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PRODUCT DATASHEET
AAT2789
SystemPowerTM
General Description
The AAT2789 is a 2-channel synchronous step-down converter operating from an input voltage range of 2.7V to 5.5V, making it the ideal choice for single-cell Lithiumion/polymer battery powered systems or low voltage 3.3V and 5V based consumer equipment. Channel 1 delivers up to 1700mA output current while Channel 2 delivers up to 800mA. Both converters incorporate a unique low noise architecture which reduces output ripple and spectral noise. The AAT2789 uses a high switching frequency to minimize external filter sizing. Peak current mode control eliminates external compensation while optimizing transient performance and stability. The AAT2789 requires a minimum of external components to realize a high efficiency dual-output step-down converter while minimizing solution size and footprint. Each of the step-down regulators has an independent input and enable pin. Externally adjustable output voltage is provided. Light load operating mode provides high efficiency over the entire load range. Low quiescent current enables excellent life for battery powered systems. The AAT2789 is available in a 3x4mm Pb-free 16-pin TDFN package and is rated over the -40C to 85C operating temperature range.
Low Noise, High Frequency Dual Step-Down Converter
Features
* * * * * VIN Range: 2.7V to 5.5V Output Voltage Range: 0.6V to VIN Low Noise Light Load Mode Low Ripple PWM Mode Output Current: Channel 1: 1700mA Channel 2: 800mA Highly Efficient Step-Down Converters Low RDS(ON) Integrated Power Switches 100% Duty Cycle High Switching Frequency Peak Current Mode Control Internal Compensation Excellent Transient Response Internal Soft Start Fast Turn-On Time Over-Temperature Protection Current Limit Protection Low Profile TDFN34-16 Package -40C to 85C Temperature Range
* * * * * * * * * * * * *
Applications
* * * * * * * * * Cellular and Smart Phones Digital Cameras Handheld Instruments Mass Storage Systems Microprocessor / DSP Core / IO Power PDAs and Handheld Computers Portable Media Players USB Devices Wireless Data Systems
Typical Application
VIN: 2.7V - 5.5V AAT2789 TDFN34-16 LX1 VP1,2 VCC1,2 EN1 PGND1 EN2 FB1 AGND1 L2 3.3H LX2 L1 1.5H C2 22F 6.3V VOUT1 1.2V, 1700mA R1 59.0k R2 59.0k
C1 10F 6.3V
VOUT2 3.3V, 800mA C3 10F 6.3V R3 267k R4 59.0k
PGND2 FB2 AGND2
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PRODUCT DATASHEET
AAT2789
SystemPowerTM
Pin Descriptions
Pin #
1 2 3 4 5 6 7,8 9 10 11 12 13 14 15 16 EP
Low Noise, High Frequency Dual Step-Down Converter
Symbol
N/C PGND2 LX2 VP2 VCC1 EN1 VP1 LX1 PGND1 FB1 AGND1 VCC2 EN2 FB2 AGND2 EP
Function
No connect. Power ground pin for Channel 2 step-down converter. Connect return of Channel 2 input and output capacitors close to this pin for best noise performance. Channel 2 step-down converter switching pin. Connect output inductor to this pin. Inductor value is determined by output voltage. Input supply voltage pin for Channel 2 step-down converter. Connect a 10F ceramic input capacitor close to this pin or connect to VP1. Operating input voltage range is 2.7V to 5.5V. Input supply pin for Channel 1. Must be closely decoupled. Enable Channel 1 input pin. Active high. Input supply voltage pin for Channel 1 step-down converter. Connect a 10F ceramic input capacitor close to this pin. Operating input voltage range is 2.7V to 5.5V. Channel 1 step-down converter switching pin. Connect output inductor to this pin. Inductor value is determined by output voltage. Power ground pin for Channel 1 step-down converter. Connect return of Channel 1 input and output capacitors close to this pin for best noise performance. Feedback pin for Channel 1. Connect an external resistor divider to this pin to program the output voltage to the desired value. Signal ground for Channel 1. Input supply pin for Channel 2. Must be closely decoupled. Enable Channel 2 input pin. Active high. Feedback pin for Channel 2. Connect an external resistor divider to this pin to program the output voltage to the desired value. Signal Ground for Channel 2. Exposed paddle. Connect to PGND1 and PGND2 as close as possible to the device. Use properly sized vias for thermal coupling to the ground plane. See PCB layout guidelines.
