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AIC1550
Low-Noise Synchronous PWM Step-Down DC/DC Converter
FEATURES
95% Efficiency or up 800mA Guaranteed Output Current. Adjustable Output Voltage from 0.75V to VIN of a range from +2.5V to 6.5V. Very Low Quiescent Current: 35A (Typ.). Fixed- 500KHz or Adjustable Frequency Synchronous PWM Operation. Synchronizable external Switching Frequency up to 1MHz. Accurate Reference: 0.75V (2%). 100% Duty Cycle in Dropout. Low Profile 8-Pin MSOP Package.
DESCRIPTION
The AIC1550 is a low-noise pulse-widthmodulated (PWM) DC-DC step-down converter. It powers logic circuits in PDAs and small wireless systems such as cellular phones, handy-terminals. The device features an internal synchronous rectifier for high conversion efficiency. Excellent noise characteristics and fixed-frequency operation provide easy post-filtering. The AIC1550 is ideally suited for Li-ion battery applications. It is also suitable for +3V or +5V fixed input applications. The device can operate in either one of the following four modes. (1) Forced PWM mode operates at a fixed frequency regardless of the load. Synchronizable PWM mode allows the synchronization by using an external switching frequency with a minimum harmonics. PWM/PFM Mode extends battery life by switching to a PFM pulseskipping mode under light loads. Shutdown mode sets device to standby, reducing supply current to 0.1A or under.
APPLICATIONS
PDAs. Digital Still Cameras. Handy-Terminals.
Cellular Phones. CPU I/O Supplies.
(2)
Cordless Phones. Notebook Chipset Supplies. Battery-Operated Devices (4 NiMH/ NiCd or 1 Li-ion Cells). (3)
(4)
The AIC1550 can deliver over 800mA output current. The output voltage can be adjusted from 0.75V to VIN ranging from +2.5V to +6.5V. Other features of the AIC1550 include low quiescent current, low dropout voltage, and a 0.75V reference of 2% accuracy. It is available in a space-saving 8-pin MSOP package.
Analog Integrations Corporation
Si-Soft Research Center 3A1, No.1, Li-Hsin Rd. I , Science Park , Hsinchu 300, Taiwan , R.O.C. TEL: 886-3-5772500 FAX: 886-3-5772510 www.analog.com.tw
DS-1550P-03 070504
1
AIC1550
TYPICAL APPLICATION CIRCUIT
VIN= 2.5V to 6.5V BP CIN 22F 1 VIN LX 8 * L1 6.8H D1 SS12 Optional VOUT = 1.8V
CBP 0.1F
2 BP 3 SHDN 4 FB
GND 7
SYNC/ 6 MODE
CF R1 820K 12P
RT
5
AIC1550
R2 560K
CO 22F
CIN: TAIYO YUDEN LMK316F226ZL-T Ceramic capacitor CO1: TAIYO YUDEN LMK316F226ZL-T Ceramic capacitor L1: TDK SLF6025-6R8M1R3 D1: GS SS12 * Note: Efficiency can boost 2% to 4% if D1 is connected.
ORDERING INFORMATION
AIC1550XXXX
PACKING TYPE TR: TAPE & REEL TB: TUBE PACKAGING TYPE O:MSOP8 C: Commercial Degree P: Lead Free
PIN CONFIGURATION
TOP VIEW
VIN 1 BP 2 SHDN 3 FB 4
8 LX 7 GND 6 SYNC/MODE 5 RT
Example: AIC1550COTR In MSOP Package & Taping & Reel Packing Type AIC1550POTR In MSOP Lead Free Package & Taping & Reel Packing Type
2
AIC1550
ABSOLUTE MAXIMUM RATINGS
VIN, BP, SHDN, SYNC/MODE, RT to GND BP to VIN LX to GND FB to GND Operating Temperature Range Junction Temperatrue Storage Temperature Range Lead Temperature (Soldering. 10 sec) -0.3 to +7V .-0.3 to 0.3V -0.3 ~ (VIN+0.3V) -0.3 ~ (VBP+0.3V) -40C ~ 85C 125C - 65C ~ 150C 260C
Absolute Maximum Ratings are those values beyond which the life of a device may be Impaired.
