AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 1
www.analogictech.com
General Description
The AAT2786 is a dual output power solution. It includes
a 1.5A, 1.4MHz, high-efficiency step-down converter and
a high performance 150mA LDO regulator. The step-
down regulator output voltage is adjustable from 0.6V to
VIN. The LDO regulator has a factory preset fixed output
voltage from 1.2V to 3.3V.
The step-down converter consumes only 42µA of no-load
quiescent current and is designed to maintain high effi-
ciency throughout the load range. The step-down con-
verter has ultra-low RDS(ON) integrated MOSFETs and can
operate up to 100% duty cycle to enable high output
voltage, high current applications which require a low
dropout threshold. The AAT2786 provides excellent tran-
sient response and high output accuracy across the
operating range. Pulling the MODE/SYNC pin high
enables “PWM Only” mode, maintaining constant fre-
quency and low output ripple across the operating range.
Alternatively, the converter may be synchronized to an
external clock input via the MODE/SYNC pin.
The MicroPower low dropout linear regulator in the
AAT2786 has been specifically designed for high-speed
turn-on and turn-off performance, fast transient response,
and good noise and power supply ripple rejection (PSRR),
making it ideal for powering sensitive circuits with fast
switching requirements.
Over-temperature and short-circuit protection safeguard
the AAT2786 and system components from damage.
Features
• VIN Range : 2.5V to 5.5V
• 1.5A Step-Down Converter
▪ V
OUT Range: 0.6V to VIN
▪ 95% Peak Efficiency
▪ High Efficiency across load range
▪ 42µA No Load Quiescent Current
▪ Optional “PWM Only” Low Noise Mode
▪ Current limit and soft start
• 150mA LDO Regulator
▪ V
OUT Range: 1.2V to 3.3V (Fixed)
▪ High Power Supply Rejection Ratio
▪ Low Output Noise
• Independent Enable Pins
• Integrated Power MOSFETs
• Over-Temperature Protection
• TDFN34-16 Package
• -40°C to +85°C Temperature Range
Applications
• Cellular and Smart Phones
• PDAs, Palmtops
• Digital Still and Video Cameras
• Portable Instruments
• Battery-Powered Applications
Typical Application
R2
Adj
R3
59k
C8
10nF
C6
1µF
3.3µH
L1
C1
opt
C7
2.2µF
BUCK OUT
LDO OUT
LDO IN
BUCK IN
C5
Opt C4
22µF
C3
1µF
R1
100
C2
10µF VIN
12
MODE/SYNC
14
VP
16
VP
15
LDOIN
11
BUCK_EN
13
LDO_EN
9
N/C
10
LDO_OUT 6
BYP
8
LX 2
LX 1
GND 4
PGND 3
LDO_GND 7
FB 5
AAT2786IRN
U1
ON/OFF
ON/OFF
ON/OFF
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2 2786.2008.04.1.0
www.analogictech.com
Pin Descriptions
Pin # Symbol Description
1, 2 LX Step-down converter switching node. Connect the output inductor to this pin. The switching node is
internally connected to the drain of both high- and low-side MOSFETs.
3 PGND Step-down converter main power ground return pin. Connect to the output and input capacitor return.
4 GND Non-power signal ground pin.
5FB
Step-down converter feedback input pin. This pin is connected either directly to the converter output or
to an external resistive divider for an adjustable output.
6 LDOOUT LDO output pin; should be decoupled with 2.2µF ceramic capacitor.
7 LDO_GND LDO ground connection pin.
8 BYP LDO bypass capacitor connection; to improve AC ripple rejection, connect a 10nF capacitor to GND. This
will also provide a soft start function.
9 LDO_EN LDO enable pin; this pin should not be left fl oating. When pulled low, the LDO PMOS pass transistor
turns off and all internal circuitry enters low-power mode, consuming less than 1µA.
10 N/C Open
11 LDOIN LDO input voltage pin; should be decoupled with 1µF or greater capacitor.
12 VIN Step-down converter power supply. Supplies power for the internal circuitry.
13 BUCK_EN Step-down converter enable pin. A logic low disables the step-down converter and it consumes less than
1µA of current. When connected high, it resumes normal operation.
14 MODE/SYNC
Connect to ground for Light-Load/PWM mode and optimized effi ciency throughout the load range. Con-
nect high for low noise PWM operation under all operating conditions. Connect to an external clock for
synchronization (PWM only).
15,16 VP Step-down converter input voltage for the power switches.
Pin Configuration
TDFN34-16
(Top View)
PGND
GND
FB
LX
LX
3
LDOOUT
LDO_GND
BYP
MODE/SYNC
BUCK_EN
VIN
VP
VP
LDOIN
N/C
LDO_EN
4
5
1
2
6
7
8
14
13
12
16
15
11
10
9
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 3
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 3
www.analogictech.com
Absolute Maximum Ratings
TA = 25°C unless otherwise noted.
Symbol Description Value Units
VIN VIN, VP to GND 6.0
V
VLX LX Pin to GND -0.3 to VIN + 0.3
VFB FB Pin to GND -0.3 to VIN + 0.3
VNMODE/SYNC, BUCK_EN to GND -0.3 to 6.0
VLDOIN VLDOIN to LDO_GND 6.0
VENIN(MAX) LDO_EN to LDO_GND -0.3 to VIN + 0.3
TJMaximum Junction Operating Temperature -40 to +150 °C
TLEAD Maximum Soldering Temperature (at leads, 10 sec) 300
Thermal Information
Symbol Description Value Units
PDMaximum Power Dissipation12.0 W
θJA Thermal Resistance 50 °C/W
VLDOIN LDO Input Voltage (VLDOUT + VDO) to 5.5 V
1. Derate 20mW/ºC above 25°C ambient temperature. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional opera-
tion at conditions other than the operating conditions specified is not implied.