Pin Configuration
TDFN34-16 (Top View)
N/C PGND2 LX2 VP2 VCC1 EN1 VP1 VP1
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10 9
AGND2 FB2 EN2 VCC2 AGND1 FB1 PGND1 LX1
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Absolute Maximum Ratings1
Symbol
VIN VLX VFB VEN TJ TLEAD
Description
VP1, VP2, VCC1, VCC2 voltages to PGND, AGND VLX1, VLX2 to PGND, AGND VFB1, VFB2 to PGND, AGND VEN1, VEN2 to PGND, AGND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to 6.0 -40 to 150 300
Units
V V V V C C
Thermal Information
Symbol
PD JA
Description
Maximum Power Dissipation Thermal Resistance1
2
Value
2.0 50
Units
W C/W
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on a FR4 board. 3. Derate 20mW/C above 25C ambient temperature.
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PRODUCT DATASHEET
AAT2789
SystemPowerTM
Electrical Characteristics1
VIN = 3.6V, TA = -40C to 85C, unless noted otherwise. Typical values are at TA = 25C. Symbol Description Conditions Min
2.7 VP1 Rising VP1 Hysteresis VP1 Falling IOUT1 = 0A to 1.7A; VP1 = 2.7V to 5.5V No load, VEN1 = VP1, VEN2 = AGND VEN1 = GND 250 1.7 0.6 -3.0 42 1800 120 85 0.5 0.2 1.12 From Enable-1 to Output-1 Regulation 2.7 VP2 Rising VP2 Hysteresis VP2 Falling IOUT2 = 0A to 800mA, VP2 = 2.7V to 5.5V No load, VEN2 = VP2, VEN1 = AGND VEN2 = GND 100 1.7 0.6 -3.0 37 900 330 275 0.5 0.1 0.9 From Enable-2 to Output-2 Regulation 2.0 150 140 15 0.6 1.4 -1.0 1.0 2.6 VP2 3.0 70 1.0 1.40 150 1.68 VP1 3.0 90 1.0
Low Noise, High Frequency Dual Step-Down Converter
Typ
Max
5.5 2.4
Units
V V mV V V % A A mA m m % %/V MHz s V V mV V V % A A mA m m % %/V MHz s C C V V A
Channel 1: 1700mA Step-Down Converter VP1, VCC1 Input Voltage VUVLO1 UVLO Threshold
Output Voltage Range VOUT1 VOUT1(TOL) Output Voltage Tolerance IQ1 Quiescent Current ISHDN1 Shutdown Current ILIM1 Current Limit RDSON(H)1 High Side On-Resistance RDSON(L)1 Low Side On-Resistance VLOADREG1 Load Regulation VLINEREG1 / Line Regulation VP1 FOSC1 Oscillator Frequency TS1 Start-Up Time Channel 2: 800mA Step-Down Converter Input Voltage VP2, VCC2 VUVLO2 UVLO Threshold
IOUT1 = 0A to 1.7A VP1 = 2.7V to 5.5V
5.5 2.7
VOUT2 Output Voltage Range VOUT2(TOL) Output Voltage Tolerance IQ2 Quiescent Current ISHDN2 Shutdown Current ILIM2 Current Limit RDSON(H)2 High Side On-Resistance Low Side On-Resistance RDSON(L)2 VLOADREG2 Load Regulation VLINEREG2 / Line Regulation VP2 FOSC2 Oscillator Frequency TS2 Start-Up Time Over-Temperature, EN Logic Over-Temperature Shutdown Threshold TSD1,2 Over-Temperature Shutdown Hysteresis VEN1,2(L) Enable Threshold Low VEN1,2(H) Enable Threshold High Input Low Current IEN(1,2
IOUT2= 0mA to 800mA VP2 = 2.7V to 5.5V
1. The AAT2789 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range, and is assured by design, characterization and correlation with statistical process controls.
