DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
General Description
Features
The AAT2138 is a high current synchronous step-down
converter with an integrated current-limited load switch
designed for high precision current control applications.
By guarding against excessive input current, AAT2138
enables the system designer to maximize the output current from the step-down converter while protecting the
input supply. It is designed for protection of USB ports
from heavy load transient conditions commonly seen
with high data rate modem applications.
• Input Voltage Range: 2.7V to 5.5 V
• Output Voltage Range: Down to 3.0V
• Current-Limited Load Switch
- ±10% High Precision
- Programmable Current Limit Range: 500mA to 3A
• High-Current Step-Down Converter
- Efficiency up to 95%
- 85mΩ High-Side; 50mΩ Low-Side FETs
- 2.8MHz Switching Frequency
- 100% Low Dropout Operation
- No External Compensation Required
- PFM/PWM or forced PWM Modes
- 1MHz to 3MHz External Clock Support
• Soft Start
• Independent Enable Pins for Switch and Converter
• Over-Temperature and Over Load Protection
• -40°C to +85°C Temperature Range
• Pb-Free, Low-Profile, 3mmx3mm 14-pin TDFN Package
AAT2138 integrates a high precision programmable current limited P-channel MOSFET load switch to protect the
input supply against large currents which may cause the
supply to fall out of regulation. The current limit threshold is programmed up to 3A by an external resistor
between RSET and ground.
The AAT2138 is a 2.8MHz current mode step-down converter. It utilizes a tiny 1μH inductor and internal compensation to enable it to have an extremely small foot
print. The regulator supports 100% duty cycle operation
for dropout conditions. An external clock in the range of
1MHz to 3MHz can be fed into MODE as a synchronous
signal for the step-down regulator. This feature offers
the flexibility of fine tuning the switching frequency and
optimizing performance in the application.
Applications
•
•
•
•
•
•
AAT2138 is available in a Pb-free, low profile, 14-pin
3mm x 3mm TDFN package. The product is rated over
the -40°C to 85°C temperature range.
Wireless Modem Data Cards
Portable Hard Drives
Portable Memory Card Readers
Barcode Scanners
Credit Card Readers
General USB Powered Devices
Typical Application
OFF
VIN
2.7V-5.5V
SYSIN
ENSYS
RSET
CIN
1μH
REGIN
CIN
Bulk Capacitor
CIN
0.1μF
ON
RSET
1μF
10μF
R1
274k Ω
AAT2138
PGND
ON
ENREG
VOUT
3.4V/3A
SW
FB
R2
59k Ω
COUT
22μF
OFF PWM
MODE
AGND
PFM/PWM
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
1
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Pin Descriptions
Pin #
Symbol
1, 2
PGND
3, 4
REGIN
5
ENREG
6
RSET
7
AGND
8
ENSYS
9
FB
10
MODE
11, 12
SYSIN
13, 14
SW
EP
GND (EP)
Function
Power Ground. PGND is internally connected to the source of the low-side N-channel MOSFET.
Input Power Supply for the Step-Down Regulator. Connect REGIN to the input power source.
Bypass REGIN to PGND with a 4.7μF or greater ceramic capacitor. REGIN internally connects to
the source of the load switch output and the step-down regulator’s high-side P-channel MOSFET
(see the Functional Block Diagram).
Step-Down Regulator Enable Input. A logic high enables the AAT2138 switching regulator. A
logic low forces the AAT2138 into shutdown mode, placing the output into a high-impedance
state and reducing the quiescent current to less than 1μA. Do NOT leave ENREG floating.
Load Switch Current-Limit Adjustment. Connect a resistor between RSET and ground to set the
input current limit threshold.
Analog Ground. AGND is internally connected to the analog ground of the control circuitry.
Load Switch Enable. Active Low Input. A logic low enables the AAT2138 load switch. A logic high
disables the AAT2138 load switch. Do NOT leave ENSYS floating.
Feedback Input / Output Voltage Sense. FB senses the output voltage for regulation control. For
adjustable output versions, connect a resistive divider network from the output to FB to GND to
set the output voltage accordingly. The FB regulation threshold is 0.6V.
Regulator Operating Mode. Pull MODE high to force the regulator into forced PWM operation.
Pull MODE low to allow automatic pulse-skipping under light-load operation. An external clock in
the range of 1MHz to 3MHz can be connected into MODE as a synchronous clock signal for the
step-down converter to bypass the internal oscillator.
System Load Switch Input. SYSIN connects to the source of the P-channel load switch that limits the system input supply current.