TEST CIRCUIT
Refer to Typical Application Circuit.
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AIC1550
ELECTRICAL CHARACTERISTICS
(VIN=+3.6V, TA=+25C, SYNC/MODE =GND, SHDN =IN, unless otherwise specified.) (Note1)
PARAMETER Input Voltage Range Output Adjustment Range Feedback Voltage Line Regulation Load Regulation SYMBOL CONDITIONS VIN VOUT VFB Duty Cycle = 100% to 23% IOUT = 0 to 800mA IFB VFB = 1.4V, VIN = 3.6V VIN = 2.5V VIN = 3.6V VIN = 2.5V 1 -50 MIN 2.5 VREF 0.735 0.75 +1 -1.3 0.01 0.32 0.38 0.32 0.38 1.5 35 0.1 -20 400 500 dutyMAX UVLO VIH VIL VIN rising, typical hysteresis is 85mV
SHDN , SYNC/MODE, LIM SHDN , SYNC/MODE, LIM SHDN , SYNC/MODE, LIM
TYP
MAX 6.5 VIN 0.765
UNITS V V V % %
FB Input Current
50 0.65 0.65
nA A A A A KHz KHz %
P-Channel On-Resistance PRDS(ON) ILX = 100mA N-Channel On-Resistance NRDS(ON) ILX = 100mA P-Channel Current-Limit Threshold Quiescent Current Shutdown Supply Current LX Leakage Current Oscillator Frequency SYNC Capture Range Maximum Duty Cycle Undervoltage Lockout Threshold Logic Input High Logic Input Low Logic Input Current SYNC/MODE Minimum Pulse Width fOSC (Note 2)
2.1 70 1 20 600 1000
SYNC/MODE = GND, VFB = 1.4V, LX unconnected
SHDN = LX = GND, includes LX
leakage current VIN = 5.5V, VLX = 0 or 5.5V
0.1 500
100 1.9 2 0.4 -1 500 0.1 1 2.0 2.1
V V V A nS
High or low
Note 1: Specifications are production tested at TA=25C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with Statistical Quality Controls (SQC). Note 2: Maximum specification is guaranteed by design, not production tested.
4
AIC1550
TYPICAL PERFORMANCE CHARACTERISTICS
(TA=25oC, VIN=3.6V, SYNC/MODE=GND, with Schottky diode D1, unless otherwise noted.)
100
100
90
VIN=2.1V
90
VIN=2.1V
Efficiency (%)
Efficiency (%)
80
80
70
70
60 50
VIN=5.0V VIN=2.3V
VIN=6.5V
VIN=5.0V
60
VIN=6.5V
VIN=3.3V VOUT=1.5V
50
VOUT=1.2V
40
40
0.1
1
10
100
1000
0.1
1
Load Current (mA) Fig. 1 Load Current vs. Efficiency (VOUT=1.2V) (Refer to typical application circuit)
100
Load Current (mA)
10
100
1000
Fig. 2 Load Current vs. Efficiency (VOUT=1.5V) (Refer to typical application circuit) (R f
100
tt
i
l
li
ti
i
it)
90
VIN=2.1V
90
VIN=3.3V
Efficiency (%)
70
Efficiency (%)
80
80
VIN=6.5V VIN=5.0V
70
VIN=6.5V VIN=5.0V
60
60
50
VIN=3.3V
0.1 1 10
VOUT=1.8V
50
VOUT=2.5V
100 1000
40
40
0.1
1
10
100
1000
Load Current (mA) Fig. 3 Load Current vs. Efficiency (VOUT=1.8V) (Refer to typical application circuit)
Fig. 4
Load Current (mA) Load Current vs. Efficiency (VOUT=2.5V) (Refer to typical application circuit)
100
100
VIN=3.6V
90 90 80
VIN=3.6V
Efficiency (%)
Efficiency (%)
80 70
VIN=6.5V
70
VIN=4.2V
VIN=5.0V
60 50
60 50
VOUT=3.0V
0.1 1 10 100 1000
VIN=4.2V
VOUT=3.3V
40
40
0.1
1
10
100
1000