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 1
Efficiency vs. Output Current
(VOUT = 3.3V)
100 1.0 90
Load Regulation vs. Output Current
(VOUT = 3.3V)
Load Regulation (%)
Efficiency (%)
80 70 60 50 40 30 0.1 1 10 100
0.5
0.0
VIN = 3.6V VIN = 4.2V VIN = 5V
1000 10000
-0.5
-1.0 0.1
VIN = 3.6V VIN = 4.2V VIN = 5V
1 10 100 1000 10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(VOUT = 1.8V)
100 1.0
Load Regulation vs. Output Current
(VOUT = 1.8V)
Load Regulation (%)
90
Efficiency (%)
80 70 60 50 40 30 0.1 1 10 100
0.5
0.0
VIN = 2.7V VIN = 3.6V VIN = 4.2V
1000 10000
-0.5
-1.0 0.1
VIN = 2.7V VIN = 3.6V VIN = 4.2V
1 10 100 1000 10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(VOUT = 1.2V)
100 1.0
Load Regulation vs. Output Current
(VOUT = 1.2V)
VIN = 2.7V VIN = 3.6V VIN = 4.2V
Load Regulation (%)
90
Efficiency (%)
80 70 60 50 40 30 0.1 1 10 100
0.5
0.0
VIN = 2.7V VIN = 3.6V VIN = 4.2V
1000 10000
-0.5
-1.0
0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 1
Quiescent Current vs. Input Voltage
(VOUT = 1.8V; No Load)
80 1.5
Output Voltage Error vs. Temperature
(VOUT = 1.8V; IOUT = 1A) Output Voltage Error (%)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
Quiescent Current (A)
70 60 50 40 30 20 10 0
85C 25C -40C
2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Input Voltage (V)
Temperature (C)
Output Voltage vs. Input Voltage
(VOUT = 1.8V; IOUT = 1A)
1.81
Switching Frequency vs. Temperature
(VOUT = 1.8V; IOUT = 1A) Switching Frequency (MHz)
1.56 1.54 1.52 1.5 1.48 1.46 1.44 1.42 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
1.82
85C
Output Voltage (V)
1.80 1.79 1.78 1.77 1.76 1.75 1.74 1.73 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
25C -40C
Input Voltage (V)
Temperature (C)
Load Transient Response
(VOUT = 1.8V) Output Voltage (AC coupled) (top)(mV)
0.20 0.10 0.00 -0.10 -0.20 -0.30 -0.40 -0.50 -0.60
Load Transient Response
(VOUT = 1.8V; CFF = 100pF) Output Voltage (AC coupled) (top)(mV)
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.20 0.10 0.00 -0.10 -0.20 -0.30 -0.40 -0.50 -0.60 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8
Output Current (bottom) (A)
Output Current (bottom) (A)
Time (100s/div)
Time (100s/div)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 1
Line Transient Response
(VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)
5.0 4.5 0.12 0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 0.40 0.30
Line Regulation
(VOUT = 1.8V; IOUT = 1A) Output Voltage (AC coupled) (bottom) (V)
VOUT Error (%)
Input Voltage (top) (V)
4.0 3.5 3.0 2.5 2.0 1.5 1.0
0.20 0.10 0.00 -0.10 -0.20 -0.30 -0.40 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Time (100s/div)
Input Voltage (V)
Light Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF) Output Voltage (AC coupled) (top)(mV) Output Voltage (AC coupled) (top)(mV)
80 40 0 -40 -80 -120 -160 -200 -240 0.7 80 40 0 -40 -80 -120 -160 -200 -240
Light Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF)
0.7
Inductor Ripple Current (bottom) (A)
Inductor Ripple Current (bottom) (A)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1
0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1
Time (5s/div)
Time (200s/div)
Light Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF) Output Voltage (AC coupled) (top)(mV)
80 40 0 -40 -80 -120 -160 -200 -240 0.7
Light Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF) Output Voltage (AC coupled) (top)(mV)
80 40 0 -40 -80 -120 -160 -200 -240 0.7
Inductor Ripple Current (bottom) (A)
Inductor Ripple Current (bottom) (A)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1
0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1
Time (5s/div)
Time (500s/div)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 1
Enable Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.7A) Output Voltage (AC coupled) (top)(mV)
20 10 0 -10 -20 -30 -40 -50 -60
Heavy Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1.7mA)
2.8
Inductor Ripple Current (bottom) (A)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2