Bias Input Supply for the System and Step-Down Regulator. SYSIN supplies power to the analog and logic control circuitry of the AAT2138.
Inductor Switching Node. SW is internally connected to the source of the high-side P-Channel
MOSFET and the drain of the low-side N-channel MOSFET. Externally connected to the power
inductor as shown in the Typical Application Circuit.
Exposed Pad. Connect directly to the ground plane to reduce the thermal impedance.
Pin Configuration
TDFN33-14
(Top View)
2
PGND
1
14
SW
PGND
2
13
SW
REGIN
3
12
SYSIN
REGIN
4
11
SYSIN
ENREG
5
10
MODE
RSET
6
9
FB
AGND
7
8
ENSYS
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Absolute Maximum Ratings1
Symbol
VIN
VSW
VFB
VRSET
VEN
ISYSIN
VGND
TJ
TA
TLEAD
Description
SYSIN, REGIN to PGND
SW to PGND
FB to AGND
RSET to AGND
ENSYS, ENREG to AGND
Load Switch Maximum RMS Current Capability
PGND to AGND
Operating Junction Temperature Range
Operating Ambient Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Value
-0.3 to +6
-0.3 to (REGIN + 0.3)
-0.3 to (SYSIN + 0.3)
-0.3 to (VIN + 0.3)
-0.3 to (VIN + 0.3)
±4.0
-0.3 to +0.3
-40 to 150
-40 to 85
300
Units
V
A
V
°C
Thermal Characteristics
Symbol
ΘJA
PD
Description
Maximum Thermal Resistance
Maximum Power Dissipation2, 3
Value
Units
60
1.67
°C/W
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.
2. Mounted on an FR4 board.
3. Derate 70mW/°C above 25°C.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
3
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Electrical Characteristics1
VSYSIN = 5.0V, ENREG = SYSIN, ENSYS = AGND = PGND, TA = -40°C to 85°C unless otherwise noted. Typical values are
at TA = 25°C.
Symbol
VSYSIN
Description
Conditions
System Input Voltage Range
SYSIN, REGIN
SYSIN Rising
Hysteresis
REGIN Rising2
Hysteresis
SYSIN Rising
Hysteresis
VSYSIN = VREGIN = 3.6V
Hysteresis = 15°C
No Load Current; Not Switching
ENREG = SW = GND,
VUVLO(SYSIN)
System Input Under-Voltage Lockout
VUVLO(REGIN)
Regulator Input Under-Voltage Lockout
VDIS(SYSIN)
Regulator Input Discharge Threshold
RDISCHRG
TSHDN
IQ
ISHDN
System Input
RDS(ON)
ΔISYSILIM
Discharge MOSFET On-Resistance
Over-Temperature Shutdown Threshold
No Load Supply Current
Shutdown Current
Load Switch
Load Switch On-Resistance
Current Limit Accuracy
Short-Circuit Current-Limit Threshold
Adjustable Current-Limit Range
Current-Limit Response Time
tSS(SYS)
Load Switch Soft-Start Period
VIH_ENSYS
ENSYS Input Logic Threshold High
ENSYS Input Logic Threshold Low
VIL_ENSYS
IENSYS
ENSYS Input Current
Step-Down Regulator
VOUT
Output Voltage Range
IFB
FB Leakage Current
VSYSIN = 5.0V, TA = 25°C
RSET = 6.4kΩ, VREGIN < VUVLO(REGIN),
TA = 20°C to 85°C
RSET = 6.4kΩ, VREGIN < VUVLO(REGIN),
TA = 20°C to 85°C
TA = 25°C
Min
2.7
FB Regulation Threshold3
ΔVOUT/IOUT
Load Regulation
ΔVOUT/VIN
Line Regulation
ILIMPK
RDS(ON)HI
RDS(ON)LO
fOSC
tSS(REG)
