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PRODUCT DATASHEET
AAT2785
SystemPowerTM
General Description
The AAT2785 is a 3-channel 1.8MHz step-down converter for applications where power efficiency and solution size are critical. The input voltage range is 2.7V to 5.5V and the outputs are adjustable from 0.6V to VIN. The AAT2785 incorporates a unique low noise architecture which reduces ripple and spectral noise. Channel 3 delivers up to 1.5A output current and channels 1 and 2 deliver up to 600mA each. The AAT2785 uses a high switching frequency to minimize the size of external components. The AAT2785 requires a minimum of external components to realize a high efficiency triple-output buck converter minimizing solution cost and PCB footprint. Each of the 3 regulators has an independent enable pin, adjustable output voltage and operates with low no load quiescent current, providing high efficiency over the entire load range. The AAT2785 is available in a Pb-free 16 pin TDFN34 package, and is rated over the -40C to +85C operating temperature range.
3-Channel Step-Down DC/DC Converter
Features
* VIN Range: 2.7 to 5.5V * Output Voltage Range: 0.6V to VIN * Output Current: Channel 3: 1.5A Channel 1: 600mA Channel 2: 600mA * Low Noise Light Load Mode * Low Ripple PWM Mode * Highly Efficient Step-Down Converters * Low RDS(ON) Integrated Power Switches * 100% Duty Cycle * 1.8MHz Switching Frequency * Internal Soft Start * Fast 150s Turn-On Time * Over-Temperature Protection * Current Limit Protection * 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 LAN
Typical Application
IN LX1
L1 4.7H
VOUT1: 3.3V, 600mA R1 133k
AAT2785
FB1 VIN: 2.7V-5.5V VP1_2 EN1 LX2 FB2 C1 10F EN2 PGND L3 1.5H VP3 EN3 FB3 C2 10F GND PGND LX3 L2 4.7H VOUT2: 3.3V, 600mA R3 133k R4 29.4k C4 4.7F
R2 29.4k
C3 4.7F
VOUT3: 1.2V, 1.5A R5 59.0k R6 59.0k
C5 10F
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Pin Descriptions
Pin #
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 EP
3-Channel Step-Down DC/DC Converter
Symbol
PGND2 FB2 EN1 EN2 AGND IN EN3 FB3 PGND3 LX3 VP3 FB1 PGND1 LX1 VP1_2 LX2 EP
Function
Power ground return pin 2. Connect to the output and input capacitor return. Feedback input pin for channel 2. Connect an external resistor divider to this pin to program the output voltage to the desired value. Enable pin for channel 1. Active high. Enable pin for channel 2. Active high. Signal ground. Input supply pin for device. Supplies bias for the internal circuitry. Enable pin for channel 3. Active high. Feedback input pin for channel 3. Connect an external resistor divider to this pin to program the output voltage to the desired value. Power ground return pin 3. Connect to the output and input capacitor return. Power switching node for channel 3. Output switching node connects to the output inductor. Input power supply pin for channel 3. Must be closely decoupled. Feedback input pin for channel 1. Connect an external resistor divider to this pin to program the output voltage to the desired value. Power ground return pin 1. Connect to the output and input capacitor return. Power switching node for channel 1 and 2. Output switching node connects to the output inductor. Input power supply pin for channels 1 and 2. Must be closely decoupled. Power switching node for channel 2. Output switching node connects to the output inductor. Exposed pad. Connect to ground directly under the device. Use properly sized vias for thermal coupling to the ground plane. See section on PCB layout guidelines.
Pin Configuration
TDFN34-16 (Top View)
PGND2 FB2 EN1 EN2 AGND IN EN3 FB3
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10 9
LX2 VP1_2 LX1 PGND1 FB1 VP3 LX3 PGND3
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PRODUCT DATASHEET
AAT2785
Units
V V V V C C
SystemPowerTM
Absolute Maximum Ratings1
Symbol
VIN, VP VLX VFB VEN TJ TLEAD
3-Channel Step-Down DC/DC Converter
Description
Input Voltages to AGND/PGND LX1, LX2, LX3 to AGND/PGND FB1, FB2, FB3 to AGND/PGND EN1, EN2, EN3 to AGND/PGND 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
Thermal Information
Symbol
PD JA
Description
Maximum Power Dissipation2 Thermal Resistance3
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 an FR4 board. 3. Derate 20mW/C above 25C ambient temperature.