EN (2V/div) VOUT (1V/div) IIN (500mA/div)
Time (100s/div)
Time (500ns/div)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 2
Efficiency vs. Output Current
(VOUT = 3.3V)
100 90 80 70 60 50 40 30 0.1 1 10 100 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0.1 1 10 100 1000
Load Regulation
(VOUT = 3.3V)
VIN = 3.6V VIN = 4.2V VIN = 5V
VIN = 3.6V VIN = 4.2V VIN = 5V
1000
Output Current (mA)
Output Error (%)
Efficiency (%)
Output Current (mA)
Efficiency vs. Output Current
(VOUT = 2.5V)
100 1 0.8
Load Regulation
(VOUT = 2.5V)
VIN = 3V VIN = 3.6V VIN = 4.2V VIN = 5V
80 70 60 50 40 30 0.1 1 10 100
Output Error (%)
90
Efficiency (%)
0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0.1 1 10 100
VIN = 3V VIN = 3.6V VIN = 4.2V VIN = 5V
1000
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(VOUT = 1.8V)
100 90 80 70 60 50 40 30 0.1 1 10 100 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0.1 1
Load Regulation
(VOUT = 1.8V)
Output Error (%)
Efficiency (%)
VIN = 2.7V VIN = 3.6V VIN = 4.2V
1000
VIN = 2.7V VIN = 3.6V VIN = 4.2V
10 100 1000
Output Current (mA)
Output Current (mA)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 2
Line Regulation
(VOUT = 1.8V)
0.4 0.3
Switching Frequency vs. Temperature
(VOUT = 1.8V; IOUT = 800mA)
1mA 400mA 600mA 800mA
Switching Frequency (MHz)
5.5
0.5
2.08 2.06 2.04 2.02 2 1.98 1.96 1.94 1.92 -40 -15 10 35 60 85
Accuracy (%)
0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 2.7 3.1 3.5 3.9 4.3 4.7
5.1
Input Voltage (V)
Temperature (C)
Frequency Variation vs. Input Voltage
4 3 2 1 0 -1 -2 -3 -4 2.7 3.1 3.5 3.9 4.3 4.7 2.0
Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.8V, IOUT = 400mA) Output Voltage Error (%)
Frequency Variation (%)
1.0
0.0
-1.0
VOUT = 1.8V VOUT = 3V
5.1 5.5
-2.0 -40
-20
0
20
40
60
80
100
Input Voltage (V)
Temperature (C)
No Load Quiescent Current vs. Input Voltage
60
P-Channel RDS(ON) vs. Input Voltage
550 500
Supply Current (A)
55 50 40 35 30 25 20 15 10 2.7 3.1 3.5 3.9 4.3 4.7 5.1 85C 25C -40C 5.5 45
RDS(ON) (m)
450 400 350 300 250 200 2.5 3 3.5 4 4.5 5
120C 100C 85C 25C
5.5
6
Input Voltage (V)
Input Voltage (V)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 2
N-Channel RDS(ON) vs. Input Voltage
550 1.9
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10F; CFF = 100pF) Output and Inductor Current (100mA/div) Output Voltage (top) (V)
500
RDS(ON) (m)
450 400 350 300 250 200 2.5
120C 100C 85C 25C
1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 300mA 1mA 300mA 1mA
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
Time (50s/div)
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 4.7F; CFF = 0pF)
1.85 1.85
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10F; CFF = 0pF) Output and Inductor Current (100mA/div) Output and Inductor Current (100mA/div) Output Voltage (top) (V)
Output Voltage (top) (V)
1.8 1.75 1.7 1.65 1.6 1.55 1.5 400mA 300mA 400mA 300mA
1.8 1.75 1.7 1.65 1.6 1.55 1.5 400mA 300mA 400mA 300mA
Time (50s/div)
Time (50s/div)
Load Transient
(VIN = 3.6V; VOUT = 1.8V; COUT = 10F; CFF = 100pF)
1.85 4.8
Line Transient
(VOUT = 1.8V; VIN = 3.6V to 4.2V; IOUT = 400mA; CFF = 0pF)
1.92 1.9 1.88 1.86 1.84 1.82 1.8 1.78 1.76
Output and Inductor Current (100mA/div)
Output Voltage (bottom) (V)
Output Voltage (top) (V)
1.8 1.75 1.7 1.65 1.6 1.55 1.5 400mA 400mA 300mA 300mA
Input Voltage (top) (V)
4.2 3.6 3 2.4 1.8 1.2 0.6 0
Time (50s/div)
Time (50s/div)
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Typical Characteristics - Channel 2
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 1mA; CFF = 0pF)
0.02 0.35 0.02
Output Ripple
(VOUT = 1.8V; VIN = 3.6V; IOUT = 400mA; CFF = 0pF)
1.4 1.2
Output Voltage (top) (V)
Output Voltage (top) (V)
0.01 0 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06
0.3
0.01 0 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
0.25 0.2 0.15 0.1 0.05 0 -0.05
1 0.8 0.6 0.4 0.2 0 -0.2
Time (10s/div)
Time (10s/div)
Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA) Enable Voltage (top) (V) Output Voltage (middle) (V)
4 3 2 1 0
Input Current (bottom) (A)
