ANALOGICTECH AAT2785

PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
General Description
Features
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.
• 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 150μs Turn-On Time
• Over-Temperature Protection
• Current Limit Protection
• TDFN34-16 Package
• -40°C to 85°C Temperature Range
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 -40°C to +85°C operating
temperature range.
Applications
•
•
•
•
•
•
•
•
•
Typical Application
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
L1
4.7μH
VOUT1: 3.3V, 600mA
LX1
IN
R1
133k
AAT2785
FB1
VIN: 2.7V–5.5V
L2
4.7μH
VP1_2
EN1
LX2
EN2
R4
29.4k
LX3
EN3
FB3
C2
10μF
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GND
C4
4.7μF
PGND
L3
1.5μH
VP3
C3
4.7μF
R3
133k
FB2
C1
10μF
VOUT2: 3.3V, 600mA
R2
29.4k
PGND
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VOUT3: 1.2V, 1.5A
R5
59.0k
R6
59.0k
C5
10μF
1
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Pin Descriptions
Pin #
Symbol
1
PGND2
2
FB2
3
4
5
6
7
EN1
EN2
AGND
IN
EN3
8
FB3
9
10
11
PGND3
LX3
VP3
12
FB1
13
14
15
16
PGND1
LX1
VP1_2
LX2
EP
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
2
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
LX2
VP1_2
LX1
PGND1
FB1
VP3
LX3
PGND3
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Absolute Maximum Ratings1
Symbol
VIN, VP
VLX
VFB
VEN
TJ
TLEAD
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
Units
6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to 6.0
-40 to 150
300
V
V
V
V
°C
°C
Value
Units
2.0
50
W
°C/W
Thermal Information
Symbol
PD
θJA
Description
Maximum Power Dissipation2
Thermal Resistance3
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 25°C ambient temperature.
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3
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Electrical Characteristics1
VIN = VP = 3.6V; TA = -40°C to 85°C, unless noted otherwise. Typical values are at TA = 25°C.
Symbol
Description
VIN
Input Voltage
Conditions
IOUT3 = 0 to 1.5A; IOUT1,2 = 0 to 600mA;
VIN = 2.7 to 5.5V
VOUT
Output Voltage Tolerance
VOUT
IQ1,2
IQ3
ISHDN
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
ILX_LEAK
ILX_LEAK
IFB
ILIM1,2
ILIM3
RDS(ON)H1,2
RDS(ON)L1,2
RDS(ON)H3
RDS(ON)L3
ΔVLOADREG
ΔVLINEREG
FOSC1,2
FOSC3
TS
TSD
THYS
VIL
VIH
IEN
Min
Typ
Max
Units
2.7
5.5
V
-3.0
3.0
%
VIN
100
90
1.0
1.0
1.0
0.2
V
μA
μA
μA
μA
μA
μA
A
A
mΩ
mΩ
mΩ
mΩ
%
%
MHz
MHz
μs
°C
°C
V
V
μA
0.6
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
50
45
1.8
3.8
400
400
150
120
0.8
0.5
1.8
1.8
150
140
15
ILOAD1,2 = 0 to 600 mA; ILOAD3 = 0 to 1.5A
VIN = 2.7 to 5.5V
From Enable to Output Regulation
0.6
VIN = VEN = 5.5V
1.4
-1.0
1.0
1. The AAT2785 is guaranteed to meet performance specifications over the –40°C to +85°C operating temperature range, and is assured by design, characterization and correlation with statistical process controls.