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Electrical Characteristics1
3-Channel Step-Down DC/DC Converter
VIN = VP = 3.6V; TA = -40C to 85C, unless noted otherwise. Typical values are at TA = 25C. Symbol
VIN VOUT VOUT IQ1,2 IQ3 ISHDN ILX_LEAK ILX_LEAK IFB ILIM1,2 ILIM3 RDS(ON)H1,2 RDS(ON)L1,2 RDS(ON)H3 RDS(ON)L3 VLOADREG VLINEREG FOSC1,2 FOSC3 TS TSD THYS VIL VIH IEN
Description
Input Voltage Output Voltage Tolerance Output Voltage Range Quiescent Current Channels 1, 2 Quiescent Current Channel 3 Shutdown Current LX Reverse Leakage Current LX Leakage Current Feedback Leakage P-Channel Current Limit P-Channel Current Limit High Side Switch On-Resistance Low Side Switch On-Resistance High Side Switch On-Resistance Low Side Switch On-Resistance Load Regulation Line Regulation Oscillator Frequency Channels 1,2 Oscillator Frequency Channel 3 Start-Up Time Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis Enable Threshold Low Enable Threshold High Enable Input Current
Conditions
IOUT3 = 0 to 1.5A; IOUT1,2 = 0 to 600mA; VIN = 2.7 to 5.5V Per Channel, No Load No Load VEN1 = VEN2 = VEN3 = GND VIN Open, VLX = 5.5V; VEN = 0V VIN = 5.5V, VLX = 0 to VIN VFB = 1.0V
Min
2.7 -3.0 0.6
Typ
Max
5.5 3.0
Units
V % V A A A A A A A A m m m m % % MHz MHz s C C V V A
50 45
VIN 100 90 1.0 1.0 1.0 0.2
ILOAD1,2 = 0 to 600 mA; ILOAD3 = 0 to 1.5A VIN = 2.7 to 5.5V
From Enable to Output Regulation
1.8 3.8 400 400 150 120 0.8 0.5 1.8 1.8 150 140 15 0.6 1.4 -1.0
VIN = VEN = 5.5V
1.0
1. The AAT2785 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
AAT2785
SystemPowerTM
Typical Characteristics
Efficiency vs. Output Current
(Channel 1, 2; VOUT = 3.3V)
105 95
3-Channel Step-Down DC/DC Converter
Load Regulation
(Channel 1, 2; VOUT = 3.3V)
3 2
Output Error (%)
Efficiency (%)
85 75 65 55 45 35 25 15 0.1 1 10 100
1 0 -1 -2 -3 -4 -5 0.1
VIN = 3.6V VIN = 4.2V VIN = 5V
1000
VIN = 5V VIN = 4.2V VIN = 3.6V
1 10 100 1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(Channel 3; VOUT = 1.2V)
100 90 80 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0.1
Load Regulation
(Channel 3; VOUT = 1.2V)
70 60 50 40 30 20 10 0 0.1 1 10 100
Output Error (%)
Efficiency (%)
VIN = 4.2V VIN = 3.6V VIN = 2.7V
1000 10000
VIN = 4.2V VIN = 3.6V VIN = 2.7V
1 10 100 1000 10000
Output Current (mA)
Output Current (mA)
Switching Frequency vs. Input Voltage
10 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -40
Output Error vs. Temperature
Switching Frequency (%)
8 4 2 0 -2 -4 -6 -8 -10 2.3 2.8 3.3 3.8 4.3 4.8
Channel 3 Channel 1, 2
5.3 5.8
Output Error (%)
6
Channel 3 Channel 1, 2
-15 10 35 60 85
Input Voltage (V)
Temperature (C)
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Typical Characteristics
Quiescent Current vs. Input Voltage
(Channel 1, 2; VOUT = 3.3V; No Load; Open Loop)
100
3-Channel Step-Down DC/DC Converter
Quiescent Current vs. Input Voltage
(Channel 3; VOUT = 1.2V; No Load; Open Loop)
100 90 80 70 60 50 40 30 20 10 5.6 0 2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5
Supply Current (A)
80 70 60 50 40 30 20 10 0 3.2 3.5 3.8 4.1 4.4 4.7 5
85C 25C -40C
5.3
Supply Current (A)
90
85C 25C -40C
5.3 5.6
Input Voltage (V)
Input Voltage (V)
P-Channel On-Resistance vs. Input Voltage
(Channel 1, 2; VOUT = 3.3V)
100C 85C 25C
P-Channel On-Resistance vs. Input Voltage
(Channel 3; VOUT = 1.2V) Switch On-Resistance (m)
300 250 200 150 100 50 0
1000
On-Resistance (m)
900 800 700 600 500 400 300 3.2 3.6 4 4.4 4.8 5.2
.
