View LTC3620EDC-1PBF_4487252.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

FEATURES
n n n n n n n n n n n n n n
LTC3620 Ultralow Power 15mA Synchronous Step-Down Switching Regulator DESCRIPTION
The LTC(R)3620 is a high efficiency, synchronous buck regulator, suitable for very low power, very small footprint applications powered by a single Li-Ion battery. The internal synchronous switches increase efficiency and eliminate the need for external Schottky diodes. Low output voltages are easily supported by the 0.6V feedback reference voltage. The LTC3620-1 option is internally programmed to provide a 1.1V output. The LTC3620 uses a unique variable frequency architecture to minimize power loss and achieve high efficiency. The switching frequency is proportional to the load current, and an internal frequency clamp forces a minimum switching frequency at light loads to minimize noise in the audio range. The user can program the frequency of this clamp by applying an external clock to the FMIN/MODE pin. The battery status output, LOBATB, indicates when the input voltage drops below 3V. To help prevent damage to the battery, an undervoltage lockout (UVLO) circuit shuts down the part if the input voltage falls below 2.8V. The LTC3620 is available in a low profile, 2mm x 2mm 8-lead DFN package.
High Efficiency: Up to 95% Maximum Current Output: 15mA Externally Programmable Frequency Clamp with Internal 50kHz Default Minimizes Audio Noise 18A IQ Current 2.9V to 5.5V Input Voltage Range Low-Battery Detection 0.6V Reference Allows Low Output Voltages Shutdown Mode Draws <1A Supply Current 2.8V Undervoltage Lockout Unique Low Noise Control Architecture Internal Power MOSFETs No Schottky Diodes Required Internal Soft-Start Tiny 2mm x 2mm 8-Lead DFN Package
APPLICATIONS
n n n n
Hearing Aids Wireless Headsets Li-Ion Cell Applications Button Cell Replacement
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 7528587.
TYPICAL APPLICATION
High Efficiency Low Power Step-Down Converter
VIN 2.9V TO 5.5V VIN VOUT 1.1V VOUT (mVP-P) RUN 1F CER LOBATB SW 22H 22pF 15 VIN = 3.6V 10 25
Output Voltage Ripple vs Load Current
VOUT = 1.1V FMIN/MODE = 0V L = 22H EFFICIENCY (%) VIN = 5.5V 100 90 80 70 60 50 40 30 1F CER 5 20 10
3620 TA01a
Efficiency vs Load Current
3.0 EFFICIENCY 2.5 POWER LOSS (mW) 2.0 1.5 VIN = 3V FMIN/MODE = 0V VOUT = 1.1V VOUT = 1.8V VOUT = 2.5V 1.0 0.5 0 1 LOAD CURRENT (mA) 10
3620 TA01c
20
LTC3620 FMIN/MODE VFB GND
432k 523k
LOSS
0
0
10 5 OUTPUT CURRENT (mA)
15
3620 TA01b
0 0.1
3620f
1
LTC3620 ABSOLUTE MAXIMUM RATINGS
(Notes 1, 2)
PIN CONFIGURATION
TOP VIEW SW 1 GND 2 FMIN/MODE 3 LOBATB 4 9 8 VIN 7 RUN 6 VFB 5 NC
Input Supply Voltage .................................... -0.3V to 6V RUN Voltage ................................. -0.3V to (VIN + 0.3V) VFB Voltage ................................... -0.3V to (VIN + 0.3V) LOBATB Voltage ........................................... -0.3V to 6V FMIN/MODE Voltage ..................... -0.3V to (VIN + 0.3V) SW Voltage .................................. -0.3V to (VIN + 0.3V) P-channel Switch Source Current (DC) ..................50mA N-channel Switch Sink Current (DC) ......................50mA Operating Junction Temperature Range (Notes 2, 4) ............................................-40C to 125C Storage Temperature Range................... -65C to 150C
DC PACKAGE 8-LEAD (2mm 2mm) PLASTIC DFN TJMAX = 125C, JA = 88.5C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3620EDC#PBF LTC3620EDC-1#PBF TAPE AND REEL LTC3620EDC#TRPBF LTC3620EDC-1#TRPBF PART MARKING LFJJ LFJK PACKAGE DESCRIPTION 8-Lead (2mm x 2mm) Plastic DFN 8-Lead (2mm x 2mm) Plastic DFN TEMPERATURE RANGE -40C to 85C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
SYMBOL VIN VFB PARAMETER Input Voltage Range Regulated Feedback Voltage (Note 3)
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise noted.