0.5 0 -0.5
Time (100s/div)
12
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2789.2008.03.1.0
PRODUCT DATASHEET
AAT2789
SystemPowerTM
Functional Block Diagram
VCC1,2
Low Noise, High Frequency Dual Step-Down Converter
OT
OSC
VP1
FB1
Comp. Error Amp
Voltage Ref 1
Logic
LX1
EN1
Control Logic
PGND1
VP2
OT OSC
FB2
Comp. Error Amp
Voltage Ref 2
Logic
LX2
EN2
Control Logic
PGND2
AGND1,2
Functional Description
The AAT2789 is a 2-channel synchronous step-down (Buck) converter operating from an input voltage range of 2.7V to 5.5V; making it the ideal choice for single-cell Lithium-ion/polymer battery powered systems or low voltage 3.3V and 5V based consumer equipment. Channel 1 delivers up to 1700mA output current while Channel 2 delivers up to 800mA. Both converters incorporate a unique low noise architecture which reduces output ripple and spectral noise. The device utilizes a high switching frequency to minimize external filter sizing. Peak current mode control eliminates external compensation while optimizing transient performance and stability. The device requires a minimum of external components to realize a high efficiency dual-output step-down converter while minimizing solution size and footprint.
Each of the step-down regulators has an independent input and enable pin. Adjustable output voltage is provided. Light load operating mode provides high efficiency over the entire load range. The enable inputs, when pulled low, force the respective converter into a low power non-switching state consuming less than 1A of current. Low quiescent current enables excellent life for battery powered systems. Additional features include integrated soft start to limit inrush current. Soft start limits the current surge seen at the input and eliminates output voltage overshoot. For overload conditions, the peak input current is limited. Also, over-temperature protection safeguards the device from damage due to high operating temperature or fault conditions. The junction over-temperature threshold is 140C with 15C of hysteresis. Under voltage lockout (UVLO) guarantees sufficient input voltage bias prior to turn-on. The AAT2789 is available in the 3x4mm Pb-free 16-pin TDFN package and is rated over the -40C to 85C operating temperature range.
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PRODUCT DATASHEET
AAT2789
SystemPowerTM
Applications Information
Inductor Selection
Both step-down converters use peak current mode control with slope compensation to maintain stability for duty cycles greater than 50%. When the duty cycle exceeds 50%, the inductor value must be selected to maintain the prescribed down-slope in accordance with the internal slope compensation requirements.
Low Noise, High Frequency Dual Step-Down Converter
Input Capacitor
Select a 10F to 22F X7R or X5R ceramic capacitor for the input. To estimate the required input capacitor size, determine the acceptable input ripple level (VPP) and solve for CIN. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage.
Channel 1
The internal slope compensation for the adjustable and low voltage fixed versions of Channel 1 is 0.75A/s. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.8V output and 1.8H inductor.
CIN =
V VO * 1- O VIN VIN
VPP - ESR * FS IO
VO V 1 * 1 - O = for VIN = 2 * VO VIN VIN 4 CIN(MIN) = 1 VPP - ESR * 4 * FS IO
m= L=
0.75 VO 0.75 1.8V A = = 0.75 L 1.8H s 0.75 VO 0.75 1.2V = = 1.2H m A 0.75 s
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value. For example, the capacitance of a 10F, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6F. The maximum input capacitor RMS current is:
The inductor should be set equal to the output voltage numeric value in microhenries (H). This guarantees sufficient internal slope compensation. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. Some inductors may meet the peak and average current ratings yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor.
IRMS = IO *
VO V * 1- O VIN VIN
The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current.