100C 85C 25C
3.1 3.6 4.1 4.6 5.1 5.6
5.6
2.6
Input Voltage (V)
Input Voltage (V)
VIH vs. Input Voltage
1.3 1.2 1.1 1.3 1.2 1.1
VIL vs. Input Voltage
VIH (V)
1 0.9 0.8 0.7 0.6 2.6 3.1 3.6 4.1 4.6 5.1
VIL (V)
1 0.9 0.8 0.7
85C 25C -40C
5.6
0.6 2.6
85C 25C -40C
3.1 3.6 4.1 4.6 5.1 5.6
Input Voltage (V)
Input Voltage (V)
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Typical Characteristics
Line Regulation
(Channel 1, 2; VOUT = 3.3V)
1 0.5
3-Channel Step-Down DC/DC Converter
Line Regulation
(Channel 3; VOUT = 1.2V)
0.5 0.4 0.3
Accuracy (%)
Accuracy (%)
0 -0.5 -1 -1.5 -2 3 3.5 4 4.5 5 5.5 6
0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 2.6 3.1 3.6 4.1 4.6
IOUT = 10mA IOUT = 150mA IOUT = 300mA IOUT = 600mA
IOUT = 10mA IOUT = 100mA IOUT = 1000mA IOUT = 1500mA
5.1 5.6
Input Voltage (V))
Input Voltage (V))
Soft Start
(Channel 1, 2; VIN = 5V; VOUT = 3.3V; IOUT = 300mA) Enable Voltage (top) (V) Output Voltage (middle) (V)
3
Soft Start
(Channel 3; VIN = 5V; VOUT = 1.2V; IOUT = 1mA) Enable Voltage (top) (V) Output Voltage (middle) (V)
4 3
4
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
2 1 0 1 0.5 0
2 1 0 1 0.5 0
Time (40s/div)
Time (40s/div)
Soft Start
(Channel 3; VIN = 5V; VOUT = 1.2V; IOUT = 1.5A) Enable Voltage (top) (V) Output Voltage (middle) (V)
4 3 0.01
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 4.6V; IOUT = 1mA) Output Voltage (top) (V)
0
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
2 1 0 1.5 1 0.5 0
-0.01
0.2 0.1 0
Time (40s/div)
Time (400ns/div)
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Typical Characteristics
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 4.6V; IOUT = 600mA)
0.01
3-Channel Step-Down DC/DC Converter
Output Ripple
(Channel 3; VOUT = 1.2V; VIN = 4.6V; IOUT = 1.5A)
0.01
Output Voltage (top) (V)
Output Voltage (top) (V)
0
0
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
-0.01
-0.01
0.8 0.6 0.4 0.2 0
2 1.5 1 0.5 0
Time (400ns/div)
Time (400ns/div)
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 3.6V; IOUT = 600mA)
0.01 0.01
Output Ripple
(Channel 3; VOUT = 1.2V; VIN = 3.6V; IOUT = 1.5A) Output Voltage (top) (V)
0
Output Voltage (top) (V)
0
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
-0.01
-0.01
0.8 0.6 0.4 0.2 0
2 1.5 1 0.5 0
Time (400ns/div)
Time (400ns/div)
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 5V; IOUT = 600mA)
0.01 0.01
Output Ripple
(Channel 3; VOUT = 1.2V; VIN = 5V; IOUT = 1.5A) Output Voltage (top) (V)
0
Output Voltage (top) (V)
0
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
-0.01
-0.01
0.8 0.6 0.4 0.2 0
2 1.5 1 0.5 0
Time (400ns/div)
Time (400ns/div)
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Typical Characteristics
Output Ripple
(Channel 3; VOUT = 1.2V; VIN = 4.2V; IOUT = 1mA)
0.04
3-Channel Step-Down DC/DC Converter
Load Transient
(Channel 1, 2; VIN = 3.6V; IOUT = 100mA to 600mA; VOUT = 3.3V) Output Current (middle) (A) Inductor Current (bottom) (A)
0.2
Output Voltage (top) (V)
0.02
Output Voltage (top) (V)
0 -0.2 100mA 600mA 100mA 600mA
Inductor Current (bottom) (A)
0 -0.02
0.4 0.2 0
Time (100s/div)
Time (200s/div)
Load Transient
(Channel 1, 2; VIN = 3.6V; IOUT = 1mA to 600mA; VOUT = 3.3V) Output Current (middle) (A) Inductor Current (bottom) (A)
0.2 0.1
Load Transient
(Channel 3; VIN = 5V; IOUT = 0.1A to 1.5A; VOUT = 1.2V) Output Current (middle) (A) Inductor Current (bottom) (A) Output Voltage (top) (V)
0 -0.1 1.5A
Output Voltage (top) (V)
0 -0.2 1mA 600mA
100mA 1.5A
600mA 1mA
100mA
Time (200s/div)
Time (400s/div)
Load Transient
(Channel 3; VIN = 5V; IOUT = 0.5A to 1.5A; VOUT = 1.2V) Output Current (middle) (A) Inductor Current (bottom) (A)
0.1 6