tOFF
VIH_ENREG
VIL_ENREG
IENREG
High-Side P-Channel MOSFET Current
Limit
High-Side P-Channel MOSFET OnResistance
Low-Side N-Channel MOSFET OnResistance
Internal Oscillator Frequency
Step-Down Regulator Soft-Start Period4
Minimum Off-time
ENREG Input Logic Threshold High
ENREG Input Logic Threshold Low
ENREG Input Current
Max
5.5
2.7
2.7
0.2
3.15
0.15
8
150
90
1
85
-10
±5
10
1.5
0.4
1.0
3.0
VFB = 0.65V
10mA Load, TA = +25°C
No Load, TA = -40°C to +85°C
IOUT = 0 to 2.5A, TA = 25°C
VIN = 3.6V to 5.5V, VOUT = 3.3V,
TA = 25°C, IOUT = 10mA
588
582
600
600
0.20
0.10
3.0
%
mA
3
-1.0
μA
mΩ
400
0.5
V
Ω
°C
5
1.5
VEN = 0V or < VIN + 0.3V
Units
0.2
tILIM
VFB
Typ
VIN
50
612
618
A
μs
ms
V
μA
V
nA
mV
%/A
0.30
%/V
4.0
A
VREGIN = 3.6V
85
mΩ
VREGIN = 3.6V
50
mΩ
2.8
1
50
MHz
ms
ns
V
V
μA
2.4
COUT = 100μF
1.5
VEN = 0V or 5.5V < VIN + 0.3V
-1.0
0.4
1.0
1. The AAT2138 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.
2. Also used for Load Switch Current-Limit Fold-back Threshold
3. The regulated feedback voltage is tested in an internal test mode that connects VFB to the output of the error amplifier.
4. This time period will keep the regulator start-up current around 300mA to 400mA.
4
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
Step-Down Converter Efficiency
Step-Down Converter Efficiency
(VOUT = 3.65V)
(VOUT = 3.4V)
90
90
80
80
Efficiency (%)
100
Efficiency (%)
100
70
60
50
VREGIN = 3.8V
40
VREGIN = 4.2V
30
VREGIN = 4.5V
20
VREGIN = 4.8V
10
VREGIN = 5.0V
70
60
50
40
VIN = 4.2V
VIN = 4.5V
VIN = 4.8V
VIN = 5.0V
VIN = 5.2V
VIN = 5.5V
30
20
10
VREGIN = 5.5V
0
0
1
10
100
1000
10000
1
10
100
1000
Output Current (mA)
Output Current (mA)
Step-Down Converter Efficiency
Step-Down Converter Load Regulation
(VOUT = 3.8V)
(VOUT = 3.4V)
100
1.2
VREGIN
VREGIN
VREGIN
VREGIN
VREGIN
VREGIN
90
0.8
Output Error (%)
80
Efficiency (%)
10000
70
60
50
40
VREGIN
VREGIN
VREGIN
VREGIN
VREGIN
VREGIN
30
20
10
= 4.2V
= 4.5V
= 4.8V
= 5.0V
= 5.2V
= 5.5V
0.4
= 3.8V
= 4.2V
= 4.5V
= 4.8V
= 5.0V
= 5.5V
0
-0.4
-0.8
0
-1.2
1
10
100
1000
0
10000
500
1000
Output Current (mA)
1500
2000
2500
3000
Output Current (mA)
Step-Down Converter Line Regulation
Step-Down Converter Load Regulation
(VOUT = 3.4V)
(VOUT = 3.8V)
1.2
1.2
0.8
0.8
Output Error (%)
Output Error (%)
1
0.4
0
VREGIN = 4.2V
VREGIN = 4.5V
VREGIN = 4.8V
VREGIN = 5.0V
VREGIN = 5.2V
VREGIN = 5.5V
-0.4
-0.8
500
1000
0.4
0.2
0
-0.2
1mA
100mA
500mA
1000mA
1500mA
2000mA
2500mA
-0.4
-0.6
-0.8
-1
-1.2
0
0.6
1500
2000
Output Current (mA)
2500
3000
-1.2
3.8
4
4.2
4.4
4.6
4.8
5
5.2
5.4
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
5
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
Step-Down Converter Line Regulation
Frequency vs Temperature
(VOUT = 3.8V)
2.9
2.85
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
Frequency (MHz)
Output Error (%)
1.2
1
0.8
0.6
1mA
100mA
500mA
1000mA
1500mA
2000mA
2500mA
-1.2
4.2
4.4
4.6
4.8
5
5.2
2.8
2.75
2.7
2.65
2.6
-40
5.4
-15
Input Voltage (V)
35
60
Feedback Voltage vs Input Voltage
0.609
0.606
0.606
VFB (V)
0.609
0.603
VFB (V)
0.603
0.600
0.600
0.597
0.597
0.594
0.594
0.591
0.591
-40
-15
10
35
60
85
3.1
3.4
3.7
4.0
4.3
4.6
4.9
5.2
Step-Down Converter Enable Threshold vs
Temperature
Step-Down Converter Enable Threshold vs
Input Voltage
1.2
1.1
1.1
1
1
VENREG (V)
1.2
0.9
0.8
0.9
0.8
VENH
0.7
4.2
4.4
4.6
4.8
5
Input Voltage (V)
6
5.2
5.4
VENH
VENL
0.7
VENL
4
5.5
Input Voltage (V)
Temperature (ºC)
0.6
3.8
85
Temperature(°C)
Feedback Voltage vs Temperature