4
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Efficiency vs. Output Current
Load Regulation
(Channel 1, 2; VOUT = 3.3V)
(Channel 1, 2; VOUT = 3.3V)
105
3
95
2
85
75
65
55
45
VIN = 3.6V
VIN = 4.2V
VIN = 5V
35
25
15
0.1
1
10
100
Output Error (%)
Efficiency (%)
Typical Characteristics
1
0
-1
-2
VIN = 5V
VIN = 4.2V
VIN = 3.6V
-3
-4
-5
1000
0.1
1
Efficiency vs. Output Current
Load Regulation
(Channel 3; VOUT = 1.2V)
(Channel 3; VOUT = 1.2V)
100
1
90
0.8
80
0.6
70
60
50
40
30
VIN = 4.2V
VIN = 3.6V
VIN = 2.7V
20
10
0
0.1
1
10
100
1000
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
10000
VIN = 4.2V
VIN = 3.6V
VIN = 2.7V
1
0.8
6
0.6
Output Error (%)
Switching Frequency (%)
1
8
4
2
0
-2
-4
Channel 3
Channel 1, 2
2.3
2.8
3.3
3.8
4.3
4.8
1000
10000
5.3
5.8
0.4
0.2
0
-0.2
-0.4
-0.6
Channel 3
Channel 1, 2
-0.8
-1
-40
Input Voltage (V)
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100
Output Error vs. Temperature
10
-8
10
Output Current (mA)
Switching Frequency vs. Input Voltage
-6
1000
0.4
Output Current (mA)
-10
100
Output Current (mA)
Output Error (%)
Efficiency (%)
Output Current (mA)
10
-15
10
35
60
85
Temperature (°C)
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5
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Quiescent Current vs. Input Voltage
Quiescent Current vs. Input Voltage
(Channel 1, 2; VOUT = 3.3V; No Load; Open Loop)
(Channel 3; VOUT = 1.2V; No Load; Open Loop)
100
100
90
90
Supply Current (µA)
Supply Current (µA)
Typical Characteristics
80
70
60
50
40
30
85°C
25°C
-40°C
20
10
0
3.2
3.5
3.8
4.1
4.4
4.7
5
5.3
80
70
60
50
40
30
85°C
25°C
-40°C
20
10
0
5.6
2.6
2.9
3.2
Input Voltage (V)
Switch On-Resistance (mΩ
Ω)
On-Resistance (mΩ
Ω)
100°C
85°C
25°C
900
800
700
600
500
400
4.4
4.8
5.2
5.6
150
.
100°C
85°C
25°C
50
0
2.6
3.1
1.2
1.1
1.1
1
1
VIL (V)
VIH (V)
4.1
4.6
5.1
5.6
VIL vs. Input Voltage
0.9
0.8
5.1
0.9
0.8
85°C
25°C
-40°C
0.7
85°C
25°C
-40°C
0.7
5.6
0.6
2.6
Input Voltage (V)
6
3.6
Input Voltage (V)
1.3
4.6
5.6
100
1.2
4.1
5.3
200
1.3
3.6
5
250
VIH vs. Input Voltage
3.1
4.7
300
Input Voltage (V)
0.6
2.6
4.4
(Channel 3; VOUT = 1.2V)
1000
4
4.1
P-Channel On-Resistance vs. Input Voltage
(Channel 1, 2; VOUT = 3.3V)
3.6
3.8
Input Voltage (V)
P-Channel On-Resistance vs. Input Voltage
300
3.2
3.5
3.1
3.6
4.1
4.6
5.1
5.6
Input Voltage (V)
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Typical Characteristics
Line Regulation
Line Regulation
(Channel 1, 2; VOUT = 3.3V)
(Channel 3; VOUT = 1.2V)
1
0.5
0.4
0.3
Accuracy (%)
Accuracy (%)
0.5
0
-0.5
IOUT = 10mA
IOUT = 150mA
IOUT = 300mA
IOUT = 600mA
-1
-1.5
3.5
4
4.5
5
5.5
0.1
0
-0.1
IOUT = 10mA
IOUT = 100mA
IOUT = 1000mA
IOUT = 1500mA
-0.2
-0.3
-0.4
-2
3
0.2
-0.5
2.6
6
4.1
4.6
5.1
5.6
Soft Start
Soft Start
(Channel 1, 2; VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
(Channel 3; VIN = 5V; VOUT = 1.2V; IOUT = 1mA)
3
1
0
1
0.5
0
3
2
1
0
1
0.5
Inductor Current
(bottom) (A)
2
4
Enable Voltage (top) (V)
Output Voltage (middle) (V)
4
0
Time (40µs/div)
Time (40µs/div)
Soft Start
Output Ripple
(Channel 3; VIN = 5V; VOUT = 1.2V; IOUT = 1.5A)
(Channel 1, 2; VOUT = 3.3V; VIN = 4.6V; IOUT = 1mA)
4
1
0
1.5
1
0.5
0
0
-0.01
0.1
0
Time (400ns/div)
Time (40µs/div)