Line Transient
(Channel 1, 2; VIN = 4V to 5V; IOUT = 600mA; VOUT = 3.3V) Output Voltage (bottom) (V) Input Voltage (top) (V)
5 4
Output Voltage (top) (V)
0 -0.1 1.5A 500mA 1.5A 500mA
0.2 0.1 0 -0.1 -0.2
Time (400s/div)
Time (1ms/div)
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Typical Characteristics
Line Transient
(Channel 3; VIN = 3.6V to 4.2V; IOUT = 1.5A; VOUT = 1.2V)
6
3-Channel Step-Down DC/DC Converter
Output Voltage (bottom) (V)
Input Voltage (top) (V)
5 4 3 0.04 0.02 0 -0.02 -0.04
Time (1ms/div)
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Functional Block Diagram
3-Channel Step-Down DC/DC Converter
VP3
FB3
Comp. Error Amp
Logic
LX 3
EN3
Control Logic
PGND3
OT OSC
VP1_2
FB2
Comp. Error Amp
Logic
LX2
EN2
Control Logic
PGND2 AGND
OSC
IN
FB1
Comp. Error Amp
Voltage Ref
Logic
LX1
EN1
Control Logic
PGND1
Functional Description
The AAT2785 is a high performance power management IC comprised of 3 buck converters. Each channel has an independent input voltage and enable pin. Operating at a switching frequency of 1.8MHz, the converter requires a minimum of small external components, reducing the solution cost and PCB footprint. All converters operate with an input voltage range of 2.7V to 5.5V. The output voltage range is 0.6V to VIN and is adjustable with an external resistor divider. Channel 3 power devices are sized for 1.5A output current. Channels 1 and 2 power devices are sized for 600mA output current while maintaining over 85% efficiency at full load. Peak efficiency is above 95%. Light load efficiency is maintained at greater than 80% down to 85%
of full load current. All channels have excellent transient response, load and line regulation. Transient response time is typically less than 20s. Soft start limits the current surge seen at the input and eliminates output voltage overshoot. The enable inputs, when pulled low, force the respective converter into a low power non-switching state consuming less than 1A of current. For overload conditions, the peak input current is limited. Also, thermal protection completely disables switching if internal dissipation becomes excessive, thus protecting the device from damage. The junction over-temperature threshold is 140C with 15C of hysteresis. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuits prior to activation.
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Control Loop
The AAT2785 is a peak current mode step-down converter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as shortcircuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor. The output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output voltage for all load and line conditions. Internal loop compensation terminates the transconductance voltage error amplifier output. The reference voltage is internally set to program the converter output voltage greater than or equal to 0.6V.
3-Channel Step-Down DC/DC Converter
threshold is 140C with 15C of hysteresis. Once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation.
Component Selection
Inductor Selection: Channels 1 and 2
The step-down converter uses peak current mode control with slope compensation to maintain stability for duty cycles greater than 50%. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. The internal slope compensation for the adjustable and low voltage fixed versions of channels 1 and 2 is 0.6A/ s. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.8V output and 2.2H inductor.
Soft Start/Enable
Soft start limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the AAT2785 into a low-power, non-switching state. The total input current during shutdown is less than 1A.