Load Current (mA) Fig. 5 Load Current vs. Efficiency (VOUT=3.0V) (Refer to typical application circuit)
Load Current (mA) Fig. 6 Load Current vs. Efficiency (VOUT=3.3V) (Refer to typical application circuit)
5
AIC1550
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
100 W/ Schottky Diode 90 80 Efficiency (%) Wo/ Schottky Diode 70 60 50 40 30 0.1 VOUT=3.0V 1 10 Load Current (mA) 100 1000 SYNC= VIN SYNC= GND
0.765 0.760
VIN=3.6V
Reference Voltage (V)
0.755 0.750 0.745 0.740 0.735 0.730 0.725 -50
-25
0
25
50
75
100
125
Fig. 7 Load Current vs. Efficiency (W/ or W/O Schottky Diode)
Temperature (C) Fig. 8 Reference Voltage vs. Temperature
550 540 530
550
VIN=3.6V
540 530
Frequency (KHz)
-40 -20 0 20 40 60 80 100 120
Frequency (KHz)
520 510 500 490 480 470 460 450
520 510 500 490 480 470 460 450 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Temperature (C) Fig. 9 Oscillator Frequency vs. Temperature
Supply Voltage (V) Fig. 10 Frequency vs. Input Voltage
0.44 0.42
1.82
RDSON (m)
0.38 0.36 0.34 0.32 0.30 0.28 0.26 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Output Voltage (V)
0.40
Main Switch
1.80
VIN=3.6V
1.78
1.76
Synchronous Switch
1.74
1.72 1 10 100 1000
Supply Voltage (V) Fig. 11 RDSON vs. Supply Voltage
Load Current (mA) Fig. 12 Output Voltage vs. Load Current
6
AIC1550
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
300
100
VOUT=1.8V
250
90 80
PWM/PFM
DC Supply Current (A)
SYNC/PWM=IN
150
Efficiency (%)
200
70 60 50 40
PWM
100
SYNC/PWM=GND
50
30 20
VIN=3.6V VOUT=1.8V
0.1 1 10 100 1000
0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
10
Supply Voltage (V) Fig. 13 DC Supply Current vs. Supply Voltage
Fig. 14
Load current (mA) Efficiency vs. Load current
1000
Operation Frequency (KHz)
900
800 700 600
500 2250 2000 1750 1500 1250 1000 750 Tuning Resistor RT (k)
500
250
Fig. 15
Operation Frequency vs. Tuning Resistor
Fig. 16 Start-up from Shutdown, RLOAD=3
VOUT=1.8V; ILOAD=50mA to 500mA; SYNC/MODE=IN
VOUT=1.8V; ILOAD=50mA to 500mA; SYNC/MODE=GND
Fig. 17 Load Transient Response
Fig. 18
Load Transient Response
7
AIC1550
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN=3.3V to 5V, SYNC/MODE=IN
IOUT=1.8V;
ILOAD=200mA to 500mA;
Fig. 19 Line Transient Response
Fig. 20 Short Circuits Protection
VIN=3.6V, VOUT=1.8V, ILOAD=800mA VOUT
VIN=3.6V; VOUT=1.8V; ILOAD=500mA SYNC/MODE=IN
VLX
Fig. 21
Switching Waveform
Fig. 22 Output Ripple voltage
8
AIC1550
BLOCK DIAGRAM
BP 0.75V REF SHDN C urrent A MP . C hip S upply
10
V IN
+ X5 -
V IN
S lope RT 500K H z O scillator Frequency S YN C S election PW M C om parator + C ontrol Logic A ntiS hootT hrough C urrent Lim it C om parator + -
5
Q1 x1 Q2 X20
C om pensation
REF
P hase C om pensation FB
FB REF
+
E rror A MP .