VO V * 1- O = VIN VIN
For VIN = 2 * VO
D * (1 - D) =
0.52 =
1 2
Channel 2
The slope compensation for Channel 2 output is set at 0.75A/us. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.8V output and 1.8H inductor:
IRMS(MAX) =
IO 2
m= L=
0.75 VO 0.75 1.8V A = = 0.75 L 1.8H s 0.75 VO 0.75 3.3V = = 3.3H m A 0.75 s
The term VIN VIN appears in both the input voltage ripple and input capacitor RMS current equations and is a maximum when VO is twice VIN. This is why the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle.
VO
V * 1- O
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
COUT =
The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2789. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function. To minimize stray inductance, the capacitor should be placed as closely as possible to the IC. This keeps the high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C1) can be seen in the evaluation board layout in the Layout section of this datasheet (see Figures 1 and 2). A laboratory test set-up typically consists of two long wires running from the bench power supply to the evaluation board input voltage pins. The inductance of these wires, along with the low-ESR ceramic input capacitor, can create a high Q network that may affect converter performance. This problem often becomes apparent in the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain measurements can also result. Since the inductance of a short PCB trace feeding the input voltage is significantly lower than the power leads from the bench power supply, most applications do not exhibit this problem. In applications where the input power source lead inductance cannot be reduced to a level that does not affect the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the low ESR/ESL bypass ceramic capacitor. This dampens the high Q network and stabilizes the system.
3 * ILOAD VDROOP * FS
Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum value for the output capacitor with respect to load transients. The internal voltage loop compensation also limits the minimum output capacitor value to 10F. This is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater phase margin. The maximum output capacitor RMS ripple current is given by:
IRMS(MAX) =
VOUT * (VIN(MAX) - VOUT) L * FS * VIN(MAX) 2* 3 1 *
Dissipation due to the RMS current in the ceramic output capacitor ESR is typically minimal, resulting in less than a few degrees rise in hot-spot temperature.
Channel 2
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7F to 10F X5R or X7R ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the ESR and ESL characteristics necessary for low output ripple.
Output Capacitor
Channel 1
The output capacitor limits the output ripple and provides holdup during large load transitions. A 10F to 22F X5R or X7R ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the ESR and ESL characteristics necessary for low output ripple. The output voltage droop due to a load transient is dominated by the capacitance of the ceramic output capacitor. During a step increase in load current, the ceramic output capacitor alone supplies the load current until the loop responds. Within two or three switching cycles, the loop responds and the inductor current increases to match the load current demand. The relationship of the output voltage droop during the three switching cycles to the output capacitance can be estimated by:
Output Voltage
The AAT2789 output voltages are programmed with external resistors R1, R2 (Channel 1) and R3, R4 (Channel 2). To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum suggested value for R2 and R4 are 59k. Although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensitive to external noise and interference. Table 1 summarizes the resistor values for various output voltages with R2 and R4 set to either 59k for good noise immunity or 221k for reduced no load input current.
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15
PRODUCT DATASHEET
AAT2789
SystemPowerTM
VOUT(V)
0.6 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.0 3.3
Low Noise, High Frequency Dual Step-Down Converter
R2, 4 = 59k R1, 3(k)
0 19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 237 267
R2, 4 = 221k R1, 3(k)
0 75.0 113 150 187 221 261 301 332 442 464 523 715 887 1000
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
PTOTAL = IO12 * RDS(ON)H1 + IQ1 * VIN1 + IO22 * RDS(ON)H2 + IQ2 * VIN2
Since RDS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. Given the total losses, the maximum junction temperature can be derived from the JA for the TDFN34-16 package, which is 50C/W.
TJ(MAX) = PTOTAL * JA + TAMB
Table 1: AAT2789 Resistor Values for Various Output Voltages.
PCB Layout
The suggested PCB layout for the AAT2789 is shown in Figures 1 and 2. The following guidelines should be used to help ensure a proper layout. 1. The input and output capacitors C1, C2, C3, and C4 should be connected as closely as possible to the input and output pins. Output capacitors and inductors (C2, C3 and L1; C4 and L2) should connect as closely as possible. The connection of the inductor (L1, L2) to the LX1 and LX2 pins should be as short as possible. The feedback traces or FB pins should be separated from any power traces and connect as closely as possible to the load point. Sensing along a highcurrent load trace will degrade DC load regulation. The resistance of the trace from the load return to PGND should be kept to a minimum. This will help to minimize any error in DC regulation due to differences in the potential of the internal signal ground and the power ground. The lower R2 (FB1) and R4 (FB2) resistor's grounds should be connected to the AGND1 and AGND2 pins. C5, C6 are optional feed forward capacitors for both channels to stabilize the output voltage during large load transitions. For good thermal coupling, PCB vias are required from the pad for the TDFN paddle to the bottom ground plane.