Low Dropout Operation
For conditions where the input voltage drops to the output voltage level, the converter duty cycle increases to 100%. As the converter approaches the 100% duty cycle, the minimum off time initially forces the high side in time to exceed the 1.8MHz clock cycle and reduce the effective switching frequency. Once the input drops below the level where the converter can regulate the output, the high side P-channel MOSFET is enabled continuously for 100% duty cycle. At 100% duty cycle the output voltage tracks the input voltage minus the I*R drop of the high side P-channel MOSFET.
m= L=
0.75 VO 0.75 1.8V A = = 0.6 L 2.2H s
0.75 VO 0.75 3.3V = = 4.1H m A 0.6 s
Current Limit and Over-Temperature Protection
For overload conditions, the peak input current is limited. To minimize power dissipation and stresses under current limit and short-circuit conditions, switching is terminated after entering current limit for a series of pulses. Switching is terminated for seven consecutive clock cycles after a current limit has been sensed for a series of four consecutive clock cycles. Thermal protection completely disables switching when internal dissipation becomes excessive. The junction over-temperature
In this case a standard 4.7H value is selected. Table 1 displays the suggested inductor values for channels 1 and 2. The 4.7H CDRH2D11 series inductor selected from Sumida has a 170m DCR and a 0.88A DC current rating. At full load the inductor DC loss is 15mW which corresponds to a 1.5% loss in efficiency for a 600mA, 3.3V output.
Inductor Selection: Channel 3
The internal slope compensation for the adjustable and low voltage fixed versions of channel 3 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.
m=
0.75 VO 0.75 1.8V A = = 0.75 L 1.8H s
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
L= 0.75 VO 0.75 1.2V = = 1.2H m A 0.75 s
3-Channel Step-Down DC/DC Converter
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 micro henries (H). This guarantees that there is 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. For channel 3, the 1.5H LQH32PN1R5NN0L series Murata inductor has a 68.4m worst case DCR and a 1.75A DC current rating. At full 1.5A load, the inductor DC loss is 154mW which gives less than 5% loss in efficiency for a 1.5A, 1.2V output.
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
IRMS(MAX) =
VO
IN
Input Capacitor
Select a 10F to 22F X7R or X5R ceramic capacitor for the VP1_2 and VP3 inputs. 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. Output Voltage
0.6V2.0V 2.5V 3.3V
Configuration
0.6V adjustable with external resistive divider
Inductor
2.2H 3.3H 4.7H
Slope Compensation
0.6A/s
Table 1: AAT2785 Inductor Values.
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
The term V V appears in both the input voltage ripple and input capacitor RMS current equations and is at 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. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2785. 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 Figure 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
IN
V * 1- O
IO 2
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
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-Channel Step-Down DC/DC Converter
immunity, the minimum suggested value for R2 and R4 are 29.4 k, and R6 is 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 2 and Table 3 summarize the resistor values for various output voltages of channel 1, channel 2, and channel 3. R2 = R4 = 29.4k R1 = R3 (k)
10 15 20 25 29 34 39 44 59 61 69 93 118 132
Output Capacitor: Channels 1 and 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. 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:
VOUT (V)
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.9 2.0 2.5 3.0 3.3
COUT =
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 4.7F. 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.
Table 2: AAT2785 Resistor Values for Various Output Voltages of Channel 1 and Channel 2. R6 = 59k VOUT (V)
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
R6 = 221k R5 (k)
75 113 150 187 221 261 301 332 442 464 523 715 887 1000
R5 (k)
19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 237 267
Output Capacitor: Channel 3
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.
Adjustable Output Resistor Selection
The output voltage for each channel of the AAT2785 is programmed with external resistors R1, R2, R3, R4, R5, and R6. To limit the bias current required for the external feedback resistor string while maintaining good noise
Table 3: AAT2785 Resistor Values for Various Output Voltages of Channel 3.
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Thermal Calculations
There are three types of losses associated with the AAT2785 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 the losses is given by:
3-Channel Step-Down DC/DC Converter
Layout
The suggested PCB layout for the AAT2785 is shown in Figures 2 and 3. The following guidelines should be used to help ensure a proper layout. 1. The power input capacitors (C5 and C8) should be connected as closely as possible to VP1_2, VP3 and PGND1,2,3 as shown in Figure 2. Due to the pin placement of VP1_2 and VP3 for all converters, proper decoupling is not possible with just one input capacitor. C1 and R7 are optional low pass filter components for the IN supply pin for the device if additional noise decupling is required in a noisy system C2 and L1, C6 and L2, C10 and L3 should be connected as closely as possible. The connection of L1, 2, 3 to the LX1, 2, 3 pin should be as short as possible. The feedback trace or FB pin should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. The resistance of the trace from the load returns to PGND1, 2 and 3 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. Connect unused signal pins to ground to avoid unwanted noise coupling. For good thermal coupling, PCB vias are required from the pad for the TDFN paddle to the bottom ground plane. The via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid.