2. Mounted on an FR4 board.
3. To calculate minimum input voltage, use the following equation: VIN(MIN) = VOUT(MAX) + VDO(MAX) as long as VIN ≥ 2.5V.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
4 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
4 2786.2008.04.1.0
www.analogictech.com
Electrical Characteristics1
VIN=3.6V; TA = -40oC to 85oC unless otherwise noted. Typical values are at TA = 25oC.
Symbol Description Conditions Min Typ Max Units
Step-Down Converter
VIN Input Voltage 2.5 5.5 V
VOUT Output Voltage Range 0.6 VIN V
VUVLO UVLO Threshold
VIN Rising 2.5 V
Hysteresis 150 mV
VIN Falling 1.7 V
VOUT Output Voltage Tolerance IOUT = 0A to 1.5A, VIN = 2.4V to 5.5V -3.0 3.0 %
IQ Quiescent Current No Load 42 90 µA
ISHDN Shutdown Current VEN = GND 1.0 µA
ILIM Current Limit 1.8 A
RDS(ON)H High Side Switch On-Resistance 0.120 Ω
RDS(ON)L Low Side Switch On-Resistance 0.085 Ω
ILXLEAK LX Leakage Current VIN = 5.5V, VLX = 0 to VIN 1.0 µA
ΔVLOADREG Load Regulation ILOAD = 0A to 1.5A 0.5 %
ΔVLINEREG/
ΔVIN Line Regulation VIN = 2.4V to 5.5V 0.2 %/V
VFB Feedback Threshold Voltage Accuracy
(Adjustable Version) No Load, TA = 25°C 0.591 0.60 0.609 V
IFB FB Leakage Current VOUT = 1.0V 0.2 µA
FOSC Internal Oscillator Frequency TA = 25°C 1.12 1.4 1.68 MHz
Synchronous Clock 0.60 3.0
TS Start-Up Time From Enable to Output Regulation 150 µs
TSD Over-Temperature Shutdown Threshold 140 °C
THYS Over-Temperature Shutdown Hysteresis 15 °C
MODE/SYNC
VMODE/SYNC(L) Enable Threshold Low 0.6 V
VMODE/SYNC(H) Enable Threshold High 1.4 V
IMODE/SYNC Enable Leakage Current VIN = VEN = 5.5V 1.0 µA
1. The AAT2786 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correla-
tion with statistical process controls.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 5
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 5
www.analogictech.com
Electrical Characteristics (continued)
VLDOIN = VOUT(NOM) + 1V for VOUT options greater than 1.5V. VIN = 2.5 for VOUT ≤1.5V. IOUT = 1mA, COUT = 2.2µF, CIN = 1µF,
TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C.