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0.2
Inductor Current
(bottom) (A)
2
Output Voltage (top) (V)
0.01
3
Inductor Current
(bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
3.6
Input Voltage (V))
Inductor Current
(bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
Input Voltage (V))
3.1
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Typical Characteristics
Output Ripple
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 4.6V; IOUT = 600mA)
(Channel 3; VOUT = 1.2V; VIN = 4.6V; IOUT = 1.5A)
0.01
0.8
0.6
0.4
0.2
0
-0.01
2
1.5
1
0.5
0
0
Time (400ns/div)
Time (400ns/div)
Output Ripple
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 3.6V; IOUT = 600mA)
(Channel 3; VOUT = 1.2V; VIN = 3.6V; IOUT = 1.5A)
0.01
0.8
0.6
0.4
0.2
0
-0.01
2
1.5
1
0.5
0
0
Time (400ns/div)
Time (400ns/div)
Output Ripple
Output Ripple
(Channel 1, 2; VOUT = 3.3V; VIN = 5V; IOUT = 600mA)
(Channel 3; VOUT = 1.2V; VIN = 5V; IOUT = 1.5A)
0.01
0.8
0.6
0.4
0.2
0
-0.01
2
1.5
1
Inductor Current
(bottom) (A)
-0.01
Output Voltage (top) (V)
0.01
0
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
Inductor Current
(bottom) (A)
-0.01
Output Voltage (top) (V)
0
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.01
0.5
0
0
Time (400ns/div)
8
Inductor Current
(bottom) (A)
-0.01
Output Voltage (top) (V)
0
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
0.01
Time (400ns/div)
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Typical Characteristics
Output Ripple
Load Transient
(Channel 3; VOUT = 1.2V; VIN = 4.2V; IOUT = 1mA)
(Channel 1, 2; VIN = 3.6V; IOUT = 100mA to 600mA; VOUT = 3.3V)
0
-0.02
0.4
0.2
0
Output Voltage (top) (V)
0.2
0.02
Inductor Current
(bottom) (A)
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
0.04
0
-0.2
600mA
100mA
600mA
100mA
Time (100µs/div)
Time (200µs/div)
Load Transient
Load Transient
(Channel 1, 2; VIN = 3.6V; IOUT = 1mA to 600mA; VOUT = 3.3V)
(Channel 3; VIN = 5V; IOUT = 0.1A to 1.5A; VOUT = 1.2V)
600mA
1mA
600mA
1mA
0.1
Output Voltage (top) (V)
Output Voltage (top) (V)
-0.2
0
1.5A
-0.1
100mA
1.5A
100mA
Time (400µs/div)
Time (200µs/div)
Load Transient
Line Transient
(Channel 3; VIN = 5V; IOUT = 0.5A to 1.5A; VOUT = 1.2V)
(Channel 1, 2; VIN = 4V to 5V; IOUT = 600mA; VOUT = 3.3V)
1.5A
500mA
1.5A
500mA
Input Voltage (top) (V)
-0.1
6
5
4
Time (400µs/div)
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0.2
0.1
0
-0.1
-0.2
Output Voltage (bottom) (V)
0
Output Current (middle) (A)
Inductor Current (bottom) (A)
0.1
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
0
Output Current (middle) (A)
Inductor Current (bottom) (A)
0.2
Time (1ms/div)
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PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Typical Characteristics
Line Transient
(Channel 3; VIN = 3.6V to 4.2V; IOUT = 1.5A; VOUT = 1.2V)
Input Voltage (top) (V)
5
4
3
0.04
0.02
0
-0.02
-0.04
Output Voltage (bottom) (V)
6
Time (1ms/div)
10
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2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Functional Block Diagram
VP3
Comp.
FB3
Error
Amp
LX 3
Logic
Control
Logic
EN3
PGND3
OT
OSC
VP1_2
Comp.
FB2
Error
Amp
Logic
LX2
Control
Logic
EN2
PGND2
AGND
OSC
IN
Comp.