PTOTAL =
IO2 * (RDS(ON)H * VO + RDS(ON)L * [VIN - VO]) VIN
2.
3.
+ (tsw * FS * IO + IQ) * VIN
IQ is the step-down converter quiescent current. The term tSW is used to estimate the full load step-down converter switching losses. For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to: 4.
5.
PTOTAL = IO2 * RDSON(H) + IQ * VIN
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. 6. 7.
TJ(MAX) = PTOTAL * JA + TAMB
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Evaluation Board Schematic
LX 3
3-Channel Step-Down DC/DC Converter
LX 2
LX 1
1
1
1
L2
Vi n
1 4.7H R7 0 C7 opt R3 133k
1
VOUT2
EN1
1 2 3 1 2 3 C5 10F C4 10F
C6 4.7F
AA T2785 1 2 3 4 5 6 7 8 C1 10F PGND2 FB2 EN1 EN2 GND VIN EN3 FB3 U1 LX 2 VP1_2 LX 1 PGND1 FB1 VP3 LX3 PGND3 16 15 14 13 12 11 10 9 L1 4.7H L3 1.5uH C9 opt R5 59K 1 C3 opt R1 133K 1
R4 29.4k
VOUT1
C2 4.7F
EN2
VOUT3
C10 10F R2 29.4K
EN3
1 2 3
C8 10F
R6 59K 1
PGND
1
PGND
Figure 1: AAT2785 Evaluation Board Schematic.
Evaluation Board Layout
Figure 2: AAT2785 Evaluation Board Component Side Layout.
Figure 3: AAT2785 Evaluation Board Solder Side Layout.
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Component
U1 L1, L2 L3 C1 C2, C6 C5, C8, C10 C9 R1, R3 R2, R4 R5, R6 R7
3-Channel Step-Down DC/DC Converter
Manufacturer
AATI Sumida Murata Generic Murata Murata Generic Generic Generic Generic Generic
Part Number
AAT2785 CDRX2D11 LQH32PN1R5NN0L GMR219R61A475KE19 GMR21BR60J106KE19
Description
3-Channel Step-Down DC/DC Converter 4.7H 0.88A 170m (3.2x3.2x1.2)mm Shielded 1.5H series Murata inductor has a 68.4m worst case DCR and a 1.75A DC Optional 4.7F 10V 0805 10F 6.3V 0805 56pF 6.3V 0402 133K 0402 29.4K 0402 59K 0402 Optional
Table 4: AAT2785 Evaluation Board Bill of Materials.
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17
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Design Example
Specifications
VO3 VO1 VO2 VIN FS TAMB 1.2V @ 3.3V @ 3.3V @ 2.7V to 1.8MHz 85C
3-Channel Step-Down DC/DC Converter
1.5A (adjustable using 0.6V version), pulsed load ILOAD = 1.5A 600mA (adjustable using 0.6V version), pulsed load ILOAD = 600mA 600mA (adjustable using 0.6V version), pulsed load ILOAD = 600mA 4.2V (3.6V nominal)
Channel 3 Output Inductor
L= 0.75 VO 0.75 1.2V = = 1.2H ; use 1.5H. (see Table 4). m A 0.75 s
Select Murata LQH32PN1R5NN0L 1.5H 1.75A DC current rating DCR = 68m.
I3 =
VO3 V 1.5V 1.5V 1 - O3 = 1= 357mA LF VIN 1.5H 1.8MHz 4.2V
IPK3 = 1.5A + 0.357A = 1.9A PL3 = IO32 DCR = 1.5A2 68m = 153mW
Channels 1 and 2 Output Inductors
L1 = L2 = 0.75 VO 0.75 3.3V = = 4.1H; use 4.7H. (see Table 4) m A 0.6 s
Select Sumida CDRH2D11 4.7H 0.88A DC current rating DCR = 170m.