CONDITIONS
l
MIN 2.9 0.594 0.588 1.089 1.078
TYP 0.6 0.6 1.1 1.1 0.05 0.5 18 0.01 0.5 35
MAX 5.5 0.606 0.612 1.111 1.122 0.15 25 1
UNITS V V V V V %/V % A A A mA kHz V
LTC3620 LTC3620 LTC3620-1 LTC3620-1 VIN = 3V to 5.5V (Note 3) (Note 3) VFB = 0.65V, FMIN/MODE = VIN RUN = 0V RUN = VIN, VIN = 2.5V
l l
VFB VLOADREG IQ IQSD IQU IPK fSW VRUN IRUN
Reference Voltage Line Regulation Output Voltage Load Regulation Quiescent Current, No Switching Quiescent Current in Shutdown Quiescent Current in UVLO Condition Peak Inductor Current
Minimum Switching Frequency (Internal) VFB = 0.65V, FIN/MODE = 0 RUN Input Voltage High RUN Input Voltage Low RUN Leakage Current
l
40 0.8
50 0.3 0.01 1
V A
3620f
2
LTC3620 ELECTRICAL CHARACTERISTICS
SYMBOL VFMIN fEXT IFMIN/MODE ISW IFB VUVLO VLOBATB RLOBATB VHLOBATB RPFET RNFET PARAMETER FMIN/MODE Input Voltage High FMIN/MODE Input Voltage Low FMIN/MODE Input Frequency FMIN/MODE Pin Leakage Current Switch Leakage Current VFB Pin Current Undervoltage Lockout (UVLO) LOBATB Threshold LOBATB Pull-Down On-Resistance LOBATB Hysteresis RDS(ON) of P-channel FET (Note 5) RDS(ON) of N-channel FET (Note 5) ISW = 50mA, VIN = 3.6V ISW = -50mA, VIN = 3.6V VRUN = 0V, VSW = 0V or 5.5V, VIN = 5.5V LTC3620, VFB = 0.6V LTC3620-1, VFB = 1.1V VIN Decreasing VIN Decreasing 2.7 2.93 20 0.01 0.01 0 1.2 2.8 3.0 15 100 2.0 1.0
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise noted.
CONDITIONS MIN 0.9 0.7 300 1 1 30 2.0 2.9 3.08 TYP MAX UNITS V V kHz A A nA A V V mV
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: TJ is calculated from the ambient temperature, TA, and power dissipation, PD, according to the following formula: TJ = TA + (PD)(88.5C/W) Note 3: The LTC3620 is tested in a proprietary test mode that connects VFB to the output of the error amplifier.
Note 4: The LTC3620E is guaranteed to meet performance specifications from 0C to 85C junction temperature. Specifications over the -40C to 85C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. Note 5: The DFN switch-on resistance is guaranteed by correlation to wafer level measurements.
3620f
3
LTC3620 TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency vs Load Current, FMIN/MODE
1000 200kHz, EXTERNAL FMIN/MODE = 0V FMIN/MODE = VIN 0.5
Load Regulation
VIN = 3.6V VOUT = 1.1V FMIN/MODE = 0V TA = 25C VFB (mV) 620 615 610 605 600 595 590 -0.3 585 -0.5 0 10 5 LOAD CURRENT (mA) 15
3620 G02
LTC3620 Feedback Voltage vs Temperature
VIN = 3.6V VOUT = 1.1V FMIN/MODE = 0V
SWITCHING FREQUENCY (kHz)
VOUT VOLTAGE CHANGE (%) TA = 25C VIN = 3.6V VOUT = 1.1V
0.3
0.1
100
-0.1
10 0.01
1 0.1 LOAD CURRENT (mA)
10 20
3620 G01
580 -50
50 0 TEMPERATURE (C)
100
130
3620 G03
LTC3620-1 Feedback Voltage vs Temperature
1.125 1.120 1.115 1.110 VFB (V) 1.105 1.100 1.095 1.090 1.085 1.080 -50 50 0 TEMPERATURE (C) 100 130
3620 G04
Quiescent Current vs Temperature
30 2.85 VOUT = 1.1V FMIN/MODE = VIN UVLO THRESHOLD (V) 2.84 2.83 2.82 2.81 2.80 2.79 2.78 2.77 2.76 50 0 TEMPERATURE (C) 100 130
3620 G05
UVLO Threshold vs Temperature
VOUT = 1.1V FMIN/MODE = 0V
VIN = 3.6V FMIN/MODE = 0V QUIESCENT CURRENT (A)
28 26 24 22 20 18 16 14 12
VIN = 5V VIN = 3.6V
10 -50
2.75 -50
50 0 TEMPERATURE (C)
100
130
3620 G06
LOBATB Threshold vs Temperature
3.05 3.04 3.03 LOBATB THRESHOLD (V) 3.02 IPEAK (mA) 3.01 3.00 2.99 2.98 2.97 2.96 2.95 -50 50 0 TEMPERATURE (C) 100 130
3620 G07
Peak Inductor Current vs Temperature
40 VOUT = 1.1V L = 22H VIN = 5.5V 38 37 36 35 34 -50 VIN = 3.6V VSW 2V/DIV 39 VOUT (AC) 20mV/DIV
Switching Waveforms at 250A Load, FMIN/MODE = 0V