Thermal Calculations
There are three types of losses associated with the AAT2789 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction losses are associated with the RDS(ON) characteristics of the power output switching devices. Switching losses are dominated by the gate charge of the power output switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of both stepdown converters is given by:
IO12 * (RDSON(HS) * VO1 + RDSON(L) * [VIN -VO1]) VIN1 IO22 * (RDSON(HS) * VO2 + RDSON(L) * [VIN -VO2]) VIN2
2.
3.
PTOTAL =
+ (tsw * FS * IO1 + IQ1) * VIN1
4.
+
+ (tsw * FS * IO2 + IQ2) * VIN2
IQ1 and IQ2 are the step-down converter quiescent currents for Channel 1 and Channel 2 respectively. The term tSW is used to estimate the full load step-down converter switching losses.
5. 6.
7.
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PRODUCT DATASHEET
AAT2789
SystemPowerTM Low Noise, High Frequency Dual Step-Down Converter
Printed Circuit Board Layout Recommendations
Figure 1: AAT2789 Evaluation Board Component Side Layout.
U1 1 NC VIN 7 VP1 8 VP1 C1 10F 5 VCC1 6 LX2 15 AAT2789 EN1 FB2 AGND1 3 L2 3.3H R3 267K FB1 12 11 LX1 9 L1 1.5H
Figure 2: AAT2789 Evaluation Board Solder Side Layout.
OUT1 C5 opt C2 10F C3 10F
R1 59K
R2 59K OUT2
C6 opt
4
VP2
AGND2
16
R4 59K
C4 10F
13 VCC2 PGND1 14 EN2 PGND2
10
2
TDFN34-16
Figure 3: AAT2789 Evaluation Board Schematic. Symbol
U1 C1, C2, C3, C4 C5 C6 L1 L2 R1-R4
Part Number
AAT2789 GRM188R60J106ME47D Generic Generic LQM2HPN1R5MG0 TFC252008MBT Generic
Description
AnalogicTech 2-Output Buck TDFN34-16 Cap, MLC, 10uF/6.3V, 0603 ( HMAX=0.9mm), Murata Cap,10nF/6.3V,0402 Cap,100pF/6.3V,0402 1.5uH, ISAT=3A, 2x2.5x0.9mm (HMAX=0.95mm), shielded chip inductor, Murata 3.3uH, ISAT=0.52A, 2x2.5x1mm (HMAX=1.0mm), non-shielded chip inductor, TDK Carbon Film resistor, 0402
Qty
1 4 1 1 1 1 1
Table 1: AAT2789 Bill of Materials.
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17
PRODUCT DATASHEET
AAT2789
SystemPowerTM
Design Example
Specifications
VO1 = 1.2V @ 1.5A (adjustable using 0.6V version), Pulsed Load ILOAD = 1.5A VO2 = 3.3V @ 500mA (adjustable using 0.6V version), Pulsed Load ILOAD = 0.5A VIN = 2.7V to 4.2V (3.6V nominal) FS1 = 1.8MHz, FS2 = 2MHz m = 0.75A/s TAMB = 85C in TDFN34-16 Package
Low Noise, High Frequency Dual Step-Down Converter
Channel 1 Inductor
L= 0.75 VO 0.75 1.2V = = 1.2H; use 1.5H m A 0.75 s
For TDK inductor LQM2PHN1R5MG0, 1.5H, DCR = 70m max.
I =
VO1 1.2V VO1 1.2V * 1= * 1= 317mA L1 FS VIN 1.5H 1.8MHz 4.2V I = 1.5A + 0.317A = 1.817A 2
IPK1 = IO1 +
PL1 = IOUTBUCK2 DCR = 1.5A2 70m = 158mW
Channel 2 Inductor
L= 0.75 VO 0.75 3.3V = = 3.3H m A 0.75 s
For TDK inductor TFC252008MBT, 3.3H, DCR = 100m max.