VENREG (V)
10
5.6
0.6
-40
-15
10
35
Temperature (°C)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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March 18, 2013
60
85
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
No Load Quiescent Current vs Temperature
No Load Quiescent Current vs Input Voltage
(Open Loop, VSYSIN = 5V)
(Open Loop)
120
Quiescent Current (μA)
Quiescent Current (μA)
120
110
100
90
80
70
85°C
60
25°C
50
-40°C
40
3.1
3.5
3.9
4.3
4.7
5.1
110
100
90
80
70
60
50
-40
5.5
-15
35
60
No Load Quiescent Current vs Input Voltage
No Load Quiescent Current vs Temperature
(Close Loop, VOUT = 3.4V)
(Close Loop, VSYSIN = 5V, VOUT = 3.4V)
16
Quiescent Current (mA)
18
15
12
9
6
85°C
25°C
3
-40°C
14
12
10
8
0
3.7
4
4.3
4.6
4.9
5.2
-40
5.5
-15
Input Voltage (V)
10
35
60
Step-Down Converter N-Channel RDS(ON) vs
Input Voltage
130
80
85°C
85°C
70
112
25°C
RDS(ON) (mΩ)
25°C
-40°C
94
76
58
60
-40°C
50
40
30
40
3.1
85
Temperature (°C)
Step-Down Converter P-Channel RDS(ON) vs
Input Voltage
RDS(ON) (mΩ)
85
Temperature (°C)
Input Voltage (V)
Quiescent Current (mA)
10
3.5
3.9
4.3
4.7
Input Voltage (V)
5.1
5.5
20
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
7
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
Soft Start
Soft Start
(VSYSIN = VREGIN = 5V, COUT = 22μF, L=1μH, VOUT = 3.4V,
IOUT = 2.5A)
(VSYSIN = VREGIN = 5V, COUT = 22μF,VOUT = 3.4V,
L = 1μH, IOUT = 0A)
VENREGIN
2V/div
VENREGIN
2V/div 0V
VOUT
2V/div 0V
0V
VOUT
2V/div 0V
IREGIN
0A
1A/div
IREGIN 0A
50mA/div
Time 1ms/div
Time 1ms/div
Output Ripple
Output Ripple
(VSYSIN = VREGIN = 5V, PFM Mode, L=1μH, COUT = 22μF,
CREGIN = 10μF + 0.1μF,VOUT = 3.4V,IOUT = 10mA)
(VSYSIN = VREGIN = 5V, PWM Mode, L = 1μH, COUT = 22μF,
CREGIN = 10μF + 0.1μF, VOUT= 3.4V, IOUT = 10mA)
VSW
2V/div
VSW
2V/div
0V
0V
VOUT(AC)
10mV/div 0V
VOUT(AC) 0V
10mV/div
0A
IIN
200mA/div
0A
IIN
200mA/div
Time 400ns/div
Time 400ns/div
Output Ripple
Step-Down Converter Load Transient
(VSYSIN = VREGIN = 5V, PWM Mode, L = 1μH, COUT = 22μF,
CREGIN = 10μF + 0.1μF, VOUT = 3.4V, IOUT = 2A)
(VSYSIN = VREGIN = 5V, VOUT = 3.8V ,
CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF)
VOUT(AC)
100mV/div
VSW
2V/div
0V
1A
0V
VOUT(AC) 0V
10mV/div
ILOAD
500mA/div
0A
100mA
IIN
1A/div
0A
Time 400ns/div
8
Time 100μs/div
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
Step-Down Converter Load Transient
Step-Down Converter Load Transient
(VSYSIN = VREGIN = 5V,VOUT = 3.8V, CCOUT = 22μF, L = 1μH,
CFF = 22pF, CREGIN = 10μF + 0.1μF)
(VSYSIN = VREGIN = 5V, VOUT = 3.4V,
CCOUT = 22μF, L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF)
VOUT(AC)
200mV/div
VOUT(AC)
100mV/div
0V
0V
1A
2.5A
ILOAD
500mA/div
100mA
0A
ILOAD
100mA
1A/div 0A
Time 100μs/div
Time 100μs/div
Step-Down Converter Load Transient
Step-Down Converter LineTransient
(VSYSIN = VREGIN = 5V, VOUT = 3.4V, CCOUT = 22μF, L = 1μH,
CFF = 22pF, CREGIN = 10μF + 0.1μF)
(IOUT = 100mA, VOUT = 3.8V, COUT = 22μF, L = 1μH, CFF = 22pF,
CREGIN = 10μF + 0.1μF)
VOUT(AC)
200mV/div
0V
5V
VREGIN
1V/div 0V
4.2V
2.5A
VOUT(AC) 0V
200mV/div
ILOAD
1A/div 0A
100mA
Time 100μs/div
Time 100μs/div
Step-Down Converter LineTransient
(IOUT = 100mA, VOUT = 3.4V, COUT = 22μF,
L = 1μH, CFF = 22pF, CREGIN = 10μF + 0.1μF)
Load Switch Enable Threshold vs Input Voltage
1.2
1.1
5V
0V
4.2V
VENSYS (V)
VREGIN
1V/div
VOUT(AC) 0V
200mV/div
1
0.9
0.8
VENH
0.7
VENL
0.6
3.1
Time 100μs/div
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. •
March 18, 2013
9