VOUT = 1.1V FMIN/MODE = 0V
IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 4s/DIV
3620 G09
0 50 TEMPERATURE (C)
100
130
3620 G08
3620f
4
LTC3620 TYPICAL PERFORMANCE CHARACTERISTICS
Switching Waveforms at 1mA Load, FMIN/MODE = 0V
VOUT (AC) 20mV/DIV VOUT (AC) 20mV/DIV VSW 2V/DIV
Switching Waveforms at 12mA Load, FMIN/MODE = 0V
VFMIN/MODE 1V/DIV VOUT (AC) 20mV/DIV VSW 2V/DIV
Switching Waveforms at 250A Load, FMIN/MODE = 200kHz Clock
VSW 2mV/DIV
IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 4s/DIV
3620 G10
IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 400ns/DIV
3620 G11
IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 2s/DIV
3620 G12
Switching Waveforms at 1mA Load, FMIN/MODE = 200kHz Clock
VFMIN/MODE 1V/DIV VOUT (AC) 20mV/DIV VSW 2V/DIV VFMIN/MODE 1V/DIV VOUT (AC) 20mV/DIV VSW 2V/DIV IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 2s/DIV
3620 G13
Switching Waveforms at 12mA Load, FMIN/MODE = 200kHz
Start-Up Waveforms
VOUT 200mV/DIV
IL 25mA/DIV
IL 25mA/DIV VOUT = 1.1V VIN = 3.6V TA = 25C 400ns/DIV
3620 G14
VOUT = 1.1V 200s/DIV VIN = 3.6V IOUT = 0mA FMIN/MODE = 0V TA = 25C
3620 G15
Transient Response, 250A to 3mA Step, FMIN/MODE = 0V
Transient Response, 1mA to 10mA Step, FMIN/MODE = 0V
2.3 2.1
PFET RDS(ON) vs Temperature
ISW = 35mA
VOUT (AC) 10mV/DIV
VOUT (AC) 20mV/DIV RDS(ON) ()
1.9 1.7 1.5 1.3 1.1 0.9 VIN = 3.6V VOUT = 1.1V TA = 25C 4ms/DIV
3620 G17
VIN = 3.6V
VIN = 5V
ILOAD 5mA/DIV VIN = 3.6V VOUT = 1.1V TA = 25C 4ms/DIV
3620 G16
ILOAD 5mA/DIV
0.7 0.5 -50 50 0 TEMPERATURE (C) 100 130
3620 G18
3620f
5
LTC3620 TYPICAL PERFORMANCE CHARACTERISTICS
NFET RDS(ON) vs Temperature
1.5 1.4 1.3 EFFICIENCY (%) 1.2 RDS(ON) () 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -50 50 0 TEMPERATURE (C) 100 130
3620 G19
Efficiency vs Load Current, VOUT
100 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 90 80 70 60 50 40 30 20 10 0 0.1 TA = 25C VIN = 3V FMIN/MODE = 0V VOUT = 1.1V VOUT = 1.8V VOUT = 2.5V 1 LOAD CURRENT (mA) 10
3620 G20
Efficiency vs Load Current, FMIN/MODE Frequency
ISW = 35mA
VIN = 3.6V
VIN = 5V
TA = 25C VIN = 3V VOUT = 1.1V FMIN = 20kHz FMIN = 100kHz FMIN = 200kHz 1 LOAD CURRENT (mA) 10
3620 G21
0 0.1
Efficiency vs Load Current, VIN
100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 LOAD CURRENT (mA) TA = 25C VOUT = 1.1V FMIN/MODE = VIN VIN = 3V VIN = 3.6V VIN = 5.5V 10
3620 G22
Efficiency vs fMIN, 1mA Load
81 80 79 INTERNAL fMIN (kHz) 78 77 76 75 74 73 72 71 0 TA = 25C VIN = 3.6V VOUT = 1.1V FMIN/MODE = EXTERNAL CLOCK 100 200 fMIN (kHz) 300
3620 G23
Internal fMIN vs Temperature
55 54 53 52 51 50 49 48 47 46 45 -50 50 0 TEMPERATURE (C) 100
3620 G24
VIN = 3.6V VIN = 5V
Spectral Content, 500A Load
-60 -80 -100 -120 -140 -160 12.5kHz VOUT = 1.1V VIN = 3.6V FMIN/MODE = 0V TA = 25C 8kHz/DIV 92.5kHz
3620 G25
Spectral Content, 5mA Load
-40 -60 -80 -100 -120 -140 1kHz 39.9kHz/DIV VOUT = 1.1V VIN = 3.6V FMIN/MODE = 0V TA = 25C 400kHz
3620 G26
POWER RATIO (dBm)
POWER RATIO (dBm)
52.5kHz -81.4dBm
355.6kHz -80.2dBm
3620f
6
LTC3620 PIN FUNCTIONS