FB1
Error
Amp
Logic
Voltage
Ref
EN1
LX1
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%
2785.2008.01.1.0
of full load current. All channels have excellent transient
response, load and line regulation. Transient response
time is typically less than 20μs.
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 1μA
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 140°C with 15°C 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
3-Channel Step-Down DC/DC Converter
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.
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 1μA.
threshold is 140°C with 15°C 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/
μs. This equates to a slope compensation that is 75% of
the inductor current down slope for a 1.8V output and
2.2μH inductor.
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.
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
12
m=
L=
0.75 ⋅ VO 0.75 ⋅ 1.8V
A
=
= 0.6
L
2.2µH
µs
0.75 ⋅ VO
0.75 ⋅ 3.3V
=
= 4.1µH
m
A
0.6 µs
In this case a standard 4.7μH value is selected. Table 1
displays the suggested inductor values for channels 1
and 2. The 4.7μH CDRH2D11 series inductor selected
from Sumida has a 170mΩ 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/μs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 1.8μH
inductor.
m=
www.analogictech.com
0.75 ⋅ VO 0.75 ⋅ 1.8V
A
=
= 0.75
L
1.8µH
µs
2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
L=
3-Channel Step-Down DC/DC Converter
0.75 ⋅ VO
0.75 ⋅ 1.2V
=
= 1.2µH
m
A
0.75 µs
The inductor should be set equal to the output voltage
numeric value in micro henries (μH). 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 1.5μH LQH32PN1R5NN0L
series Murata inductor has a 68.4mΩ 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.
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF, 6.3V, X5R
ceramic capacitor with 5.0V DC applied is actually about
6μF. The maximum input capacitor RMS current is:
IRMS = IO ·
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 ⎠
IRMS(MAX) =
Configuration
0.6V adjustable
with external
resistive divider
Output
Voltage
0.6V2.0V
2.5V
3.3V
Inductor
Slope
Compensation
2.2μH
3.3μH
4.7μH
0.6A/μs
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) =
2785.2008.01.1.0
1
⎛ VPP
⎞
- ESR · 4 · FS
⎝ IO
⎠
0.52 =
1
2
⎛
V ⎞
· 1- O
IO
2
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
Select a 10μF to 22μF 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.
D · (1 - D) =
for VIN = 2 · VO
VO
Input Capacitor
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
www.analogictech.com
IN
13
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
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.
Output Capacitor: Channels 1 and 2
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7μF to
10μF 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:
COUT =
3 · ΔILOAD
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 4.7μF. 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.
Output Capacitor: Channel 3
The output capacitor limits the output ripple and provides holdup during large load transitions. A 10μF to
22μF 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
immunity, the minimum suggested value for R2 and R4
are 29.4 kΩ, and R6 is 59kΩ. 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.
VOUT (V)
R2 = R4 = 29.4kΩ
R1 = R3 (kΩ)
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
10
15
20
25
29
34
39
44
59
61
69
93
118
132
Table 2: AAT2785 Resistor Values for Various
Output Voltages of Channel 1 and Channel 2.
R6 = 59kΩ
R6 = 221kΩ
VOUT (V)
R5 (kΩ)
R5 (kΩ)
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
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
267
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
Table 3: AAT2785 Resistor Values for Various
Output Voltages of Channel 3.
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
14
www.analogictech.com
2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Thermal Calculations
Layout
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:
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.
PTOTAL =
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
VIN
1.
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 θJA for the TDFN34-16 package, which
is 50°C/W.
6.
7.
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.
TJ(MAX) = PTOTAL · ΘJA + TAMB
2785.2008.01.1.0
www.analogictech.com
15
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
LX 3
LX 2
LX 1
1
1
1
Evaluation Board Schematic
L2
Vi n
1
1
4.7μH
R7
0
EN1
EN2
EN3
1
2
3
1
2
3
1
2
3
R3
133k
AA T2785
C5
10μF
1
2
3
4
5
6
7
8
C4
10μF
C1
10μF
PGND2
FB2
EN1
EN2
GND
VIN
EN3
FB3
LX 2
VP1_2
LX 1
PGND1
FB1
VP3
LX3
PGND3
16
15
14
13
12
11
10
9
1
1.5uH
C9
opt
U1
R5
59K
C6
4.7μF
1
4.7μH
L3
VOUT2
R4
29.4k
L1
C8
10μF
PGND
C7
opt
C3
opt
R1
133K
C2
4.7μF
VOUT3
C10
10μF
VOUT1
R2
29.4K
R6
59K
1
1
PGND
Figure 1: AAT2785 Evaluation Board Schematic.