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Typical Characteristics
Load Switch RDS(ON) vs Input Voltage
Load Switch Enable Threshold vs Temperature
1.3
140
85°C
25°C
120
RDS(ON) (mΩ)
VENSYS (V)
1.2
1.1
1
0.9
-40°C
100
80
VENH
0.8
60
VENL
40
3.1
0.7
-40
-15
10
35
60
85
Load Switch Turn on
VENSYS
2V/div
4
4.3
4.6
4.9
5.2
5.5
Load Switch Turn off
(VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF, RL = 100Ω)
(VOUT = 3.4V)
VENSYS
2V/div
0V
VREGIN
2V/div
VOUT
2V/div 0V
VOUT
2V/div
VREGIN
2V/div
3.7
Input Voltage (V)
Temperature (°C)
(VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF, RL = 100Ω)
(VOUT = 3.4V)
3.4
0V
0V
0V
0V
IIN
50mA
/div
IIN
0A
500mA/div
0A
Time 1ms/div
Time 400μS/div
Load Switch Turn On
REGIN Discharge Function
(VSYSIN = 5V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF + 680μF, RL = 100Ω)
(VOUT = 3.4V)
(VSYSIN from 5V to 3V, CSYSIN = 1μF, CREGIN = 10μF + 0.1μF)
VENSYS
2V/div
VSYSIN
2V/div
0V
VREGIN
5V/div
0V
VOUT
2V/div
0V
VREGIN 0V
2V/div
IIN
500mA/div
0A
0V
Time 2ms/div
10
Time 400μs/div
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DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Functional Block Diagram
SYSIN
1.0V
Ref.
RSET
Current
Limit
AGND
Load Switch
Control
EN
REGIN
Discharge
Control
ENSYS
FB
VP
VP
Current
Limit
600mV
Reference
Step-Down
Converter
EN
ENREG
SW
MODE
PGND
MODE
Functional Description
The AAT2138 is a high performance, 2.8MHz, synchronous step-down converter with a high-precision, programmable current limited P-channel load switch up to
3A.
The P-channel load switch is adopted to limit the system
input current. The current limit value is programmed by
external resistor between RSET and GND. Its fast transient response time makes it ideal to protect the system
from input power surges.
The AAT2138 employs internal error amplifier and compensation. It provides excellent transient response, load
and line regulation. Its output voltage is programmed by
an external resistor divider from 3.0V to converter input
voltage. Soft start eliminates any output voltage overshoot when the enable or input voltage is applied.
Dropout mode makes the converter increase the switch
duty cycle to 100% and the output voltage tracks the
input voltage minus the RDS(ON) drop of the P-channel
high-side MOSFET of the converter.
The AAT2138's input voltage range is 2.7V to 5.5V. Two
independent enable pins control the load switch and
step-down converter separately. The converter efficiency
has been optimized for a 1μH inductor.
trol scheme. The converter senses the current through
the high-side P-channel MOSFET for current loop control
as well as overload protection. A fixed slope compensation signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The input
current of the step-down converter is limited by the load
switch.
The feedback amplifier compares the FB voltage against
the 0.6V reference voltage. The error amplifier’s transconductance output is internally compensated, and programs the current-mode loop for the necessary peak
switch current to force a constant output voltage over all
load and line conditions.
Enable/Soft Start
AAT2138 has two independent enable pins: ENSYS and
ENREG. When ENSYS is pulled high, the current limit
load switch is turned off and REGIN drops to zero. When
ENREG is pulled low, the step-down converter is forced
into the low-power, no-switching, high impedance state.
The total input current during shutdown is less than 1uA.