Symbol Description Conditions Min Typ Max Units
LDO Regulator
VLDOOUT Output Voltage Tolerance ILDOOUT = 1mA
to 150mA
TA = 25°C -1.5 1.5 %
TA = -40°C to 85°C -2.5 2.5
ILDOOUT Output Current VLDOOUT > 1.2V 150 mA
VDO Dropout Voltage2, 3 ILDOOUT = 150mA 200 300 mV
ISC Short-Circuit Current VLDOOUT < 0.4V 600 mA
IQ Ground Current VIN = 5V, No Load, EN = VIN 70 125 µA
ISD Shutdown Current VIN = 5V, EN = 0V 1 µA
ΔVOUT/
VOUT*ΔVIN Line Regulation VIN = VOUT + 1 to 5.0V 0.09 %/V
ΔVOUT(line) Dynamic Line Regulation VIN = VLDOOUT + 1V to VLDOOUT + 2V,
ILDOOUT = 150mA, TR/TF = 2µs 2.5 mV
ΔVOUT(load) Dynamic Load Regulation ILDOOUT = 1mA to 150mA, TR < 5µs 30 mV
PSRR Power Supply Rejection Ratio ILDOOUT = 10mA,
CBYP = 10nF
1 kHz 67
dB10kHz 47
1MHz 45
eN Output Noise Noise Power BW = 300Hz - 50kHz 50 µVrms
TSD Over-Temperature Shutdown Threshold 145 °C
THYS Over-Temperature Shutdown Hysteresis 12 °C
TC Output Voltage Temperature Coeffi cient 22 ppm/°C
Enable
VIL Enable Threshold Low 0.6 V
VIH Enable Threshold High 1.4 V
IEN_BUCK Enable Leakage Current VEN_BUCK = 5V 1 µA
IEN_LDO Enable Leakage Current VEN_LDO = 5V 1.0 µA
tENDLY Enable Delay Time BYP = Open 15 µs
1. VDO is defined as VLDOIN - VLDOOUT when VLDOOUT is 98% of nominal.
2. For VLDOOUT < 2.3V, VDO = 2.5V - VLDOOUT
.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
6 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
6 2786.2008.04.1.0
www.analogictech.com
Typical Characteristics–Step-Down Converter
Efficiency vs. Output Current
(Light-Load Mode; VOUT = 3.3V)
Output Current (mA)
Efficiency (%)
40
50
60
70
80
90
100
0.1 1 10 100 1000 10000
VIN = 4.2V
VIN = 3.6V
VIN = 5.0V
Load Regulation
(Light-Load Mode; VOUT = 3.3V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 3.6V VIN = 4.2V
VIN = 5.0V
Efficiency vs. Output Current
(PWM Mode; VOUT = 3.3V)
Output Current (mA)
Efficiency (%)
0
20
40
60
80
100
1.0 10 100 1000 10000
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
Load Regulation
(PWM Mode; VOUT = 3.3V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 3.6V VIN = 5.0V
VIN = 4.2V
Efficiency vs. Output Current
(Light-Load Mode; VOUT = 2.5V)
Output Current (mA)
Efficiency (%)
50
60
70
80
90
100
0.1 1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V VIN = 4.2V
Load Regulation
(Light-Load Mode; VOUT = 2.5V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 2.7V VIN = 3.6V
VIN = 4.2V
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 7
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 7
www.analogictech.com
Typical Characteristics–Step-Down Converter
Output Current (mA)
Efficiency (%)
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
Efficiency vs. Output Current
(PWM Mode; VOUT = 2.5V)
Output Current (mA)
Efficiency (%)
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VIN = 5.0V
Efficiency vs. Output Current
(PWM Mode; VOUT = 2.5V)
Efficiency vs. Output Current
(Light-Load Mode; VOUT = 1.8V)
Output Current (mA)
Efficiency (%)
40
50
60
70
80
90
100
0.1 1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V VIN = 4.2V
Load Regulation
(Light-Load Mode; VOUT = 1.8V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000
VIN = 3.6V VIN = 2.7V
VIN = 4.2V
Efficiency vs. Output Current
(PWM Mode; VOUT = 1.8V)
Output Current (mA)
Efficiency (%)
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
Load Regulation
(PWM Mode; VOUT = 1.8V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 2.7V VIN = 3.6V
VIN = 4.2V
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
8 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
8 2786.2008.04.1.0
www.analogictech.com
Typical Characteristics–Step-Down Converter
Efficiency vs. Output Current
(Light-Load Mode; VOUT = 1.2V)
Output Current (mA)
Efficiency (%)
30
40
50
60
70
80
90
100
0.1 1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
Load Regulation
(Light-Load Mode; VOUT = 1.2V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 2.7V VIN = 3.6V
VIN = 4.2V
Efficiency vs. Output Current
(PWM Mode; VOUT = 1.2V)
Output Current (mA)
Efficiency (%)
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
Load Regulation
(PWM Mode; VOUT = 1.2V)
Output Current (mA)
VOUT Error (%)
-0.50
-0.25
0.00
0.25
0.50
0.1 1 10 100 1000 10000
VIN = 2.7V VIN = 3.6V
VIN = 4.2V
Output Voltage vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A)
Temperature (°
°
C)
Output Voltage Change (%)
-1.0
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1.0
-40 -20 0 20 40 60 80
Supply Current vs. Supply Voltage
(VOUT = 1.8V; No Load; Light-Load Mode)
Supply Voltage (V)
Supply Current (µA)
30
35
40
45
50
55
60
65
70
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
85°C 25°C
-40°C
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 9
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 9
www.analogictech.com
Typical Characteristics–Step-Down Converter
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A)
Temperature (°
°
C)
Switching Frequency (MHz)
1.24
1.26
1.28
1.30
1.32
1.34
1.36
1.38
1.40
-40 -20 0 20 40 60 80
Line Regulation
(VOUT = 1.8V; IOUT = 1A)
Supply Voltage (V)
Output Voltage Error (%)
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
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
Switching Frequency vs. Input Voltage
(IOUT = 1A)
Input Voltage (V)
Switching Frequency (MHz)
1.32
1.33
1.34
1.35
1.36
1.37
1.38
1.39
1.40
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
VOUT = 2.5V
VOUT = 3.3V
VOUT = 1.8V
Enable Soft Start
(VOUT = 3.6V; IOUT = 1.5A)
Time (100µs/div)
VOUT
(1V/div)
EN
(2V/div)
IIN
(500mA/div)
P-Channel RDS(ON) vs. Input Voltage
Input Voltage (V)
RDS(ON) (mΩ
Ω
)
90
100
110
120
130
140
150
160
170
180
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
120°C
85°C
25°C
N-Channel RDS(ON) vs. Input Voltage
Input Voltage (V)
RDS(ON) (mΩ
Ω
)
60
70
80
90
100
110
120
130
140
150
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
120°C
85°C
25°C
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
10 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