SW (Pin 1): Switch Node Connection to Inductor. This pin connects to the internal power MOSFET Switches. GND (Pin 2): Ground Connection for Internal Circuitry and Power Path Return. Tie directly to local ground plane. FMIN/MODE (Pin 3): Frequency Clamp Select Input. Driving this pin with a 20kHz to 300kHz external clock sets the minimum switching frequency. Pulling this pin low sets the minimum switching frequency to the internally set 50kHz. Pulling this pin high defeats the minimum switching frequency and allows the part to switch at arbitrarily low frequencies dependent on the load current. LOBATB (Pin 4): Low-Battery Status Output. This opendrain output pulls low when VIN falls below 3V. NC (Pin 5): No Connect. VFB (Pin 6): Regulator Feedback Pin. This pin receives the feedback voltage from the resistive divider across the output. For the LTC3620-1, this pin must be connected directly to VOUT . VOUT is internally divided from VOUT to the reference voltage of 0.6V as seen in the Block Diagram. RUN (Pin 7): Regulator Enable Pin. Apply a voltage greater than 0.8V to enable the regulator. Do not float this pin. VIN (Pin 8): Input Supply Pin. Must be locally bypassed. Exposed Pad (Pin 9): GND. Must be soldered to PCB.
BLOCK DIAGRAM
VIN 8 LOBAT 3V RUN 7 UVLO FMIN/MODE 3 SELECT 50kHz PFD SWITCH DRIVER SW 1 SHUTDOWN ICMP LOBATB 4 PEAK INDUCTOR CURRENT ADJUST VIN 8
0.6V VFB (LTC3620) 6 VFB (LTC3620-1) 6 EAMP RCMP
2 GND
3620 BD
3620f
7
LTC3620 OPERATION
The LTC3620 is a variable frequency buck switching regulator with a maximum output current of 15mA. At high loads the LTC3620 will supply constant peak current pulses through the output inductor at a frequency dependent on the load current. A switching cycle is initiated by a pulse from the error amplifier, EAMP The top FET is turned on and remains on . until the peak current threshold is sensed by ICMP (35mA at full loads). When this occurs, the top FET it is turned off and the bottom FET is turned on. The bottom FET remains on until the inductor current drops to 0A, as sensed by the reverse-current comparator, RCMP The time interval before . another switching cycle is initiated is adjusted based on the output voltage error, measured by the EAMP to be the difference between VFB and the 0.6V reference. As the load current decreases, the EAMP will decrease the switching frequency to match the load, until the minimum switching frequency (internally or externally set) is reached. With the FMIN/MODE pin pulled low, the minimum frequency is internally set to 50kHz. Further decreasing the load will cause the phase frequency detector (PFD) to decrease the peak inductor current in order to maintain the switching frequency at 50kHz. The minimum switching frequency can be externally set by clocking the FMIN/MODE pin at the desired minimum switching frequency. The load current below which the
1000
switching frequency will be clamped is dependent on the externally set frequency and the value of the inductor used. A higher externally set minimum frequency will result in a higher load current threshold below which the part will lock to this minimum frequency. The relationship between load current and minimum frequency is described by the following equation: IMAX(LOCK)
( VIN )( fMIN )(L )(35mA )2 =
2VOUT ( VIN - VOUT )
The LTC3620 will switch at this externally set frequency at load currents below this threshold; though in general, neither this minimum nor this synchronization will be maintained during load transients. At very light loads, the minimum PFET on time will be reached and the peak inductor current can no longer be reduced. In this situation, the LTC3620 will resume decreasing the regulator switching frequency to prevent the output voltage from climbing uncontrollably. For those applications which are not sensitive to the spectral content of the output ripple, the minimum frequency clamp can be defeated by pulling the FMIN/MODE pin high. In this mode the inductor current peaks will be held at 35mA and the switching frequency will decrease without limit.