LX
Q3
P W M/P FM C ontrol Zero C ross C om parator
+ GND
REF
+
P FM C om parator
PIN DESCRIPTIONS
PIN 1: VINSupply Voltage Input ranging from +2.5V to +6.5V. Bypass with a 22F capacitor. Supply Bypass Pin internally connecting to VIN. Bypass with a 0.1F capacitor. PIN 6: SYNC/MODEOscillator Sync and Low-Noise, Mode-Control Input. SYNC/MODE = VIN (Forced PWM Mode) SYNC/MODE = GND (PWM/PFM Mode) An external clock signal connecting to this pin allows LX switching synchronization. PIN 7: GND- Ground. PIN 8: LXInductor connecting to the Drains of the Internal Power MOSFETs
PIN 2: BP-
Shutdown-Control PIN 3: SHDN - Active-Low, Input reducing supply current to 0.1A in shutdown mode. PIN 4: FBFeedback Input. PIN 5: RTFrequency Adjustable Pin connecting to GND through a resistor to increase frequency. (Refer to Fig. 15)
9
AIC1550
APPLICATION INFORMATIONS
Introduction
AIC1550 is a low-noise, pulse-width-modulated (PWM), DC-DC step-down converter. It features an internal synchronous rectifier, which eliminates external Schottky diode. AIC1550 is suitable for Li-lon battery applications, or can be used at 3V or 5V fixed input voltage. It operates in one of following four modes. (1) Forced PWM mode operates at a fixed frequency regardless of the load. (2) Synchronizable PWM mode allows the synchronization by using an external switching frequency with a minimum harmonics. (3) PWM/PFM Mode extends battery life by switching to a PFM pulseskipping mode under light loads. (4) Shutdown mode sets device to standby, reducing supply current to 0.1A or under. Continuous output current of AIC1550 can be upward to 800mA and output voltage can be adjusted from 0.75V to VIN with an input range from 2.5V to 6.5V by a voltage divider. AIC1550 also features high efficiency, low dropout voltage, and a 0.75V reference with 2% accuracy. It is available in a space-saving 8-pin MSOP package. increase efficiency. When control logic block turns Q2 on, Q3 will turn off through anti-short-through block. Similarly, when Q3 is on, Q2 will turn off. AIC1550 provides current limit function by using a 5 resistor. When Q1 turns on, current follows through the 5 resistor. And current amplifier senses the voltage, which crosses the resistor, and amplifies it. When the sensed voltage gets bigger than reference voltage, control logic shuts the device off.
PWM/PFM Function
When connecting SYNC/MODE pin to VIN, the device is forced into PWM (Pulse-WidthModulated) mode with constant frequency. Advantage of constant frequency is easily reducing noise without complex post-filter. But its disadvantage is low efficiency at light loading. Therefore, AIC1550 provides a function to solve this problem. When connecting SYNC/MODE pin to GND, device is able to get into PWM/PFM (Pulse-Frequency-Modulated) modes. Under a light loading condition, the device turns to PFM mode, which results in a higher efficiency. PWM mode is on when heavy loading applies and the noise is reduced.
Frequency Synchronization
Connecting an external clock signal to SNYC/MODE pin can control switching frequency. The acceptable range is from 500kHz to 1MHz. This mode exhibits low output ripple as well as low audio noise and reduces RF interference while providing reasonable low current efficiency.
Operation
When power on, control logic block detects SYNC/MODE pin connecting to VIN or GND to determine operation function and gives a signal to PWM/PFM control block to determine the proper comparator (ref. Block Diagram). AIC1550 works with an internal synchronous rectifier Q3, to
10
AIC1550
100% Duty Cycle Operation When the input voltage approaches the output voltage, the converter continuously turns Q2 on. In this mode, the output voltage is equal to the input voltage minus the voltage, which is the drop across Q2. If input voltage is very close to output voltage, the switching mode goes from pure PWM mode to 100% duty cycle operation. During this transient state mentioned above, large output ripple voltage will appear on output terminal.
V V OUT = OUT (2) 2 x Ma 2 x 0.27 Note that output voltage can be defined according L1 >
to user's requirement to get a suitable inductor value.