10 2786.2008.04.1.0
www.analogictech.com
Typical Characteristics–Step-Down Converter
Heavy Load Switching Waveform
(PWM Mode; VIN = 3.6V; VOUT = 1.8V; 1.5A Load)
Time (2.5µs/div)
Output Voltage (AC coupled)
(top) (mV)
Inductor Ripple Current
(bottom) (mA)
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Light Load Switching Waveform
(PWM Mode; VIN = 3.6V; VOUT = 1.8V; 1mA Load)
Time (2.5µs/div)
Output Voltage (AC coupled)
(top) (mV)
Inductor Ripple Current
(bottom) (mA)
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
-400
-200
0
200
400
600
800
1000
1200
Light Load Switching Waveform
(Light-Load Mode; VIN = 3.6V; VOUT = 1.8V; 1mA Load)
Time (100µs/div)
Output Voltage (AC coupled)
(top) (mV)
Inductor Ripple Current
(bottom) (mA)
-24.0
-20.0
-16.0
-12.0
-8.0
-4.0
0.0
4.0
8.0
-100
0
100
200
300
400
500
600
700
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; CFF = 100pF)
Time (50µs/div)
Output Voltage
(top) (V)
Load Current
(bottom) (A)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; No CFF)
Time (50µs/div)
Output Voltage
(top) (V)
Load Current
(bottom) (A)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Line Transient Response
(VOUT = 1.8V; 1.5A Load)
Time (200µs/div)
Input Voltage
(top) (V)
Output Voltage
(bottom) (V)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 11
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 11
www.analogictech.com
Typical Characteristics–LDO
Dropout Voltage vs. Temperature
0
20
40
60
80
100
120
140
160
180
200
220
240
260
-40-30-20-100 102030405060708090100110120
Temperature (°C)
Dropout Voltage (mV)
IL = 150mA
IL = 100mA
IL = 50mA
Dropout Characteristics
2.00
2.20
2.40
2.60
2.80
3.00
3.20
2.70 2.80 2.90 3.00 3.10 3.20
Input Voltage (V)
Output Voltage (V)
IOUT = 150mA
IOUT = 100mA
IOUT = 50mA
IOUT = 10mA
IOUT = 0mA
Dropout Voltage vs. Output Current
0
50
100
150
200
250
300
0 25 50 75 100 125 150
Output Current (mA)
Dropout Voltage (mV)
85°C
25°C
-40°C
Ground Current vs. Input Voltage
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
2 2.5 3 3.5 4 4.5
Input Voltage (V)
Ground Current (µA)
IOUT = 0mA
IOUT = 10mA
IOUT = 50mA
IOUT = 150mA
Quiescent Current vs. Temperature
0
10
20
30
40
50
60
70
80
90
100
-40-30-20-10 0 10203040506070 8090100110120
Temperature (°C)
Quiescent Current (µA)
Output Voltage vs. Temperature
1.196
1.197
1.198
1.199
1.200
1.201
1.202
1.203
-40-30-20-10 0 1020 30 40 5060 7080 90100
Temperature (°C)
Output Voltage (V)
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
12 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
12 2786.2008.04.1.0
www.analogictech.com
Typical Characteristics–LDO
Initial Power-Up Response Time
(CBYP = 10nF)
Time (400µs/div)
VEN (5V/div)
VOUT (1V/div)
Time (50µs/div)
VEN (5V/div)
VOUT (1V/div)
Turn-Off Response Time
(CBYP = 10nF)
Turn-On Time From Enable (VIN present)
(CBYP = 10nF)
Time (5µs/div)
VEN (5V/div)
VOUT (1V/div)
Over-Current Protection
-200
0
200
400
600
800
1000
1200
Time (20ms/div)
Output Current (mA)
Line Transient Response
2.98
2.99
3.00
3.01
3.02
3.03
3.04
Time (100µs/div)
Input Voltage (V)
0
1
2
3
4
5
6
Output Voltage (V)
VOUT
VIN
Load Transient Response
2.60
2.65
2.70
2.75
2.80
2.85
2.90
Time (100µs/div)
Output Voltage (V)
-100
0
100
200
300
400
500
Output Current (mA)
VOUT
IOUT
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 13
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 13
www.analogictech.com
Typical Characteristics–LDO
AAT2786 Self Noise
(COUT = 10µF, ceramic)
0.001
0.01
0.1
1
10
0.01 0.1 1 10 100 1000 10000
Frequency (kHz)
Noise Amplitude (µV/rtHz)
Band Power:
300Hz to 50kHz = 44.6µVrms/rtHz
100Hz to 100kHz = 56.3µVrms/rtHz
VIH and VIL vs. VIN
1.050
1.075
1.100
1.125
1.150
1.175
1.200
1.225
1.250
2.5 3.0 3.5 4.0 4.5 5.0 5.5
Input Voltage (V)
VIH
VIL
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
14 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
14 2786.2008.04.1.0
www.analogictech.com
Functional Description
The AAT2786 is a dual output power solution. It includes
a 1.5A, 1.4MHz, high-efficiency step-down converter and
a high performance 150mA LDO regulator. The step-
down regulator output voltage is adjustable from 0.6V to
VIN. The LDO regulator has a factory preset fixed output
voltage from 1.2V to 3.3V. The on and off states of the
step-down converter and the LDO regulator are indepen-
dently controlled by separate enable pins.
Step-Down Converter
The step-down converter in the AAT2786 is a high perfor-
mance 1.5A monolithic step-down converter operating at
1.4MHz switching frequency. It minimizes external compo-
nent size and optimizes efficiency over the complete load
range. Apart from the small bypass input capacitor, only a
small L-C filter is required at the output. Typically, a 3.3µH
inductor and a 22µF ceramic capacitor are recommended
for a 3.3V output (see table of recommended values).
Functional Block Diagram
Bias
Oscillator
Control Circuit
RLDOFB1
RLDOFB2
VP
LX
LDO_GND
LDOOUT
VIN
LDOIN
MODE/SYNC
FB
PGND
BUCK_EN
LDO_EN
GND
BYP
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 15
www.analogictech.com
At dropout, the step-down converter duty cycle increases
to 100% and the output voltage tracks the input voltage
minus the RDS(ON) drop of the P-channel high side MOSFET
(plus the DC drop of the external inductor). The device
integrates extremely low RDS(ON) MOSFETs to achieve low
dropout voltage during 100% duty cycle operation. This is
advantageous in applications requiring high output volt-
ages (typically > 2.5V) at low input voltages.
The integrated low-loss MOSFET switches can provide
greater than 95% efficiency at full load. Light-Load
operation maintains high efficiency under light load con-
ditions (typically <150mA). The MODE/SYNC pin allows
optional “PWM only” mode. This maintains constant fre-
quency and low output ripple across all load conditions.
Alternatively, the IC can be synchronized to an external
clock via the MODE/ SYNC input. External synchroniza-
tion is maintained between 0.6MHz and 3.0MHz.