Control Scheme
When ENSYS is pulled low, the system will turn on the
load switch in a soft start process. To avoid a big inrush
current during the startup, the REGIN voltage will be
charged from SYSIN with limited input current under
500mA. The startup time depends on the capacitance
between REGIN and GND.
The AAT2138 uses a peak current-mode step-down con-
When ENREG is pulled high, the step-down converter is
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11
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
active with soft start. When the REGIN voltage is above
UVLO and reaches 95% of VSYSIN voltage, the reference
voltage is charged from 0 to 0.6V slowly by an internal
current source to avoid a large input inrush current.
During the soft start, the output voltage increases with
the reference voltage. The soft start time is about 1ms
with a 100μF output capacitor.
REGIN Discharge
The voltage on the capacitor between REGIN and GND is
discharged quickly when the power on SYSIN is removed
quickly (unplugged adaptor). When ENSYS is pulled low
and SYSIN is below 3.0V, AAT2138 automatically turns
on the discharge MOSFET to discharge the REGIN
capacitor storage. Discharge resistance is typical 8ohm
and the specific discharge time depends on the capacitance between REGIN and GND.
Application Information
Load Switch
Current Limit Setting
The AAT2138's load switch current limit can be programmed by an external resistor RSET from RSET to GND.
In most applications, the variation in ILIM must be taken
into account when determining RSET. The ILIM variation is
due to processing variations from part to part, as well as
variations in the voltages at SYSIN and REGIN, plus the
operating temperature. Together, these three factors add
up to a ±5% tolerance (see load switch ILIM specification
in Electrical Characteristics section). Table 1 gives 1%
standard metal film resistor example values for PMOS
current limit programming.
ILIM (A)
RSET (kΩ)
0.4
0.482
0.51
0.553
0.613
0.798
0.908
1.01
1.078
1.245
1.538
2.07
2.48
2.64
2.9
3
13.3
10
9.53
8.88
8.06
6.34
5.6
5.1
4.75
4.02
3.16
2.2
1.74
1.6
1.4
1.3
Input Under-Voltage Lockout
Under-voltage lockout (UVLO) guarantees sufficient
VREGIN bias and proper operation of all internal circuitry
prior to activation.
MODE Function
When the MODE pin is pulled high, the part runs in
forced PWM mode operation using the internal oscillator.
When the MODE pin is pulled low, a light-load mode
operation (PFM/PWM) is designed to reduce the dominant switching losses at low output voltage and lightload condition.
The Step-Down converter can also be synchronized to an
external clock signal fed into the MODE pin. In this case,
the internal oscillator is bypassed. The frequency of the
external clock must be in the range from 1MHz to 3MHz.
Table 1: Examples of 1% Standard Resistor Value
of RSET.
3.3
Protection Circuitry
3.0
The AAT2138 includes protection for overload and overtemperature conditions. The overload protection turns
off the high-side switch when the inductor current
exceeds the current-limit threshold (3A minimum).
2.4
The AAT2138 includes thermal protection that disables
the regulator and the load switch when the die temperature reaches 150°C. The REGIN pin is maintained in a
high-impendance state. It automatically restarts when
the temperature drops by 15°C or more.
ILIMIT (A)
2.7
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0.0
1
2
3
4
5
6
7
8
9
10
11
12
13
RSET (kΩ)
Figure 1: Load Switch Current Limit vs RSET.
12
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14
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Dropout Voltage
Dropout voltage is determined by RDS(ON) and the current
passing through it. AAT2138 load switch typical RDS(ON) is
80mΩ for USB application. So, for a 750mA load switch
current limit setting, the load switch dropout voltage can
be calculated by:
VSYSIN - VDROPOUT_SWITCH · η
Next, calculate the maximum current CREGIN should provide:
VDROPOUT_SWITCH = 750mA · 80mΩ = 60mV
Step-Down Converter
ICREGIN = IBUCKIN - ILIM
Finally, derive the CSYSOUT at certain load on period TON
Inductor Selection
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 inductor should be set equal to half the output voltage numeric value in 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.
VOUT · IOUT
IBUCKIN =
ICREGIN · TON
CREGIN_MIN =
∆VREGIN
Example: A 2A, 217Hz, 12.5% load pulse is applied on
3.8V VOUT in 5V VIN and 750mA load switch current limit,
under the condition, VDROPOUT_SWITCH is 0.06V, VDROPOUT_BUCK
is 0.17V. Therefore, considering the step-down converter
at 2A, 3.8V, VOUT is 90%.
