the network and greatly reduce the ringing. A tantalum, OS-CON, or electrolytic capacitor can be used in place of the ceramic and resistor, as their higher ESR reduces the Q, thus reducing the voltage ringing. Use 4.7F to 10F for the USB pin, and 1F or larger for the wall adapter input. Programming Switching Regulator Output Voltage The output voltage for each switching regulator is programmed using a resistor divider from the output connected to the feedback pins (FB1 and FB2): R2 VOUT = 0.8 V * 1 + R1 Typical values for R1 are in the range of 80k to 400k.
VOUT R2 FB1, FB2 LTC3455 GND 25
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EFFICIENCY (%)
1, 18 R1
Figure 6. Setting the Output Voltage
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Burst ModeTM Operation For highest efficiencies at light loads, both DC/DC converters are capable of operating in Burst Mode. In this mode, energy is delivered to the outputs in shorts bursts, which minimizes switching losses and quiescent-current losses. Output voltage ripple is slightly higher in this mode, but efficiency is greatly improved. As shown in Figure 7, the efficiency at low load currents increases significantly when Burst Mode operation is used.
100 90 80 70 60 50 40 30 20 1 VBAT = 3.6V 10 100 LOAD CURRENT (mA) 1000
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Burst Mode 3.3V
1.8V Burst Mode
3.3V PWM Mode
1.8V PWM Mode
Figure 7. PWM and Burst Mode Efficiency
Tie the MODE pin to VMAX to always allow automatic Burst Mode operation. Even when the MODE pin is high, the LTC3455 will only enter Burst Mode when the load current is low. For many noise-sensitive systems, Burst Mode operation might be undesirable at certain times (i.e. during a transmit or receive cycle of a wireless device), but highly desirable at others (i.e. when the device is in low-power standby mode). The MODE pin can be used to enable or disable Burst Mode operation at any time, offering both low-noise and low-power operation when they are needed the most. Burst Mode is disabled initially at startup (for the first 200ms) and also whenever external power is available, even if the MODE pin is pulled high. Figure 8 shows the switching waveforms for switcher 1 (both PWM mode and Burst Mode Operation) with VIN = 3.6V, VOUT1 = 1.8V, and IOUT1 = 25mA.
Burst Mode is a registered trademark of Linear Technology Corporation.
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Burst Mode
VSW1 2V/DIV VOUT1 50mV/DIV AC COUPLED
IL1 100mA/DIV 5s/DIV
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PWM Mode
VSW1 2V/DIV VOUT1 10mV/DIV AC COUPLED IL1 100mA/DIV 1s/DIV
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Figure 8. Burst Mode and PWM Mode Waveforms
Soft-Start for each Switcher Soft-start is accomplished by gradually increasing the peak inductor current for each switcher. This allows each output to rise slowly, helping minimize the battery in-rush current. Figure 9 shows the battery current during startup. A soft-start cycle occurs whenever each switcher first turns on, or after a fault condition has occurred (thermal shutdown or UVLO).
VMAX 2V/DIV VOUT1 (1.8V) 2V/DIV VOUT2 (3.3V) 2V/DIV IBAT 500mA/DIV
100s/DIV
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Figure 9. In-Rush Current at Startup
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In-Rush Current Limiting When the LTC3455 is battery-powered, an internal 0.15 PMOS switch connects the battery (VBAT pin) to the VMAX pin to provide power for both switchers and other internal circuitry. This PMOS switch is turned off in shutdown, and the VMAX pin discharges to ground, providing output disconnect for all outputs. At startup, this PMOS must first charge up any capacitance present on the VMAX pin to the battery voltage. To minimize the in-rush current needed from the battery, the PMOS switch is current-limited to 900mA and both switchers are disabled while the VMAX voltage is ramping up. Once VMAX reaches the battery voltage, the PMOS current-limit increases to 4A and both switchers are allowed to turn on. Figure 9 shows the startup battery current for the LTC3455, which stays wellcontrolled while VMAX is ramping up and while both switchers outputs are rising. Battery Charger General Information The battery charger and Switcher 1 will always be enabled whenever USB or wall power is present (as sensed by the USB and WALLFB pins). This ensures that the battery can be charged and that the microcontroller is alive whenever external power is available. For some applications, it may be undesirable for the charger to become active immediately when external power is applied. For such applications, an NMOS switch can be used to disconnect the RPROG resistor and allow the PROG pin to float high, turning off the charger. In this manner, charging occurs only when allowed by the microcontroller. The LTC3455 battery charger is a constant-current, constant-voltage charger. In constant-current mode, the maximum charge current is set by a single external resistor. When the battery approaches the final float voltage, the charge current begins to decrease as the charger switches to constant-voltage mode. The charge cycle is terminated only by the charge timer.