In battery-powered applications, as VIN decreases, the
converter dynamically adjusts the operating frequency
prior to dropout to maintain the required duty cycle and
provide accurate output regulation. Output regulation is
maintained until the dropout voltage, or minimum input
voltage, is reached. At 1.5A output load, dropout voltage
headroom is approximately 200mV.
The step-down converter in the AAT2786 typically
achieves better than ±0.5% output regulation across the
input voltage and output load range. A current limit of
2.0A (typical) protects the IC and system components
from short-circuit damage. Typical no load quiescent cur-
rent is 42µA.
Thermal protection completely disables switching when
the maximum junction temperature is detected. The
junction over-temperature threshold is 140°C with 15°C
of hysteresis. Once an over-temperature or over-current
fault condition is removed, the output voltage automati-
cally recovers.
Peak current mode control and optimized internal com-
pensation provide high loop bandwidth and excellent
response to input voltage and fast load transient events.
Soft start eliminates output voltage overshoot when the
step-down converter is enabled. Under-voltage lockout
prevents spurious start-up events.
Control Loop
The step-down converter in the AAT2786 is a peak cur-
rent mode step-down converter. The current through the
P-channel MOSFET (high side) is sensed for current loop
control, as well as short-circuit 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 con-
verter output voltage greater than or equal to 0.6V.
Soft Start/Enable
Soft start limits the current surge seen at VIN and elim-
inates output voltage overshoot. When EN_BUCK input is
pulled low the step-down converter is forced into a low-
power, non-switching state. The total input current dur-
ing shutdown is less than 1µA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current in the
step-down converter is limited. To minimize power dis-
sipation and stresses under current limit and short-cir-
cuit conditions, switching is terminated after entering
current limit for a series of pulses. Switching is termi-
nated 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 threshold is 140°C with
15°C of hysteresis. Once an over-temperature or over-
current fault conditions 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) in the step-down con-
verter guarantees sufficient VIN bias and proper opera-
tion of all internal circuitry prior to activation.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
16 2786.2008.04.1.0
www.analogictech.com
LDO Functional Description
The LDO regulator in the AAT2786 is intended for applica-
tions with output current load requirements from no load
to 150mA. The advanced circuit design of the AAT2786
LDO regulator has been specifically optimized for very
fast start-up and shutdown timing. This proprietary
CMOS LDO has also been tailored for superior transient
response characteristics. These traits are particularly
important for applications that require fast power supply
timing, such as GSM cellular telephone handsets.
The high-speed turn-on capability of the LDO regulator
is enabled through the implementation of a fast start
control circuit, which accelerates the turn-on behavior of
fundamental control and feedback circuits. Fast turn-off
response time is achieved by an active output pull-down
circuit, which is enabled when the LDO regulator is
placed in shutdown mode. This active fast shutdown cir-
cuit has no adverse effect on normal device operation.
The AAT2786 LDO regulator has very fast transient
response characteristics, which is an important feature for
applications in which fast line and load transient response
are required. This rapid transient response behavior is
accomplished through the implementation of an active
error amplifier feedback control. This proprietary circuit
design is unique to this MicroPower LDO regulator.
The LDO regulator output has been specifically optimized
to function with low-cost, low-ESR ceramic capacitors.
However, the design will allow for operation over a wide
range of capacitor types.
A bypass pin has been provided to allow the addition of
an optional voltage reference bypass capacitor to reduce
output self noise and increase power supply ripple rejec-
tion. Device self noise and PSRR will be improved by the
addition of a small ceramic capacitor to this pin. However,
increased CBYPASS values may slow down the LDO
regulator turn-on time.
Enable Function
The AAT2786 features an LDO regulator enable/ disable
function. This pin (EN) is active high and is compatible
with CMOS logic. To assure the LDO regulator will switch
on, the EN turn-on control level must be greater than
1.5V. The LDO regulator will go into the disable shut-
down mode when the voltage on the EN pin falls below
0.6V. If the enable function is not needed in a specific
application, it may be tied to VIN to keep the LDO regula-
tor in a continuously on state.
When the LDO regulator is in shutdown mode, an inter-
nal 1.5kΩ resistor is connected between VOUT and GND.
This is intended to discharge COUT when the LDO regula-
tor is disabled. The internal 1.5kΩ has no adverse effect
on device turn-on time.
Short-Circuit Protection
The AAT2786 contains an internal short-circuit protection
circuit that will trigger when the output load current
exceeds the internal threshold limit. Under short-circuit
conditions, the output of the LDO regulator will be cur-
rent limited until the short-circuit condition is removed
from the output or LDO regulator package power dissi-
pation exceeds the device thermal limit.
Thermal Protection
The AAT2786 has an internal thermal protection circuit
which will turn on when the device die temperature
exceeds 150°C. The internal thermal protection circuit
will actively turn off the LDO regulator output pass
device to prevent the possibility of over temperature
damage. The LDO regulator output will remain in a shut-
down state until the internal die temperature falls back
below the 150°C trip point.
The combination and interaction between the short cir-
cuit and thermal protection systems allows the LDO
regulator to withstand indefinite short-circuit conditions
without sustaining permanent damage.
No-Load Stability
The AAT2786 is designed to maintain output voltage
regulation and stability under operational no load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero.
Reverse Output-to-Input
Voltage Conditions and Protection
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO regula-
tor. The input voltage should always remain greater than
the output load voltage, maintaining a reverse bias on
the internal parasitic diode. Conditions where VOUT
might exceed VIN should be avoided since this would
forward bias the internal parasitic diode and allow exces-
sive current flow into the VOUT pin, possibly damaging
the LDO regulator.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 17
www.analogictech.com
In applications where there is a possibility of VOUT
exceeding VIN for brief amounts of time during normal
operation, the use of a larger value CIN capacitor is
highly recommended. A larger value of CIN with respect
to COUT will effect a slower CIN decay rate during shut-
down, thus preventing VOUT from exceeding VIN. In appli-
cations where there is a greater danger of VOUT exceed-
ing VIN for extended periods of time, it is recommended
to place a Schottky diode across VIN to VOUT (connecting
the cathode to VIN and anode to VOUT). The Schottky
diode forward voltage should be less than 0.45V.
This LDO regulator has complete short-circuit and ther-
mal protection. The integral combination of these two
internal protection circuits gives the AAT2786 LDO regu-
lator a comprehensive safety system to guard against
extreme adverse operating conditions. Device power
dissipation is limited to the package type and thermal
dissipation properties. Refer to the Thermal Considerations
section of this datasheet for details on device operation
at maximum output current loads.
Component Selection For
Step-Down Converter
Inductor Selection
The step-down converter uses peak current mode con-
trol 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 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.
0.75 · V
O
m = = = 0.75
L
0.75 · 1.8V
1.8µH
A
µs
0.75 · V
O
L = =
= 3.3µH
m
0.75
·
3.3V
0.75
A
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 satura-
tion 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.
The 3.3μH CDRH4D28 series Sumida inductor has a
49.2mΩ worst case DCR and a 1.57A DC current rating.
At full 1.5A load, the inductor DC loss is 97mW which
gives less than 1.5% loss in efficiency for a 1.5A, 3.3V
output.
Input Capacitor
Select a 10μF to 22μF 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 C. The calculated value varies with input volt-
age and is a maximum when VIN is double the output
voltage.
⎛⎞
· 1 -
⎝⎠
VO
VIN
CIN =
VO
VIN
⎛⎞
- ESR · FS
⎝⎠
VPP
IO
⎛⎞
· 1 - = for VIN = 2 · VO
⎝⎠
VO
VIN
VO
VIN
1
4
CIN(MIN) = 1
⎛⎞
- ESR · 4 · FS
⎝⎠
VPP
IO
Always examine the ceramic capacitor DC voltage coeffi-
cient 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 · · 1 -
⎝⎠
VO
VIN
VO
VIN
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
18 2786.2008.04.1.0
www.analogictech.com
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.
⎛⎞
· 1 - = D · (1 - D) = 0.52 =
⎝⎠
VO
VIN
VO
VIN
1
2
For VIN = 2 · VO
IO
RMS(MAX)
I2
=
The term
⎛⎞
· 1 -
⎝⎠
VO
VIN
VO
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.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2786. 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 evalu-
ation 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 dur-
ing 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 sup-
ply, most applications do not exhibit this problem.
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum 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
The output capacitor limits the output ripple and pro-
vides 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 charac-
teristics necessary for low output ripple.
The output voltage droop due to a load transient is dom-
inated 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 volt-
age droop during the three switching cycles to the output
capacitance can be estimated by:
COUT =
3 · ∆ILOAD
VDROOP · FS
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion 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 10μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
Adjustable Output Resistor Selection
The output voltage on the AAT2786 is programmed with
external resistors R1 and R2. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
gested value for R2 is 59kΩ. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensi-
tive to external noise and interference. Table 1 summa-
rizes the resistor values for various output voltages with
R2 set to either 59kΩ for good noise immunity or 221kΩ
for reduced no load input current.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 19
www.analogictech.com
VOUT(V) R2 = 59kΩ
R1(kΩ)R2 = 221kΩ
R1(kΩ)
0.8 19.6 75
0.9 29.4 113
1.0 39.2 150
1.1 49.9 187
1.2 59.0 221
1.3 68.1 261
1.4 78.7 301
1.5 88.7 332
1.8 118 442
1.85 124 464
2.0 137 523
2.5 187 715
3.0 237 887
3.3 267 1000
Table 1: AAT2786 Resistor Values for Various
Output Voltages.
Component Selection For LDO
Input Capacitor
Typically, a 1µF or larger capacitor is recommended for
CIN in most applications. A CIN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2786 is physically located more than three centime-
ters from an input power source, a CIN capacitor will be
needed for stable operation. CIN should be located as
close to the device VIN pin as practically possible. CIN
values greater than 1µF will offer superior input line
transient response and will assist in maximizing the
highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for CIN. There is no specific capacitor
ESR requirement for CIN. However, for 150mA LDO regu-
lator output operation, ceramic capacitors are recom-
mended for CIN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources, such as batteries in por-
table devices.
Output Capacitor
For proper load voltage regulation and operational sta-
bility, a capacitor is required between pins VOUT and GND.
The COUT capacitor connection to the LDO regulator
ground pin should be made as direct as practically pos-
sible for maximum device performance.
The AAT2786 has been specifically designed to function
with very low ESR ceramic capacitors. For best perfor-
mance, ceramic capacitors are recommended.
Typical output capacitor values for maximum output cur-
rent conditions range from 1µF to 10µF. Applications
utilizing the exceptionally low output noise and optimum
power supply ripple rejection characteristics of the
AAT2786 should use 2.2µF or greater for COUT
. If desired,
COUT may be increased without limit.
In low output current applications where output load is
less than 10mA, the minimum value for COUT can be as
low as 0.47µF.
Bypass Capacitor and
Low Noise Applications
A bypass capacitor pin is provided to enhance the low
noise characteristics of the AAT2786 LDO regulator. The
bypass capacitor is not necessary for operation of the
AAT2786. However, for best device performance, a small
ceramic capacitor should be placed between the bypass
pin (BYP) and the device ground pin (GND). The value of
CBYP may range from 470pF to 10nF. For lowest noise and
best possible power supply ripple rejection performance,
a 10nF capacitor should be used. To practically realize
the highest power supply ripple rejection and lowest out-
put noise performance, it is critical that the capacitor
connection between the BYP pin and GND pin be direct
and PCB traces should be as short as possible. Refer to
the PCB Layout Recommendations section of this
datasheet for examples.
There is a relationship between the bypass capacitor value
and the LDO regulator turn-on time and turn-off time. In
applications where fast device turn-on and turn-off time
are desired, the value of CBYP should be reduced.
In applications where low noise performance and/ or
ripple rejection are less of a concern, the bypass capac-
itor may be omitted. The fastest device turn on time will
be realized when no bypass capacitor is used.
DC leakage on this pin can affect the LDO regulator output
noise and voltage regulation performance. For this rea-
son, the use of a low leakage, high quality ceramic (NPO
or C0G type) or film capacitor is highly recommended.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
20 2786.2008.04.1.0
www.analogictech.com
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT2786. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lower cost, has a smaller PCB footprint, and is non-
polarized. Line and load transient response of the LDO
regulator is improved by using low-ESR ceramic capaci-
tors. Since ceramic capacitors are non-polarized, they
are not prone to incorrect connection damage.
Equivalent Series Resistance
ESR is a very important characteristic to consider when
selecting a capacitor. ESR is the internal series resis-
tance associated with a capacitor that includes lead
resistance, internal connections, size and area, material
composition, and ambient temperature. Typically, capac-
itor ESR is measured in milliohms for ceramic capacitors
and can range to more than several ohms for tantalum
or aluminum electrolytic capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically made
from NPO or C0G materials. NPO and C0G materials
generally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materials.
Large ceramic capacitors (i.e., greater than 2.2µF) are
often available in low-cost Y5V and Z5U dielectrics.
These two material types are not recommended for use
with LDO regulators since the capacitor tolerance can
vary more than ±50% over the operating temperature
range of the device. A 2.2µF Y5V capacitor could be
reduced to 1µF over temperature; this could cause prob-
lems for circuit operation. X7R and X5R dielectrics are
much more desirable. The temperature tolerance of X7R
dielectric is better than ±15%.
Capacitor area is another contributor to ESR. Capacitors
that are physically large in size will have a lower ESR
when compared to a smaller sized capacitor of an equiv-
alent material and capacitance value. These larger devic-
es can improve circuit transient response when compared
to an equal value capacitor in a smaller package size.
Consult capacitor vendor datasheets carefully when
selecting capacitors for LDO regulators.
Thermal Calculations
There are three types of losses associated with the
AAT2786 step-down converter: switching losses, con-
duction 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 con-
duction mode (CCM), a simplified form of the step-down
converter and LDO losses is given by:
PTOTAL
IO
2 · (RDS(ON)H · VOBUCK + RDS(ON)L · [VINBUCK - VOUTBUCK])
VIN(BUCK)
=
+ (tsw · FS · IOBUCK + IQBUCK) · VINBUCK + (VINLDO - VOUTLDO) · IOLDO
IQBUCK and IQLDO are the step-down converter and LDO qui-
escent currents respectively. The term tSW is used to esti-
mate 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:
PTOTAL = IOBUCK
2 · RDS(ON)H + (tSW · FS · IBUCK + IQBUCK) · VINBUCK + (VINLDO - VOUTLDO) · IOLDO
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the TDFN34-16
package, which is 50°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 21
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 21
www.analogictech.com
PCB Layout
The suggested PCB layout for the AAT2786 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 (C2, C6, and C7)
should connect as closely as possible to the input and
output pins.
2. R1 and C3 are optional low pass filter components
for the IN supply pin for the BUCK if additional noise
decupling is required in a noisy system.
3. The connection of L1 to the LX pin should be as short
as possible.
4. The feedback trace or FB pin should be separated
from any power trace and connect as closely as pos-
sible to the load point. Sensing along a high-current
load trace will degrade DC load regulation.
5. 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 differ-
ences in the potential of the internal signal ground
and the power ground.
6. Connect unused signal pins to ground to avoid
unwanted noise coupling.
7. For low output noise and highest possible power sup-
ply ripple rejection performance, it is critical to con-
nect the bypass capacitor (C8) and output capacitor
(C7) directly to the LDO regulator ground pin. This
method will eliminate any load noise or ripple current
feedback through the LDO regulator.
8. For good thermal coupling, PCB vias are required
from the pad for the TDFN paddle to the bottom
ground plane.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
22 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
22 2786.2008.04.1.0
www.analogictech.com
Printed Circuit Board Layout Recommendations
Figure 1: AAT2786 Evaluation Board Component Side Layout.
Figure 2: AAT2786 Evaluation Board Solder Side Layout.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 23
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 23
www.analogictech.com
3.3µH
L1
C1
opt
C5
opt
BUCK OUT
LDO OUT
LDO IN
BUCK IN
C2
10µF C3
1µF
C6
1µF
C8
10nF
C7
2.2µF
C4
22µF
R1
100 R2
Adj
R3
59k
VIN
12
MODE/SYNC
14
VP
16
VP
15
LDOIN
11
BUCK_EN
13
LDO_EN
9
N/C
10
LDOOUT 6
BYP
8
LX 2
LX 1
GND 4
PGND 3
LDO_GND 7
FB 5
AAT2786IRN
EN LDO
MODE/SYNC
U1 TDFN34-16
Figure 3: AAT2786 Evaluation Board Schematic.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
24 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
24 2786.2008.04.1.0
www.analogictech.com
Step-Down Converter Design Example
Specifications
VOBUCK = 3.3V @ 1.5A, Pulsed Load ∆ILOAD = 1.5A
VOLDO = 2.5V @ 150mA
VIN = 2.7V to 4.2V (3.6V nominal)
FS = 1.2MHz
m = 0.75A/µs
TAMB = 85°C in TDFN34-16 Package
3.3V Buck Output Inductor
0.75 · V
O
L = =
= 3.3µH (see Table 2)
m
0.75
·
3.3V
0.75
A
µs
For Sumida inductor CDRH4D28, 3.3µH, DCR = 49.2mΩ max.
V
OBUCK
V
OBUCK
3.3
V
3.3V
∆I =
· 1 - = · 1 - = 179m
A
L1 · F
S
V
INBUCK
3.3µH · 1.2MHz
4.2V
I
PK
= I
OBUCK
+ ∆I
1
= 1.5A + 0.089A = 1.59A
2
P
L1
= I
OUTBUCK
2
· DCR = 1.5A
2
· 49.2mΩ = 110mW
3.3V Buck Output Capacitor
VDROOP = 0.2V
1
23
1 3.3V · (4.2V - 3.3V)
3.3µH · 1.2MHz · 4.2V
23
RMS(MAX)
IL · FS · VIN(MAX)
= ·
·
3 · ∆ILOAD
VDROOP · FS
3 · 1.5A
0.2V · 1.2MHz
COUT = = = 18.8µF; use 22µF
· = 52mArms
·
(VOUT) · (VIN(MAX) - VOUT)=
PRMS = ESR · IRMS2 = 5mΩ · (52mA)2 = 13.3µW
3.3V Buck Input Capacitor
Input Ripple VPP = 50mV
CIN = = = 7.3µF; use 10µF
1
- ESR · 4 · FS
VPP
IOBUCK
1
- 5mΩ · 4 · 1.2MHz
50mV
1.5A
IOBUCK
RMS(MAX)
I
P = ESR · (IRMS
2) = 5mΩ · (0.75A)2 = 3mW
2
= = 0.75Arms
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 25
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 25
www.analogictech.com
AAT2786 Losses
Total losses can be estimated by calculating the dropout (VIN = VOBUCK) losses where the power MOSFET 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.
PTOTAL = IOBUCK
2 · RDS(ON)H + (VINLDO - VOUTLDO) · IOLDO
= 1.5A2 · 0.16Ω + (4.2 - 2.5) · 150mA
= 615mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 615mW = 116°C
The total losses are also investigated at the nominal lithium-ion battery voltage (3.6V). The simplified version of the
RDS(ON) losses assumes that the N-channel and P-channel RDS(ON) are equal.
PTOTAL
= 1.5A2 · 152mΩ + (5ns · 1.2MHz · 1.5A + 50µA) · 3.6V + (4.2V - 2.5V) · 150m
A
= 630mW
= IOBUCK
2 · RDS(ON)H + (tsw · FS · IBUCK + IQBUCK) · VINBUCK + (VINLDO - VOUTLDO) - IOLDO
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 6300mW = 117°C
VOUT (V) Inductance
(µH) Part
Number Manufacturer Size (mm)
Rated
Current
(A) IRMS
(A) ISAT
(A) DCR (Ω)
3.3 3.3 CDRH4D28 Sumida 5x5x3 1.57 36.4
2.5 2.2 CDRH4D28 Sumida 5x5x3 2.04 23.2
1.8 1.8 CDRH4D28 Sumida 5x5x3 2.2 20.4
1.5 1.8 CDRH4D28 Sumida 5x5x3 2.2 20.4
1.2 1.2 CDRH4D28 Sumida 5x5x3 2.56 17.5
1.0 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
0.8 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
0.6 1.0 SD3114-1.0 Cooper 3.1x3.1x1.45 1.67 2.07 0.042
Table 2: Surface Mount Inductors.
Manufacturer Part Number Value Voltage Temp. Co. Case
Murata GRM21BR60J106KE19 10µF 6.3V X5R 0805
Murata GRM21BR60J226ME39 22µF 6.3V X5R 0805
Table 3: Surface Mount Capacitors.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
26 2786.2008.04.1.0
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
26 2786.2008.04.1.0
www.analogictech.com
Ordering Information
Package Part Marking1LDO Output Voltage Part Number (Tape and Reel)2
TDFN34-16 3KXYY E = 1.2V AAT2786IRN-AE-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.
Package Information3
TDFN34-16
3.000 ± 0.050 1.600 ± 0.050
0.050 ± 0.050 0.229 ± 0.051
(4x)
0.850 MAX
4.000 ± 0.050
3.300 ± 0.050
Index Area
Detail "A"
Top View Bottom View
Side View
0.350 ± 0.100
0.230 ± 0.0500.450 ± 0.050
Detail "A"
Pin 1 Indicator
(optional)
C0.3
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. 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.
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 27
www.analogictech.com
AAT2786
1.5A Step-Down Converter and 150mA LDOSystemPowerTM
PRODUCT DATASHEET
2786.2008.04.1.0 27
www.analogictech.com
Advanced Analogic Technologies, Inc.
3230 Scott Boulevard, Santa Clara, CA 95054
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
property rights are implied. AnalogicTech reserves the right to make changes to their products or specifi cations 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 fi tness 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. Specifi c 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.