Components Selection
Inductor The inductor selection depends on the operating frequency of AIC1550. The internal switching frequency is 500KHz, and the external synchronized frequency ranges from 500KHz to 1MHz. A higher frequency allows the uses of smaller inductor and capacitor values. But, higher frequency results lower efficiency due to the internal switching loss. The ripple current IL interrelates with the inductor value. A lower inductor value gets a higher ripple current. Besides, a higher VIN or VOUT can also get the same result. The inductor value can be calculated as the following formula. V 1 (1) L= VOUT 1 - OUT (f )(IL ) VIN Users can define the acceptable IL to gain a suitable inductor value. Since AIC1550 can be used in ceramic capacitor application, the component selection will be different from the one for the application above. AIC1550 has a built-in slope compensation, which acitvates when duty cycle is larger than 0.45. The slope Ma, 0.27V/s, has to be larger than half of M2. M2 is equal to output voltage divided by L1. The formula of inductor is shown as below:
Output Capacitor The selection of output capacitor depends on the suitable ripple voltage. Lower ripple voltage corresponds to lower ESR (Equivalent Series Resistor) of output capacitor. Typically, once the ESR is satisfied with the ripple voltage, the value of capacitor is adequate for filtering. The formula of ripple voltage is as below: 1 (3) VOUT = IL ESR + 8fC OUT Besides, in buck converter architecture frequency stands at 1/(LC) when a double pole formed by the inductor and output capcitor occurs. This will reduce phase margin of circuit so that the stability gets weakened. Therefore, a feedforward capacitor that is parallel with R1 can be added to reduce output ripple voltage and increase circuit stability. The output capacitor can be calculated as the following formula.
1 L1 x C O 1 R1 x CF
(4)
For more reduction in the ripple voltage, a 12pF ceramic capacitor, which is parallel with output capacitor, is used. External Schottky Diode AIC1550 has an internal synchronous rectifier, instead of Schottky diode in buck converter. However, a blank time, which is an interval when both of main switch, Q2, and synchronous rectifier, Q3, are off; occurs at each switching cycle. At the moment, AIC1550 has a decreasing efficiency. Therefore, an external Schottky diode is needed to reinforce the efficiency.
11
AIC1550
Since the diode conducts during the off time, the peak current and voltage of converter is not allowed to exceed the diode ratings. The ratings of diode can be calculated by the following formulas:
5. The FB pin should connect to feedback resistors directly. And the route should be away from the noise source, such as inductor of LX line. 6. Grounding all components at the same point may effectively reduce the occurrence of loop. A stability ground plane is very important for gaining higher efficiency. When a ground plane is cut apart, it may cause disturbed signal and noise. If possible, two or three through-holes can ensure the stability of grounding. Fig.24 to 27 show the layout diagrams of AIC1550.
VD,MAX( OFF ) = VIN
(5)
ID,MAX( ON) = IOUT,MAX +
IL 2
(6)
ID,AVG( ON)
= IOUT - IIN = IOUT - D x IOUT
= (1 - D) x IOUT
(7)
Adjustable Output Voltage
AIC1550 appears a 0.75V reference voltage at FB pin. Output voltage, ranging from 0.75V to VIN, can be set by connecting two external resistors, R1 and R2. VOUT can be calculated as: R1 (8) ) VOUT = 0.75 V x (1 + R2 Applying a 12pF capacitor parallel with R1 can prevent stray pickup. They should sit as close to AIC1550 as possible. But load transient response is degraded by this capacitor.
Example
Here are two examples to prove the components selector guide above. 1. Tantalum capacitors application: Assume AIC1550 is used for mobile phone application, which uses 1-cell Li-ion battery with 2.7V to 4.2V input voltage for power source. The required load current is 800mA, and the output voltage is 1.8V. Substituting VOUT=1.8V, VIN=4.2V, I=250mA, and f=500KHz to equation (1)
L= 1 .8 V 1.8 V 1 - = 8.23H 4 .2 V 500KHz x 250mA
Layout Consideration
To ensure a proper operation of AIC1550, the following points should be given attention to: 1. Input capacitor and Vin should be placed as close as possible to each other to reduce the input ripple voltage. 2. The output loop, which is consisted of inductor, Schottky diode and output capacitor, should be kept as small as possible. 3. The routes with large current should be kept short and wide. 4. Logically the large current on the converter, when AIC1550 is on or off, should flow at the same direction.