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Charge and Recharge Cycles
When external power is first applied, a new charge cycle is always initiated. The battery will continue charging until the programmed charge time is reached. If the battery voltage is below 4.05V at the end of this cycle, the LTC3455 will start a new charge cycle. This action will continue until the battery voltage exceeds the 4.05V threshold. This operation is typically seen only when charging from USB power. Because the charge current can vary dramatically when the LTC3455 is USB powered, it takes considerably longer to charge a battery using the USB supply (as compared to a wall adapter). If the timer capacitor is chosen correctly, the battery should be fully charged on one cycle when wall power is available. If the battery is above the 4.05V threshold when a charge cycle has expired, charging will stop. At this point, a recharge cycle is initiated if any of the following occurs: The battery voltage drops below 4.05V, external power is removed and reapplied, the PROG pin is floated temporarily, or the SUSPEND pin is temporarily pulled high (if the LTC3455 is under USB power). Programming Charge Current The maximum charge current is programmed using one external resistor connected between the PROG pin and GND (use the closest 1% resistor value): RPROG = 1000 * 1.23V / IBAT If only USB power is used (no wall adapter), select the RPROG value to be 2.49k (or larger) to set the maximum charge current at 500mA. If a wall adapter is also used, ICHARGE can be programmed up to 1A (with a 1.24k RPROG value), and the USB power manager will automatically throttle back the charge current to below 500mA when under USB power. Monitoring Charge Current The voltage on the PROG pin is an accurate indication of the battery charge current under all charging conditions. IBAT = 1000 * 1.23V / RPROG
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Capacitance on the PROG pin should be minimized to ensure loop stability when in constant-current mode. Do not place a capacitor directly from the PROG pin to ground. Adding an external R-C network (see Figure 10) allows the monitoring of average, rather than instantaneous, battery charge current. Average charge current is typically of more interest to the user, especially when the LTC3455 is USB powered, as the battery charge current varies significantly with normal load transients.
LTC3455 PROG GND 25 2 RPROG 10k CFILTER CHARGE CURRENT MONITOR CIRCUITRY
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Figure 10. Monitoring Average Charge Current
Programming the Battery Charger Timer An external capacitor on the TIMER pin sets the total charge time. When this timer elapses the charge cycle terminates and the CHRG pin assumes a high impedance state. The total charge time is programmed as: TTIMER (hours) = CTIMER * (3 hours) / (0.1F) Trickle Charge and Defective Battery Detection If the battery voltage is below 2.85V at the beginning of the charge cycle, the charger goes into trickle charge mode, reducing the charge current to 10% of its programmed full-scale value. If the low battery voltage remains for one quarter of the programmed total charge time, the battery is assumed to be defective, the charge cycle is terminated, and the CHRG pin goes to a high impedance state. This fault is cleared if any of the following occurs: The battery voltage rises above 2.85V, external power is removed and reapplied, the PROG pin is floated temporarily, or the SUSPEND pin is temporarily pulled high (if the LTC3455 is under USB power). The device will still operate normally from USB or wall power even if the charger has turned off due to a trickle-charge timeout.