I1 = I2 =
VO1 V 3.3V 3.3V 1 - O1 = 1= 84mA LF VIN 4.7H 1.8MHz 4.2V
IPK1 = IPK2 = 0.6A + 0.084A = 0.7A PL1 = PL2 = IO12 DCR = 0.62 170m = 61.2mW
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
Channel 3 Output Capacitor
COUT3 = 3 * ILOAD1 3 * 1.5A = = 12.5F; use 10 to 22F 0.2V * 1.8MHz VDROOP * FS
3-Channel Step-Down DC/DC Converter
IRMS(MAX) =
VOUT * (VIN(MAX) - VOUT) 1 1.2V * (4.2V - 1.2V) * = 92mA = L * FS * VIN(MAX) 2 * 3 1.5H * 1.8MHz * 4.2V 2* 3 1 *
PESR = ESR * IRMS2 = 5m * 92mA2 = 0.04mW
Channels 1 and 2 Output Capacitors
COUT1 = COUT2 = IRMS(MAX) = 1 3 * ILOAD1 3 * 0.6A = = 5F; use 5.6F VDROOP * FS 0.2V * 1.8MHz *
VOUT1 * (VIN(MAX) - VOUT1) 1 3.3V * (4.2V - 3.3V) * = 24mA = L * FS * VIN(MAX) 2 * 3 4.7H * 1.8MHz * 4.2V 2* 3
PESR = ESR * IRMS2 = 5m * 28.9mA2 = 2.9W
Channel 3 Input Capacitor
Input Ripple VPP = 30mV
CIN3 =
VPP IO3
1 1 = = 9.3F; use 10F 30mV - 5m * 4 * 1.8MHz - ESR * 4 * FS 1.5A
IRMS(MAX) =
IO = 0.75A 2
PESR = ESR * IRMS2 = 5m * (0.75A)2 = 3mW
Channels 1 and 2 Input Capacitors
Input Ripple VPP = 15mV
CIN1 = CIN2 =
VPP IO1
1 1 = = 7F; use 10F 15mV - 5m * 4 * 1.8MHz - ESR * 4 * FS 0.6A
IRMS(MAX) =
IO = 0.3A 2
PESR = ESR * IRMS2 = 5m * (0.3A)2 = 0.45mW
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19
PRODUCT DATASHEET
AAT2785
SystemPowerTM
AAT2785 Losses
3-Channel Step-Down DC/DC Converter
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. PLOSS = IO32 * RDS(ON)H3 +2 * (IO12 * RDS(ON)H1,2) = 1.5A2 * 120m +2 * (0.6A2 * 400m) = 0.558W TJ(MAX) = TAMB + JA * PLOSS = 85C + 50C * 0.558W = 113C. Inductance (H)
1.5 2.2 3.3 4.7 1.0 2.2 4.7 2.2
Manufacturer
Sumida Sumida Sumida Sumida Taiyo Yuden Taiyo Yuden Taiyo Yuden Taiyo Yuden
Part Number
CDRH2D11 CDRH2D11 CDRH2D11 CDRH2D11 CBC2518T CBC2518T CBC2518T CBC2016T
Max DC Current (A)
1.48 1.27 1.02 0.88 1.2 1.1 0.92 0.83
DCR ()
0.068 0.098 0.123 0.170 0.08 0.13 0.2 0.2
Size (mm) LxWxH
3.2x3.2x1.2 3.2x3.2x1.2 3.2x3.2x1.2 3.2x3.2x1.2 2.5x1.8x1.8 2.5x1.8x1.8 2.5x1.8x1.8 2.0x1.6x1.6
Type
Shielded Shielded Shielded Shielded Wire Wound Chip Wire Wound Chip Wire Wound Chip Wire Wound Chip
Table 5: Typical Surface Mount Inductors.
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Ordering Information
Voltage Package
TDFN34-16 Channel 1 0.6 Channel 2 0.6 Channel 3 0.6
3-Channel Step-Down DC/DC Converter
Marking1
2NXYY
Part Number (Tape and Reel)2
AAT2785IRN-AAA-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 Voltage
Adjustable (0.6V)
Code
A
1. XYY = assembly and date code. 2. Sample stock is generally held on all part numbers listed in BOLD.
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21
PRODUCT DATASHEET
AAT2785
SystemPowerTM
Package Information
3-Channel Step-Down DC/DC Converter
TDFN34-16
3.000 0.050 1.600 0.050 Detail "A" Index Area
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)
2785.2008.01.1.0

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