Therefore, 10H is proper for the inductor. And the inductor of series number SLF6025-100M1R0 from TDK with 57.3m series resistor is recommended for the best efficiency. For output capacitor, the ESR is more important than its capacity. Assuming ripple voltage V=100mV, then the ESR can be calculated as: V 100mV ESR= = = 0.4 I 250mA Therefore, a 33H/10V capacitor, MCM series from NIPPON, is recommend. Schottky selection is calculated as following. VD,MAX( OFF ) = VIN = 4.2V
12
AIC1550
ID,MAX(ON) = IOUT,MAX +
IL 2 250mA = 800mA + 2 = 925mA
VOUT is substituted by 1.8V in equation (2) as
L1 > V OUT = 1.8 = 3.33 H 0.54 0.54
ID,avg( ON) = (1 - D) x IOUT
Let L1 = 6.8H, and choose CF = 12pF, R1 = 820k. Co calculated by the following formula can improve circuit stability.
1 L1 x C O 1 R1 x CF
= (1 -
1 .8 ) x 800mA 4 .2 = 457 .14mA
According the datas above, the Schottky diode, SS12, from GS is recommend. For feedback resistors, choose R2=390k and R1 can be calculated as follow:
1 .8 V R1 = - 1 x 390k = 546k ; use 560k 0.75
Therefore,
C
O
(R1 x CF )2 = (820k x 12pF)2 =
L1 6.8.
= 12 F
Fig. 22 shows the application circuit of AIC1550, and Fig. 23 to 26 show the layout diagrams of it. 2. Ceramic capacitors application: Of the same AIC1550 application above, except for ceramic capacitor used, Co, R1, and R2 can be calculated as following formulas. And the same values of load current and output voltage at 800mA and 1.8V respectively are used.
Say, CO is 22F. Then, R2 can be decided by equation (8) as
R1 R2 = VOUT 1.8 -1= - 1 = 1.4 Vref 0.75
So, R2 = 560k. Note: Schottky diode, SS12 from GS, is still required in this application.
VIN= 2.5V to 6.5V BP + CIN 10F
1
VIN
LX
8 **
L1 10F D1 SS12 CF R1 560K 10P +
VOUT = 1.8V
CBP 0.1F
2 BP 3 SHDN 4 FB
GND 7
SYNC/ 6 MODE
RT
5
Optional
AIC1550
R2 390K
*CO1 33F
*CO2 4.7F
* Note: CO1 can be omitted if CO2 in 10F Ceramic CIN: NIPPON 10F/6V Tantalum capacitor CO1: NIPPON 33F/6V Tantalum capacitor L: TDK SLF6025-100M1R0 D1: GS SS12 ** Note: Efficiency can boost 2% to 4% if D1 is connected.
13
AIC1550
Fig. 23 AIC1550 Application Circuit (Tantalum capacitor application)
Fig. 24 Top Layer
Fig. 25 Bottom Layer
Fig. 26 Top Over Layer
Fig. 27 Bottom Over Layer
14
AIC1550
PHYSICAL DIMENSIONS
MSOP 8 (CO) (PO) (unit: mm)
D
SYMBOL A
E E1
MIN 0.05 0.75 0.25 0.13 2.90
MAX 1.10 0.15 0.95 0.40 0.23 3.10 4.90 BSC 3.10 0.65 BSC 0.70 6
A1 A2 b c D E
c L
e A A2
0.25
E1 e L
2.90 0.40 0
b
Note:
Information provided by AIC is believed to be accurate and reliable. However, we cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AIC product; nor for any infringement of patents or other rights of third parties that may result from its use. We reserve the right to change the circuitry and specifications without notice. Life Support Policy: AIC does not authorize any AIC product for use in life support devices and/or systems. Life support devices or systems are devices or systems which, (I) are intended for surgical implant into the body or (ii) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user.
A1
15

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