SWITCHING FREQUENCY (kHz)
200kHz, EXTERNAL FMIN/MODE = 0V FMIN/MODE = VIN
100
10 0.01
TA = 25C VIN = 3.6V VOUT = 1.1V 0.1 1 LOAD CURRENT (mA) 10 20
3620 F01
Figure 1. Switching Frequency vs Load Current, FMIN/MODE
3620f
8
LTC3620 APPLICATIONS INFORMATION
Choosing an Inductor There are a number of different values, sizes and brands of inductors that will work well with this part. Table 1 has a number of recommended inductors, though there are many other manufacturers and devices that may also be suitable. Consult each manufacturer for more detailed information and for their entire selection of related parts.
Table 1: Representative Surface Mount Inductors
PART NUMBER VALUE (H) MAX DC CURRENT DCR () (mA) 70 190 185 270 380 320 WxLxH (mm3) 0.8 x 1.6 x 0.8 1.6 x 2 x 0.9 2.8 x 2.8 x 1.1 2.95 x 2.95 x 0.9
The part is optimized to get 35mA peaks for VIN = 3.6V and VOUT = 1.1V with an 18H inductor. If the falling slope is too steep the NFET will continue to conduct shortly after the inductor current reaches zero, causing a small reverse current. This means the net power delivered with every pulse will decrease. To mitigate this problem the inductor can be resized. Table 2 shows recommended inductors and output capacitors for commonly used output voltages.
Table 2. Recommended Inductor and Output Capacitor Sizes for Different VOUT
VOUT (V) 0.9 1.1 1.1 (LTC3620-1) 1.8 2.5 L (H) 15 22 22 33 47 COUT (F) 2.2 1 2.2 2.2 4.7
VENDOR Taiyo Yuden Murata
CBMF1608T 22 10% 1.3 Max LQH2MC_02 18 20% 1.8 30% 22 20% 2.1 30%
Wurth 744028220 22 30% 1.48 Max Electronics Coilcraft LPS3010 18 20% 1.0 Max 22 20% 1.2 Max
There is a trade-off between physical size and efficiency; The inductors in Table 1 are shown because of their small footprints, choose larger sized inductors with less core loss and lower DCR to maximize efficiency. The ideal inductor value will vary depending on which characteristics are most critical to the designer. Use the equations and recommendations in the next sections to help you find the correct inductance value for your design. Avoiding Audio Range Switching In order to best avoid switching in the audio range at the lowest possible load current, the minimum frequency should be set as low as is acceptable, and the inductor value should be minimized. For a 1.1V output the smallest recommended inductor value is 15H. Adjusting for VOUT The inductor current peak and zero crossing are dependent on the dI/dt. The equations for the rising and falling slopes are as follows: Rising dI/dt = (VIN-VOUT)/L Falling dI/dt = VOUT/L
Because the rising dI/dt decreases for increased VOUT and increased L, the inductor current peaks will decrease, causing the maximum load current to decrease as well. Figure 2 shows typical maximum load current versus output voltage.