Evaluation Board Layout
Figure 2: AAT2785 Evaluation Board
Component Side Layout.
16
Figure 3: AAT2785 Evaluation Board
Solder Side Layout.
www.analogictech.com
2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Component
Part Number
Manufacturer
U1
L1, L2
AAT2785
CDRX2D11
AATI
Sumida
L3
LQH32PN1R5NN0L
Murata
C1
C2, C6
C5, C8, C10
C9
R1, R3
R2, R4
R5, R6
R7
GMR219R61A475KE19
GMR21BR60J106KE19
Generic
Murata
Murata
Generic
Generic
Generic
Generic
Generic
Description
3-Channel Step-Down DC/DC Converter
4.7μH 0.88A 170mΩ (3.2x3.2x1.2)mm Shielded
1.5μH series Murata inductor has a 68.4mΩ worst case DCR and a
1.75A DC
Optional
4.7μF 10V 0805
10μF 6.3V 0805
56pF 6.3V 0402
133KΩ 0402
29.4KΩ 0402
59KΩ 0402
Optional
Table 4: AAT2785 Evaluation Board Bill of Materials.
2785.2008.01.1.0
www.analogictech.com
17
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Design Example
Specifications
VO3
VO1
VO2
VIN
FS
TAMB
1.2V @
3.3V @
3.3V @
2.7V to
1.8MHz
85°C
1.5A (adjustable using 0.6V version), pulsed load ΔILOAD = 1.5A
600mA (adjustable using 0.6V version), pulsed load ΔILOAD = 600mA
600mA (adjustable using 0.6V version), pulsed load ΔILOAD = 600mA
4.2V (3.6V nominal)
Channel 3 Output Inductor
L=
0.75 ⋅ VO
0.75 ⋅ 1.2V
=
= 1.2µH ; use 1.5μH. (see Table 4).
m
A
0.75 µs
Select Murata LQH32PN1R5NN0L 1.5μH 1.75A DC current rating DCR = 68mΩ.
ΔI3 =
⎛
VO3 ⎛
V ⎞
1.5V
1.5V ⎞
1 - O3 =
⋅ 1= 357mA
L⋅F ⎝
VIN ⎠ 1.5µH ⋅ 1.8MHz ⎝
4.2V ⎠
IPK3 = 1.5A + 0.357A = 1.9A
PL3 = IO32 ⋅ DCR = 1.5A2 ⋅ 68mΩ = 153mW
Channels 1 and 2 Output Inductors
L1 = L2 =
0.75 ⋅ VO
0.75 ⋅ 3.3V
=
= 4.1µH; use 4.7μH. (see Table 4)
m
A
0.6 µs
Select Sumida CDRH2D11 4.7μH 0.88A DC current rating DCR = 170mΩ.