I =
VO1 3.3V VO2 3.3V * 1= * 1= 107mA L1 FS VIN 3.3H 1.8MHz 4.2V I = 0.5A + 0.054A = 0.55A 2
IPK1 = IO1 +
PL1 = IOUTBUCK2 DCR = 0.55A2 100m = 30.1mW
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2789.2008.03.1.0
PRODUCT DATASHEET
AAT2789
SystemPowerTM
Channel 1 Output Capacitor
VDROOP= 0.12V
Low Noise, High Frequency Dual Step-Down Converter
COUT3 =
3 * ILOAD 3 * 1.5A = = 20.8F; use 22F 0.12V * 1.8MHz VDROOP * FS VOUT * (VIN(MAX) - VOUT) 1 1.2V * (4.2V - 1.2V) * = 92mA = 1.5H * 1.8MHz * 4.2V L * FS * VIN(MAX) 2* 3 2* 3 1 *
IRMS(MAX) =
PESR = ESR * IRMS2 = 5m * 24mA2 = 3W
Channel 2 Output Capacitor
VDROOP= 0.1V
COUT3 =
3 * ILOAD 3 * 0.5A = = 7.5F; use 10F 0.1V * 2MHz VDROOP * FS VOUT * (VIN(MAX) - VOUT) 1 3.3V * (4.2V - 3.3V) * = 30mA = 3.3H * 2MHz * 4.2V L * FS * VIN(MAX) 2* 3 2* 3 1 *
IRMS(MAX) =
PESR = ESR * IRMS2 = 5m * 30mA2 = 4.5W
Input Capacitor
Input Ripple VPP1 = 50mV, VPP2 = 25mV CIN1 =
VPP1 IO1
1 1 = = 4.9F 50mV - 5m * 4 * 1.8MHz - ESR * 4 * FS 1.5A 1 1 = = 3F 25mV - 5m * 4 * 2MHz - ESR * 4 * FS 0.5A
CIN2 =
VPP2 IO2
CIN = CIN1+ CIN2 = 4.9F+3F= 7.9F; use 10F
IRMS(MAX) = IO1 + IO2 = 1A 2
P = ESR * IRMS2 = ESR * (1A)2 = 5mW
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19
PRODUCT DATASHEET
AAT2789
SystemPowerTM
AAT2789 Losses
Total loss can be estimated by calculating the dropout (VIN = VO) losses where the power MOSFETs RDS (ON) will be at the maximum value. All values assume an 85C ambient temperature and a 120C junction temperature with the TDFN 50C/W package.
Low Noise, High Frequency Dual Step-Down Converter
PTOTAL = IO12 * RDS(ON)H1 + IQ1 * VIN1 + IO22 * RDS(ON)H2 + IQ2 * VIN2 PTOTAL = 1.5A2 * 120m + 70A * 4.2 + 0.52 * RDS(ON)H2 + 70A * 4.2V = 270mW TJ(MAX)= TAMB + JA * PLOSS = 85C + (50C/W) * 270mW = 99C
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2789.2008.03.1.0
PRODUCT DATASHEET
AAT2789
SystemPowerTM
Ordering Information
Output Voltage Package
TDFN34-16 Channel 1 Adjustable (0.6) Channel 2 Adjustable (0.6)
Low Noise, High Frequency Dual Step-Down Converter
Marking1
3JXYY
Part Number (Tape and Reel)2
AAT2789IRN-AA-T1
All AnalogicTech products are offered in Pb-free packaging. The term "Pb-free" means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/about/quality.aspx.
Legend
Output Voltage Adjustable (0.6) Code A
1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD.
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21
PRODUCT DATASHEET
AAT2789
SystemPowerTM
Package Information1
TDFN34-16
3.000 0.050 1.600 0.050 Detail "A" Index Area
Low Noise, High Frequency Dual Step-Down Converter
4.000 0.050
3.300 0.050
0.350 0.100
Top View
Bottom View
C0.3 0.230 0.050
(4x) 0.850 MAX
0.050 0.050
0.229 0.051
Side View Detail "A"
All dimensions in millimeters.
1. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection.
Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611
(c) Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Except as provided in AnalogicTech's terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer's applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.
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0.450 0.050
Pin 1 Indicator (optional)
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