20 19 MAXIMUM LOAD CURRENT (mA) 18 17 16 15 14 13 12 11 10 0.6 1.6 1.1 2.1 OUTPUT VOLTAGE (V) 2.6
3620 F02
TA = 25C
Figure 2. Maximum Output Current vs VOUT , VIN = 3.6V
Output Voltage Ripple The quantity of charge transferred from VIN to VOUT per switching cycle is directly proportional to the inductor value. Consequently, the output voltage ripple is directly proportional to the inductor value, and the switching frequency for a given load is inversely proportional to the inductor value. For a given load current, higher switching frequency will typically lower the efficiency because of the
3620f
9
LTC3620 APPLICATIONS INFORMATION
increase in switching losses internal to the part. This can be partially offset by using inductors with lower loss. The peak-to-peak output voltage ripple can be approximated by: V = 2 (COUT ) ( VOUT ) ( VIN - VOUT ) from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current and proportional to frequency. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. The RDS(ON) for both the top and bottom MOSFETS can be obtained from the Typical Performance Characteristics curves. The I2R losses per pulse will be proportional to the peak current squared times the sum of the switch resistance and the inductor resistance: Loss IPK 2 IR = R Pulse 3 EFF
2
(I )(L)( V
PK 2
IN
)
The output ripple is a strong function of the peak inductor current, IPK. When the LTC3620 is locked to the minimum switching frequency, IPK is decreased to maintain regulation. Consequently, VOUT is reduced in and below the lock range. Efficiency The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in the LTC3620's circuits: VIN quiescent current and I2R losses. VIN quiescent current loss dominates the efficiency loss at low load currents, whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of little consequence, as illustrated on the front page of this data sheet. The quiescent current is due to two components: the DC bias current, IQ, as given in the Electrical Characteristics, and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves
where REFF = RL + RPFET D + RNFET (1 - D), and D is the ratio of the top switch on-time to the total time of the pulse. Additional losses incurred from the inductor DC resistance and core loss may be significant in smaller inductors. Capacitor Selection Higher value, lower cost, ceramic capacitors are now widely available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3620's control loop does not depend on the output capacitor's ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. The output voltage ripple is inversely proportional to the output capacitor. The larger the capacitor, the smaller the ripple, and vice versa. However, the transient response time is directly proportional to COUT , so a larger COUT means slower response time. To maintain stability and an acceptable output voltage . ripple, values for COUT should range from 1F to 5F
3620f
10
LTC3620 APPLICATIONS INFORMATION
Setting Output Voltage The output voltage is set by tying VFB to a resistive divider using the following formula (refer to Figure 3): VOUT = 0.6V (R1+ R2) R2 the frequency clamp loop in returning the peak inductor current to its maximum. Thermal Considerations The LTC3620 requires the package backplane metal to be well soldered to the PC board. This gives the DFN package exceptional thermal properties, making it difficult in normal operation to exceed the maximum junction temperature of the part. In most applications the LTC3620 does not dissipate much heat due to its high efficiency and low current. In applications where the LTC3620 is running at high ambient temperatures and high load currents, the heat dissipated may exceed the maximum junction temperature of the part if it is not well thermally grounded. Design Example This example designs a 1.1V output using a Li-Ion battery with voltages between 2.8V to 4.2V, and an average of 3.6V. The internally provided 50kHz clock will be used for the minimum switching frequency, so the FMIN/MODE pin will be pulled low. For a 1.1V output, an 18H inductor should be used (refer to Table 2). COUT can be chosen from Table 2 or can be based on a desired maximum output voltage ripple, VOUT . For this case let's use a maximum VOUT equal to 1% of VOUT , or 11mV. COUT = 2VOUT (1.1V ) ( 3.6V - 1.1V )
R1 and R2 should be large to minimize standing load current and improve efficiency. The fixed output version, the LTC3620-1, includes an internal resistive divider, eliminating the need for external resistors. The resistor divider is chosen such that the VFB input current is approximately 1A. For this version, the VFB pin should be connected directly to VOUT . Maximum Load Current and Maximum Frequency The maximum current that the LTC3620 can provide is calculated to be just slightly less than half the maximum peak current. The inductor value will determine how much energy is delivered to the output for each switching cycle, and thus the duration of each pulse and the maximum frequency. Larger inductors will have slower ramp rates, longer pulses, and thus lower maximum frequencies. Conversely, smaller inductors will result in higher maximum frequencies. When using a frequency clamp, large abrupt increasing load steps from levels below the locking range to levels near the maximum output may result in a large drop in the output voltage. This is due to the low bandwidth of
VIN 2.9V TO 5.5V VIN RUN 1F CER
(35mA )(22H)(3.6V )
2
LOBATB
= 1.6F 1.5F
1M L SW 22pF VOUT 1.1V
LOBATB
LTC3620
FMIN/MODE VFB GND R2
R1 COUT
3620 F03
Figure 3. Design Example Schematic
3620f
11
LTC3620 APPLICATIONS INFORMATION
A larger capacitor could be used to reduce this number. Keep in mind that while a larger output capacitor will decrease voltage ripple, it will also increase the transient settling time. The optimal range for COUT should be between 1F and 5F . The best way to select the feedback resistors is to select a target combined resistance, and try different standard 1% resistor sizes to see which combination will give the least error. For this example a target combined resistance of around 1M will be used. By checking R1 values between 422k and 475k, and calculating R2 using the formula: R2 = VOUT - 0.6V Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3620: 1. The power traces consisting of GND, SW and VIN should be kept short, direct and wide. 2. The VFB pin should connect directly to the respective feedback resistors, which should also have short, direct paths to VOUT and GND respectively. 3. Keep COUT and CIN as close to the LTC3620 as possible. 4. All parts connecting to ground should have their ground terminals in close proximity to the LTC3620 GND connection. 5. Keep the SW node and external clock, if used, away from the sensitive VFB node. Also, minimize the length and area of all traces connected to the SW pin, and always use a ground plane under the switching regulator to minimize interplane coupling.