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Battery Charger Thermal Limit
CHARGE CURRENT (mA)
An internal thermal limit reduces the charge current if the die temperature attempts to rise above approximately 105C. This protects the LTC3455 from excessive temperature, and allows the user to push the limits of the power handling capability of a given circuit board without risk of damaging the LTC3455. Another benefit of the thermal limit is that charge current can be set according to typical, not worst-case, ambient temperatures for a given application with the assurance that the charger will automatically reduce the current under worst-case conditions. CHRG Status Output The CHRG pin is pulled low with an internal N-channel MOSFET whenever the battery charger is enabled, and is forced into a high impedance state whenever it is disabled. This NMOS device is capable of driving an external LED. This pin does not provide any C/10 information. Special Charger Features while USB Powered The LTC3455 has several special features that help make the most of the power available from the USB power supply. The internal USB power controller automatically throttles back the battery charge current to help keep the total system current under the strict 500mA/100mA USB limit. The graph in Figure 11 shows how charge current, IBAT, decreases as the current needed for the rest of the system increases (both switchers and all other external devices pull current from the VMAX pin). The total USB current, IUSB, always stays below 500mA.
500
CHARGE AND USB CURRENT (mA)
400 300 200 100
IUSB
IBAT
0 USB HIGH POWER MODE VUSB = 5V VBAT = 3.6V -100 200 300 400 500 0 100 TOTAL SYSTEM CURRENT (mA)
600
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Figure 11. Charge Current vs Total System Current
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As the USB voltage drops below 4.5V, the charge current gradually reduces (and eventually shuts off around 4V). This helps prevent "chattering" and stability problems when using long, resistive USB cables. Figure 12 shows this reduction in charge current.
500 USB HIGH POWER MODE VBAT = 3.6V 400 IBAT 300 200 100 0 3.75 4.00 4.25 4.50 4.75 VUSB (V) 5.00 5.25
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Figure 12. Charge Current vs USB Voltage
Because the charge current can vary dramatically when the LTC3455 is USB powered, battery charging can take considerably longer using the USB supply (as compared to a wall adapter). Constant-Current-Only Charger/Disabling the Charger Timer To use the charger in a constant-current-only mode, connect the TIMER pin to VMAX to disable the timer, voltage amplifier, and trickle charge function. To disable only the timer function and leave all others intact, connect the TIMER pin to GND. Since the charge cycle is terminated only by the charge timer, external charge termination is required when using either of these methods. Use an external NMOS to float the PROG pin and disable charging. Constant-current-only mode is a good choice for systems that are always powered by a USB supply or wall adapter, and the charger can be used to charge a super-cap or backup battery. Disabling the voltage amplifier allows the super-cap/backup battery to charge up fully to the available USB or wall adapter voltage.
LTC3455
APPLICATIO S I FOR ATIO
Hot Swap Output
A current limited Hot Swap output is provided for powering memory cards or other external devices that can be hot-plugged into the system. Typically connected to the 3.3V supply, this output provides isolation to prevent the external device from disturbing the 3.3V supply when inserted. The Hot Swap output can only operate when the LTC3455 is on, and is enabled using the HSON pin. If this hot-plugging protection is not needed, this output can be used as a load switch for other devices within the system. The HSO pin is discharged to ground when the LTC3455 is shut down. Gain Block The LTC3455 contains a gain block (pins AI and AO) that can be used as either a low-battery indicator, or as an LDO with the addition of an external PNP. Both circuits are shown in Figure 13. The LDO is convenient for applications needing a third output (possibly a low current 2.5V or a quiet 3V supply). The AO pin can sink around 1mA, which typically limits the LDO current to 100mA or less (due to the current gain of the PNP). An external PMOS can be used for the LDO, but a much larger output capacitor is needed to ensure stability at light loads. The gain block is alive whenever switcher 1 is enabled, and is turned off during shutdown to minimize battery drain. This means that the low-battery detector will not report a low-battery condition until the LTC3455 is turned on. This
TYPICAL APPLICATIO S
Standalone USB Power Supply with Temporary Backup Power Although designed primarily for Li-Ion powered portable applications, the LTC3455 is also a good choice for systems that are always powered by a USB supply or wall adapter. The battery charger can then be used to charge up a large capacitor or backup battery, which briefly provides power to the system after the external power has been removed. This gives the microcontroller enough time to follow proper shutdown procedures when the main power source is abruptly removed. Figure 14 shows a standalone power supply for USB high power applications (500mA maximum USB current) using the LTC3455. The total system power should be kept below 1.8W to ensure clean operation even under worst-case USB conditions. With the resistor values shown, the low-battery indicator (AI and AO pins) triggers when the VMAX pin voltage drops to 4V, notifying the microcontroller of an impending dropout condition. The 1M resistor connected between the AI
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LOW-BATTERY DECTECTOR 1.8V 1M LBO VBAT 2.49M 16 806k AI 10F 80.6k
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LDO 3.3V 100k
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AO 2.5V 169k
17
AO
LTC3455
100pF LTC3455 16 AI
Figure 13. Low-Battery Detector and LDO Using the Gain Block
is not a problem for most applications since the LTC3455 usually powers the microcontroller and all other intelligence in the system. PCB Layout Considerations As with all DC/DC converters, careful attention must be paid to the printed circuit board (PCB) layout and component placement. The VBAT capacitor, VMAX capacitor, and both inductors must all be placed as close as possible to the LTC3455. These components, along with both DC/DC converter output capacitors, should be placed on the same side of the circuit board as the LTC3455, with their connections made on that top layer. Place a local, unbroken ground plane below these components that is tied to the exposed pad of the LTC3455. The exposed pad (pin 25) must be soldered to the PCB (to system ground) for proper operation.
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LTC3455
TYPICAL APPLICATIO S
USB 5V USB 5V 1 C6 4.7F USB CONTROLLER 8 6 5 USB SUSPEND USBHP MODE HSON ON2 PWRON RST VMAX PBSTAT 21 15 19 22 20 23 1M LTC3455 4 11 3 2 9 C4 4700F 1.8V 10k DROPOUT VMAX 82.5k 1M 16 20k AI 17 AO SW1 FB1 GND
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10 C5 10F
CHRG WALLFB TIMER PROG VBAT
ON HSO
VMAX 2.49k
HSI SW2 FB2
C1, C2, C3, C5, C6: X5R OR X7R CERAMIC L1, L2: TOKO DB318C ALL RESISTORS 1%
Figure 14. Standalone USB Power Supply with Temporary Backup Power
and AO pins provides 150mV of hysteresis (the dropout indicator stays low until the VMAX pin rises back above 4.15V). A 4700F backup capacitor connected to the VBAT pin briefly provides power to the system after the USB supply has been removed, and also helps support transient loads that slightly exceed the USB current limit. Connecting this large capacitance to the VBAT pin has several advantages. It provides a large energy reservoir that is isolated from both the USB pin (the USB specification limits capacitance on the USB supply pin to 10F or less) and the VMAX pin (using a very large capacitance on this pin will delay the system turn-on), and it prevents large inrush currents by using the battery charger to slowly charge this capacitor (normally using such a large capacitor would result in very large inrush currents). With the TIMER pin tied to VMAX, the battery charger operates in constant-current mode (the voltage-loop and timer function are disabled), so the 4700F capacitor is always fully charged to the available USB voltage.
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24 14 13 12 18 7 1 25
8 6 USB CONTROLLER 5
USB SUSPEND USBHP
MODE HSON ON2 PWRON RST PBSTAT
21 15 19 22 20 23 1M 1M 1.8V ON/OFF 3.3V, HS C3 1F 1k C
1 C6 4.7F C WALL 5V 1 C7 1F 1.8V ON/OFF
10 C5 D1 10F 3.32k 1k 4 11 C8, 0.1F 1.24k 2.49k 9 3 2
VMAX
1M
LTC3455 CHRG WALLFB TIMER PROG AO VBAT HSI SW2 AI FB2 17 13 12 16 18 L2, 4.7H 100pF ON HSO 24 14
3.3V, HS C3 1F L2, 4.7H 10pF 249k C2 10F 80.6k L1, 4.7H 10pF 100k C1 10F 80.6k 1.8V 0.2A
VMAX M1 FDN304P OR Si2305DS 3.3V 1.2A C2 2x10F 2.49k
3.3V 0.4A
SINGLE CELL Li-ION 3.3V TO 4.2V
+
C4 4.7F
249k
80.6k 7 1 25 L1, 4.7H 10pF C1 TO C8: X5R OR X7R CERAMIC L1, L2: TOKO DB318C D1: ON SEMI MBRM110E ALL RESISTORS 1% FB1 GND
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SW1
100k
1.8V 0.4A C1 10F
80.6k
Figure 15. LTC3455 Application with 3.3V Output Current Increased to 1.2A
Increasing 3.3V Output Current to 1.2A With an internal current limit of 900mA, Switcher 2 typically provides a 3.3V, 600mA output. While this output current is sufficient for many portable devices, some applications need a 3.3V supply capable of providing more than 1A. Figure 15 shows how to implement a higher current 3.3V output using the LTC3455. By adding one tiny SOT23 PMOS and using the AI/AO amplifier as an LDO, the 3.3V output now provides 1.2A of output current. Switcher 2 is programmed for an output voltage of 3.3V, and the LDO is programmed for an output voltage of 3.2V (3% lower). As long as the load current is low enough for Switcher 2 to provide, the LDO is turned off completely. This circuit is ideal for applications that need the higher 3.3V output current for only a brief time. Switcher 2 will normally provide all of the output current, and the LDO will turn on briefly to provide the higher load currents.
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LTC3455
TYPICAL APPLICATIO S
When the load current exceeds what Switcher 2 can provide, the 3.3V output droops slightly and the LDO provides the additional current needed. Figure 16 shows the transient response when the 3.3V output current is stepped from 0.5A to 1.2A. More output capacitance can be added to improve the 3.3V transient response during these high current load steps.
VOUT2 (3.3V) 100mV/DIV AC COUPLED
PACKAGE DESCRIPTIO
4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05
PIN 1 TOP MARK (NOTE 6)
NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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IOUT2 0.5A/DIV 0.5A TO 1.2A STEP M1 GATE 2V/DIV 500s/DIV
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Figure 16. Load Current Step (0.5A to 1.2A) for 3.3V Output
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC BOTTOM VIEW--EXPOSED PAD 0.23 TYP R = 0.115 (4 SIDES) TYP 23 24 0.38 0.10 1 2 2.45 0.10 (4-SIDES)
4.00 0.10 (4 SIDES)
(UF24) QFN 1103
0.200 REF 0.00 - 0.05
0.25 0.05 0.50 BSC
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LTC3455
TYPICAL APPLICATIO
USB 5V 1 C6 4.7F
USB CONTROLLER
WALL 5V 1 C7 1F REMOVE THESE COMPONENTS IF WALL ADAPTER IS NOT USED D1 3.32k C5 10F 1k
1.24k 2.49k
SINGLE CELL Li-ION 3.3V TO 4.2V
+
RELATED PARTS
PART NUMBER LT1616 LTC1879 LTC3405/LTC3405A LTC3406/LTC3406B LTC3407 LTC3412 LTC3414 LTC3440/LTC3441 DESCRIPTION 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter Dual 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA/1A (IOUT), 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS 90% Efficiency, VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 1.9mA, ISD <1A, ThinSOT 95% Efficiency, VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IQ = 15A, ISD <1A, TSSOP16 95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 20A, ISD <1A, ThinSOT 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD <1A, ThinSOT 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, MS10E 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD <1A, TSSOP16E 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64A, ISD <1A, TSSOP16E 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 2.5V, IQ = 25A/50A, ISD <1A, MS/DFN
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
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8 6 5 USB SUSPEND USBHP MODE HSON ON2 PWRON RST VMAX PBSTAT 21 15 19 22 20 23 1M LTC3455 4 11 C8, 0.1F 3 2 9 C4 4.7F 1.8V 1M LBO VBAT 2.49M 16 806k AI FB1 GND
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C
10
1M 1.8V ON/OFF 3.3V, HS C3 1F
CHRG WALLFB TIMER PROG VBAT
ON HSO
24 14
HSI SW2 FB2
13 12 18 L2, 4.7H 10pF 249k 3.3V 0.5A C2 10F 80.6k
17
AO SW1
7 1 25
L1, 4.7H 10pF 100k 1.8V 0.4A C1 10F 80.6k
C1 TO C8: X5R OR X7R CERAMIC L1, L2: TOKO DB318C D1: ON SEMI MBRM110E ALL RESISTORS 1%
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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