ΔVREGIN = 5 - 3.8 - 0.06 - 0.17 = 0.970V
3.8 · 2
IBUCKIN =
= 1.71A
5 - 0.06 · 0.9
ICREGIN = 1.71 - 0.75 = 0.96A
tON is 576μs for a 217Hz, 12.5% duty cycle load pulse.
CREGIN_MIN =
0.96 · 576
0.970
= 570μF
CREGIN Selection
CREGIN is not only the load switch output capacitor but
also the step-down converter input capacitor. It is
designed to provide the additional input current and
maintain the SYSOUT voltage for the step-down converter when load switch limits the input current from
SYSIN. If the input voltage of the step-down converter
is lower than the VOUT plus the dropout voltage, the
AAT2138 enters dropout mode. CREGIN minimum value
can be calculated by the following steps:
First, calculate the allowed maximum delta voltage on
CREGIN to keep VOUT in regulation:
ΔVREGIN = VIN - VOUT - VDROPOUT_SWITCH - VDROPOUT_BUCK
Second, calculate the required input current at SYSOUT
for the step-down converter:
VOUT
500mV/div
VREGIN
2V/div
ISYSIN
1A/div
IOUT
1A/div
0
0
0
800μs/div
Figure 2: AAT2138 Operation Waveform
2A, 577μs Load Pulse (Applied)
VIN = 5V, VOUT = 3.8V.
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March 18, 2013
13
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Considering 20% capacitance tolerance, the minimum
capacitance should be 684μF. So select a 680μF tantalum capacitor as CREGIN, as well as an additional 10μF
ceramic capacitor to closely filter the input voltage VREGIN
of the step-down converter on the PCB board.
Input Capacitor Selection
Select a 1μF X7R or X5R ceramic capacitor for the system input. To estimate the required input capacitor size,
determine the acceptable input ripple level and solve for
CIN. The calculated value varies with input voltage and is
a maximum when VIN is twice the output voltage.
CIN =
V
VOUT
·
VIN
1 - VOUT
IN
VPP
IOUT
- ESR · fS
for VIN = 2 · VOUT,
VOUT
·
VIN
1-
1
VOUT
=
4
VIN
and
CIN(MIN) =
1
VPP
IOUT
- ESR · 4 · fS
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2138. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function due to their low ESR and ESL. To minimize
stray parasitic inductance, place the capacitor as close
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 4).
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the eval-
14
uation 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.
Feedback Resistor Selection
The output voltage on the AAT2138 is adjustable 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 suggested 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 sensitive to external noise and interference. The maximum
value of R1 should be below 1Mohm to keep reference
voltage normal and avoid noise coupling.
The external resistor R1, combined with an external
capacitor up to 22pF feed-forward capacitor (C4 in
Figure 3), delivers enhanced transient response for
extreme pulsed load applications and reduces ripple in
light load conditions. The addition of the feed forward
capacitor typically requires a larger output capacitor
C31-C32 for stability. The external resistors set the output voltage according to the following equation:
VOUT
VOUT
R1 = V
- 1 · R2 =
0.6V - 1 · R2
REF
Table 2 shows the standard 1% metal film resistor
examples for different step-down ouput voltages
VOUT (V)
R2 = 59kΩ,
R1 (kΩ)
3.3
3.4
3.6
3.8
267
274
294
316
Table 2: Resistor Selections for Different Output
Voltage Settings.
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March 18, 2013
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Printed Circuit Board
Layout Recommendations
For best performance of the AAT2138, the following
guidelines should be followed when designing the PCB
layout:
1. Reliably solder the exposed pad (EP) to the GND
plane. A GND pad below EP is strongly recommended.
2. Keep the power traces, including the GND trace, the
SW trace and the SYSIN, REGIN short, direct and wide
to allow large current flow. Keep the L connection to
the SW pins as short as possible. Do not put any signal lines under the inductor.
3. Connect the input capacitors (C1 and C21) as close as
possible between SYSIN and REGIN and GND to get
good power filtering.
4. Connect the input capacitor C23 as close as possible
between REGIN and AGND to get good power filtering
and load regulation.
5. Keep the switching node, SW away from the sensitive
FB node.
6. Separate the feedback trace from any power trace and
connect as close as possible to the load point. Sensing
along a high-current load trace will degrade DC load
regulation. Place external feedback resistors as close
as possible to the FB pin to minimize the length of the
high impedance feedback trace.
7. Minimize the resistance of the trace from the load
return to GND. This will help minimize any error in DC
regulation due to differences in the potential of the
internal signal ground and the power ground.
REGIN
SW
13
14
9
6
1
2
7
L1
VOUT
3.4V
C32
Option
C31
22μF
1uH
R1
274K
C21
10μF
U1
AAT2138
C4
22pF
RSET
SW
SW
FB
RSET
PGND
PGND
AGND
REGIN
REGIN
SYSIN
SYSIN
ENREG
ENSYS
MODE
3
4
11
12
5
8
10
C22
680μF
C23
0.1μF
GND
SYSIN
C1
1μF
2.7V-5.5V
R2
59K
GND
GND
J3
SYSIN
J2
GND
SYSIN
J1
GND
SYSIN
GND
Figure 3. AAT2138 Evaluation Board Schematic.
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15
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
(a) Top Layer
(b) Bottom Layer
Figure 4. AAT2138 Evaluation Board Layout
Manufacturer
Part Number
Inductance
(μH)
Max DC
Current
(A)
DCR (mΩ)
Size (mm)
LxWxH
Type
Samsung
Murata
CIG22H1R0MNE
LQH44PN1R0NPO
1
1
3.3
2.95
83
30
2x2.5x1.2
4x4x1.65
Shielded
Shielded
Table 3: Recommended Inductor Selection
Manufacturer
Murata
KEMET
Part Number
Value
(μF)
Voltage
(V)
Temperature
Range
Case Size
GRM188R71H104KA93D
GRM21BR71E105KA99L
GRM21BR61C106KE15L
GRM21BR60J226ME39
GRM0335C1H220GD01D
T495D687M006ZTE150
0.1
1
10
22
22pF
680
50
25
16
6.3
50
6.3
X7R
X7R
X5R
X5R
COJ
X5R
0603
0805
0805
0805
0603
7343
Table 4: Recommended Capacitor Selection.
Component
Part Number
Description
U1
AAT2138IWO-0.6
C1
C21
C23
C31
C4
C22
R1
R2
RSET
L
GRM21BR71E105KA99L
GRM21BR61C106KE15L
GRM21BR71E105KA99L
GRM21BR60J226ME39
GRM0335C1H220GD01D
T495D687M006ZTE150
RC0603FR-07274KL
RC0603FR-0759KL
RC0603FR-076K34L
CIG22H1R0MNE
Manufacturer
High Current Step-Down Converter with Adjustable Current
Limit Load Switch
Cap Ceramic 1uF 0805 X7R 25V 10%
Cap Ceramic 10uF 0805 X5R 16V 10%
Cap Ceramic 0.1uF 0603 X7R 50V 10%
Cap Ceramic 22uF 0805 X5R 6.3V 10%
Cap Ceramic 22pF 0603 C0J 50V 10%
Cap 680uF 7343 X5R 6.3V 20%
Res 274KΩ 1/10W 1% 0603 SMD
Res 59KΩ 1/10W 1% 0603 SMD
Res 6.34KΩ 1/10W 1% 0603 SMD
Power Inductor 1uH 3.3A 83mΩ SMD
Skyworks
Murata
KEMET
Yageo
Samsung
Table 5: Evaluation Board BOM List
16
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March 18, 2013
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Ordering Information
Output Voltage
Package
Marking1
Part Number (Tape and Reel)
Adjustable ≥ 0.6V
TDFN33-14
S6XYY
AAT2138IWO-0.6-T12
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information
TDFN33-143
Detail "A"
3.000 ± 0.050
2.500 ± 0.050
Index Area
3.000 ± 0.050
1.650 ± 0.050
Top View
Bottom View
+ 0.100
- 0.000
Pin 1 Indicator
(Optional)
0.180 ± 0.050
Side View
0.400 BSC
0.000
0.203 REF
0.750 ± 0.050
0.425 ± 0.050
Detail "A"
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.
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March 18, 2013
17
DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Copyright © 2012, 2013 Skyworks Solutions, Inc. All Rights Reserved.
Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a
service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no
responsibility whatsoever for conflicts, incompatibilities, or other difficulties arising from any future changes.
No license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. Skyworks assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of Skyworks products, information or materials, except as may be provided in Skyworks Terms and Conditions of Sale.
THE MATERIALS, PRODUCTS AND INFORMATION ARE PROVIDED “AS IS” WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS, IMPLIED, STATUTORY, OR OTHERWISE, INCLUDING FITNESS FOR A PARTICULAR
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Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product
design, or damage to any equipment resulting from the use of Skyworks products outside of stated published specifications or parameters.
Skyworks, the Skyworks symbol, and “Breakthrough Simplicity” are trademarks or registered trademarks of Skyworks Solutions, Inc., in the United States and other countries. Third-party brands and names are for
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March 18, 2013
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