(0.6V )R1
it can be found that a value of R2 = 523k and R1 = 432k minimizes the error in this range. The error can be checked by solving for VOUT and finding the percent error from the desired 1.1V. Using these resistor values will result in VOUT = 1.096V, and an error of around 0.4%. Using different target resistor sums is acceptable, but a smaller sum will decrease efficiency at lower loads, and a larger sum will increase noise sensitivity at the VFB pin.
COUT CIN L SW 1 GND 2 FMIN/MODE LOBATB 3 4 8 VIN
VIN
COUT CIN L SW 1 GND 2 FMIN/MODE 3 4
* *
*** *** ***
8 VIN
7 RUN 6 VFB 5 NC R2 R1 CFF*
LOBATB
* *
*** *** ***
7 RUN 6 VFB 5 NC
VOUT *CFF = 22pF FEEDFORWARD CAPACITOR
3620 F04
VOUT
3620 F05
LTC3620 Layout Diagram
LTC3620-1 Layout Diagram
12
+
+
VIN
3620f
LTC3620 TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter, FMIN/MODE = 0
VIN 2.9V TO 5.5V VIN RUN 1F CER LOBATB 22H SW LTC3620 22pF VOUT 1.1V 1M
LOBATB
FMIN/MODE VFB GND
432k 523k
3620 TA02a
1F CER
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 TA = 25C VOUT = 1.1V FMIN/MODE = 0V VIN = 3V VIN = 3.6V VIN = 5.5V 1 LOAD CURRENT (mA) 10
3620 TA02b
Efficiency vs VIN
100 90 80 70 60 50 40 30 20 10 0 2.5 3.5 TA = 25C VOUT = 1.1V IOUT = 500A IOUT = 1mA IOUT = 10mA 4.5 VIN (V) 5.5 6.5
3620 TA02c
3620f
13
LTC3620 TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter, Externally Programmed fMIN
VIN 2.9V TO 5.5V 1F CER RUN VIN LOBATB 22H SW LTC3620 22pF VOUT 1.1V 1M LOBATB
FMIN/MODE
FMIN/MODE VFB GND
432k 523k
3620 TA03a
1F CER
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 TA = 25C VIN = 3.6V VOUT = 1.1V fMIN = 20kHz fMIN = 100kHz fMIN = 200kHz 1 LOAD CURRENT (mA) 10
3620 TA03b
Efficiency vs VIN
100 90 80 70 60 50 40 30 20 TA = 25C 10 VOUT = 1.1V fMIN = 200kHz 0 3.5 2.5 IOUT = 500A IOUT = 1mA IOUT = 10mA 4.5 VIN (V) 5.5 6.5
3620 TA03c
Spectral Content, FMIN/MODE = 20kHz Clock
-40 -60 -80 -100 -120 -140 1kHz RBW = 3Hz VOUT = 1.1V VIN = 3.6V IOUT = 500A TA = 25C 2.99kHz/DIV 30kHz
3620 TA03d
Spectral Content, FMIN/MODE = 100kHz Clock
-40 -60 -80 -100 -120 -140 1kHz VOUT = 1.1V VIN = 3.6V IOUT = 1mA TA = 25C 14.9kHz/DIV 150kHz
3620 TA03e
Spectral Content, FMIN/MODE = 200kHz Clock
-40
POWER RATIO (dBm)
POWER RATIO (dBm)
POWER RATIO (dBm)
20.0kHz -64.9dBm
99.9kHz -59.9dBm
-60 -80 -100 -120 -140 1kHz VOUT = 1.1V VIN = 3.6V IOUT = 1mA TA = 25C 21.9kHz/DIV
199.7kHz
220kHz
3620 TA03f
3620f
14
LTC3620 PACKAGE DESCRIPTION
DC Package 8-Lead Plastic DFN (2mm x 2mm)
(Reference LTC DWG # 05-08-1719 Rev A)
0.70 0.05 2.55 0.05 1.15 0.05 0.64 0.05 (2 SIDES)
PACKAGE OUTLINE
0.25 0.45 BSC 1.37 0.05 (2 SIDES)
0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.05 TYP 2.00 0.10 (4 SIDES)
R = 0.115 TYP 5
8 0.40 0.10
PIN 1 BAR TOP MARK (SEE NOTE 6)
0.64 0.10 (2 SIDES)
PIN 1 NOTCH R = 0.20 OR 0.25 45 CHAMFER
(DC8) DFN 0106 REVO
4 0.200 REF 0.75 0.05 1.37 0.10 (2 SIDES) 0.00 - 0.05
0.23 0.45 BSC
1
0.05
BOTTOM VIEW--EXPOSED PAD
NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 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 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
3620f
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.