ΔI1 = ΔI2 =
⎛
VO1 ⎛
V ⎞
3.3V
3.3V ⎞
1 - O1 =
⋅ 1= 84mA
L⋅F ⎝
VIN ⎠ 4.7µH ⋅ 1.8MHz ⎝
4.2V ⎠
IPK1 = IPK2 = 0.6A + 0.084A = 0.7A
PL1 = PL2 = IO12 ⋅ DCR = 0.62 ⋅ 170mΩ = 61.2mW
18
www.analogictech.com
2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Channel 3 Output Capacitor
COUT3 =
3 · ΔILOAD1
3 · 1.5A
=
= 12.5µF; use 10 to 22µF
0.2V · 1.8MHz
VDROOP · FS
IRMS(MAX) =
VOUT · (VIN(MAX) - VOUT)
1
1.2V · (4.2V - 1.2V)
·
= 92mA
=
L · FS · VIN(MAX)
2 · 3 1.5µH · 1.8MHz · 4.2V
2· 3
1
·
PESR = ESR · IRMS2 = 5mΩ · 92mA2 = 0.04mW
Channels 1 and 2 Output Capacitors
COUT1 = COUT2 =
IRMS(MAX) =
3 · ΔILOAD1
3 · 0.6A
=
= 5µF; use 5.6µF
VDROOP · FS
0.2V · 1.8MHz
VOUT1 · (VIN(MAX) - VOUT1)
1
3.3V · (4.2V - 3.3V)
·
= 24mA
=
L · FS · VIN(MAX)
2 · 3 4.7µH · 1.8MHz · 4.2V
2· 3
1
·
PESR = ESR · IRMS2 = 5mΩ · 28.9mA2 = 2.9µW
Channel 3 Input Capacitor
Input Ripple VPP = 30mV
CIN3 =
⎛ VPP
⎝ IO3
IRMS(MAX) =
1
1
=
= 9.3µF; use 10µF
⎞
⎛ 30mV
⎞
- 5mΩ · 4 · 1.8MHz
- ESR · 4 · FS
⎠
⎝ 1.5A
⎠
IO
= 0.75A
2
PESR = ESR · IRMS2 = 5mΩ · (0.75A)2 = 3mW
Channels 1 and 2 Input Capacitors
Input Ripple VPP = 15mV
CIN1 = CIN2 =
IRMS(MAX) =
⎛ VPP
⎝ IO1
1
1
=
= 7µF; use 10µF
⎞
⎛ 15mV
⎞
- 5mΩ · 4 · 1.8MHz
- ESR · 4 · FS
⎠
⎝ 0.6A
⎠
IO
= 0.3A
2
PESR = ESR · IRMS2 = 5mΩ · (0.3A)2 = 0.45mW
2785.2008.01.1.0
www.analogictech.com
19
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
AAT2785 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 85°C ambient temperature and a 120°C junction temperature with the TDFN
50°C/W package.
PLOSS = IO32 · RDS(ON)H3 +2 · (IO12 · RDS(ON)H1,2) = 1.5A2 · 120mΩ +2 · (0.6A2 · 400mΩ) = 0.558W
TJ(MAX) = TAMB + θJA · PLOSS = 85°C + 50°C · 0.558W = 113°C.
Manufacturer
Part Number
Inductance
(μH)
Max DC
Current (A)
DCR
(Ω)
Size (mm)
LxWxH
Type
Sumida
Sumida
Sumida
Sumida
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
Taiyo Yuden
CDRH2D11
CDRH2D11
CDRH2D11
CDRH2D11
CBC2518T
CBC2518T
CBC2518T
CBC2016T
1.5
2.2
3.3
4.7
1.0
2.2
4.7
2.2
1.48
1.27
1.02
0.88
1.2
1.1
0.92
0.83
0.068
0.098
0.123
0.170
0.08
0.13
0.2
0.2
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
Shielded
Shielded
Shielded
Shielded
Wire Wound Chip
Wire Wound Chip
Wire Wound Chip
Wire Wound Chip
Table 5: Typical Surface Mount Inductors.
20
www.analogictech.com
2785.2008.01.1.0
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Ordering Information
Voltage
Package
Channel 1
Channel 2
Channel 3
Marking1
Part Number (Tape and Reel)2
TDFN34-16
0.6
0.6
0.6
2NXYY
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
Code
Adjustable
(0.6V)
A
1. XYY = assembly and date code.
2. Sample stock is generally held on all part numbers listed in BOLD.
2785.2008.01.1.0
www.analogictech.com
21
PRODUCT DATASHEET
AAT2785
SystemPowerTM
3-Channel Step-Down DC/DC Converter
Package Information
TDFN34-16
3.000 ± 0.050
1.600 ± 0.050
Detail "A"
3.300 ± 0.050
4.000 ± 0.050
Index Area
0.350 ± 0.100
Top View
0.230 ± 0.050
Bottom View
C0.3
(4x)
0.050 ± 0.050
0.450 ± 0.050
0.850 MAX
Pin 1 Indicator
(optional)
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.
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Phone (408) 737-4600
Fax (408) 737-4611
© 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
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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
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22
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2785.2008.01.1.0