15
LTC3620 TYPICAL APPLICATIONS
High Efficiency Low Power Step-Down Converter, LTC3620-1 Internally Programmed, 1.1VOUT
VIN 2.9V TO 5.5V VIN RUN LOBATB LTC3620-1 SW FMIN/MODE VFB GND
3620 TA04a
Efficiency vs Load Current
100 90 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 2.5
Efficiency vs VIN
LOBATB 1M 22H VOUT 1.1V 2.2F CER EFFICIENCY (%)
80 70 60 50 40 30 20 10 0 0.1 TA = 25C VOUT = 1.1V FMIN/MODE = 0V VIN = 3V VIN = 3.6V VIN = 5.5V 1 LOAD CURRENT (mA) 10
3620 G21
1F CER
TA = 25C VOUT = 1.1V FMIN/MODE = 0V IOUT = 500A IOUT = 1mA IOUT = 10mA 3.5 4.5 VIN (V) 5.5 6.5
3620 TA04c
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, I Q = 20A, ISD < 1A, ThinSOT Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD < 1A, ThinSOT Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD < 1A, MS10E, DFN Packages LTC3405A/LTC3405AB 300mA IOUT, 1.5MHz, Synchronous Step-Down DC/DC Converter LTC3406A/LTC3406AB 600mA IOUT, 1.5MHz, Synchronous Step-Down DC/DC Converter LTC3407A/LTC3407A-2 Dual 600mA/800mA IOUT, 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converter LTC3409 LTC3410/LTC3410B LTC3411A LTC3548 LTC3561A
600mA IOUT, 2.25MHz, Synchronous Step-Down DC/DC Converter 96% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.6V, IQ = 65A, ISD < 1A, DFN Package 300mA IOUT, 2.25MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26A, ISD < 1A, SC70 Package 1.25A IOUT, 4MHz, Synchronous Step-Down DC/DC Converter Dual 400mA/800mA IOUT , 2.25MHz, Synchronous Step-Down DC/DC Converter 1A IOUT, 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD < 1A, MS10, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD < 1A, MS10, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 240A, ISD < 1A, 3mm x 3mm DFN Package VIN: 4.5V to 45V (60VMAX), VOUT(MIN) = 0.8V, IQ = 12A, ISD < 1A, 3mm x 3mm DFN Package, MSOP-8E VIN: 4.5V to 50V (60VMAX), VOUT(MIN) = 0.8V, IQ = 12A, ISD < 1A, 3mm x 3mm DFN Package, MSOP-8E VIN: 4.5V to 45V (60VMAX), VOUT(MIN) = 0.8V, IQ = 12A, ISD < 1A, 3mm x 3mm DFN Package, MSOP-8E
LTC3631/LTC3631-3.3/ 45V, 100mA (IOUT), Ultralow Quiescent Current Synchronous Step-Down DC/DC Converter LTC3631-5 LTC3632 50V, 20mA (IOUT), Ultralow Quiescent Current Synchronous Step-Down DC/DC Converter
LTC3642/LTC3642-3.3/ 45V, 50mA (IOUT), Ultralow Quiescent Current Synchronous Step-Down DC/DC Converter LTC3642-5
3620f
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0809 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

▲Up To Search▲

Price & Availability of LTC3620EDC-1PBF

	To Download LTC3620EDC-1PBF Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .