202015A.pdf

DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
General Description
Features
The AAT2500M is a high efficiency 400mA step-down
converter and 300mA low dropout (LDO) linear regulator
for applications where power efficiency and solution size
are critical. The typical input power source can be a
single-cell Lithium-ion/polymer battery or a 5V or 3.3V
power bus.
• VIN Range: 2.7V to 5.5V
• Output Current:
▪ Step-Down Converter: 400mA
▪ LDO: 300mA
• Low Quiescent Current
▪ 130μA Combined for Both Step-Down Converter
plus LDO
• 90% Efficient Step-down Converter (at 100mA)
• Integrated Power Switches
• 100% Duty Cycle
• 1.8MHz Switching Frequency
• Current Limit Protection
• Automatic Soft-Start
• Over Temperature Protection
• TSOPJW-12 Package
• -40°C to +85°C Temperature Range
The step-down converter is capable of delivering up to
400mA output current, uses a typical switching frequency of 1.8MHz to greatly reduce the size of external components, offers high speed turn-on and maintains a low
25μA no load quiescent current.
The LDO is capable of delivering up to 300mA output
current.
The AAT2500M is available in the Pb-free, space-saving
12-pin TSOPJW package and is rated over the -40°C to
+85°C operating temperature range.
Applications
•
•
•
•
•
•
•
•
Cellular Phones
Digital Cameras
Handheld Instruments
Micro Hard Disc Drives
Microprocessor / DSP Core / IO Power
Optical Storage Devices
PDAs and Handheld Computers
Portable Media Players
Typical Application
AAT2500M
2.7V to 5.5V
Input Supply
IN_BUCK
LX
4.7μF
IN_LDO
1μF
VOUT_BUCK
2.2μH
R1
FB_BUCK
R2
VOUT(LDO)
OUT_LDO
C2
4.7μF
Enable Buck
EN_BUCK
Enable LDO
EN_LDO
C1
2.2μF
AGND PGND
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
1
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Pin Descriptions
Pin #
Symbol
Function
1
2
3
4
5
6
7
8, 9, 10, 11
12
LX
PGND
EN_BUCK
EN_LDO
FB_BUCK
OUT_LDO
IN_LDO
AGND
IN_BUCK
Step-down converter switching node.
Power ground for step-down converter.
Enable pin for step-down converter.
Enable pin for LDO.
Feedback input pin for step-down converter. Regulated at 0.6V for adjustable version.
LDO power output.
Input supply voltage for LDO.
Analog signal ground.
Input supply voltage for step-down converter.
Pin Configuration
TSOPJW-12
(Top View)
LX
PGND
EN_BUCK
EN_LDO
FB_BUCK
OUT_LDO
2
1
12
2
11
3
10
4
9
5
8
6
7
IN_BUCK
AGND
AGND
AGND
AGND
IN_LDO
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Absolute Maximum Ratings1
Symbol
VP
AGND, PGND
VEN, VFB
IOUT
TJ
TS
TLEAD
Description
Input Voltage
Ground Pins
Enable and Feedback Pins
Maximum DC Output Current (continuous)
Operating Temperature Range
Storage Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Value
Units
-0.3 to 6.0
-0.3 to +0.3
VIN + 0.3
1000
-40 to 150
-65 to 150
300
V
V
V
mA
°C
°C
°C
Value
Units
110
909
°C/W
mW
Thermal Information
Symbol
JA
PD
Description
Thermal Resistance2
Maximum Power Dissipation
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
3
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Electrical Characteristics1
VIN_BUCK = VIN_LDO = 5.0V. TA = -40°C to +85°C unless noted otherwise. Typical values are at TA = +25°C.
Symbol
Description
Conditions
Power Supply
VINBUCK,
Input Voltage
VINLDO
VUVLO
Under-Voltage Lockout
IQ
Quiescent Current
Shutdown Current
ISHDN
Step-Down Converter
VFB
Feedback Voltage Tolerance
ILXLEAK
LX Reverse Leakage Current
IFB
Feedback Leakage
P-Channel Current Limit
ILIM
RDS(ON)H
High Side Switch On Resistance
Low Side Switch On Resistance
RDS(ON)L
VOUT/VOUT Load Regulation
VOUT/VOUT Line Regulation
FOSC
Oscillator Frequency
TS
Start-Up Time
LDO (VOUT = 3.3V)
VOUT
Output Voltage Tolerance
VOUT
Output Voltage Range
VIN
Typ
2.7
VIN Rising
VIN Falling
VEN = VIN, No Load
VEN = GND
No Load, TA = 25°C
IOUT = 0 to 400mA; VIN = 2.7 to 5.5V
VIN = 5.5V, VLX = 0 to VIN, VEN = GND
VFB = 1.0V
Units
5.5
V
2.7
V
V
μA
μA
1.0
0.591
-3
-1.0
0.609
+3
1.0
0.2
V
%
μA
μA
A


%
%
MHz
μs
3.36
3
V
%
5.5
V
1.2
0.4
0.25
0.25
0.3
1.8
120
From Enable to Output Regulation
No Load, 25°C
IOUT = 0 to 300mA
Max
2.35
130
ILOAD = 0 to 400mA
VIN = 2.7V to 5.5V
Input Voltage
Output Current
IOUT
ILIM
Current Limit
VDO
Dropout Voltage3
VOUT/VOUT Load Regulation
VOUT/VOUT Line Regulation
Start-Up Time
TS
Logic Signals
VEN(L)
Enable Threshold Low
VEN(H)
Enable Threshold High
IEN(H)
Enable Current Consumption
Over-Temperature Shutdown Threshold
TSD
THYS
Over-Temperature Shutdown Hysteresis
Min
3.24
-3
VOUT +
VDO2
300
3.30
1
160
1.2
0.6
100
IOUT = 300mA
ILOAD = 0 to 300mA
VIN = 3.7V to 5.5V
From Enable to Output Regulation
240
0.6
1.5
-1.0
1.0
150
15
1. Specification over the -40°C to +85°C operating temperature ranges is assured by design, characterization and correlation with statistical process controls.
2. To calculate the minimum LDO input voltage, use the following equation: VIN(MIN) = VOUT(MAX) + VDO(MAX).
3. VDO is defined as VIN - VOUT when VOUT is 98% of nominal.
4
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
mA
A
mV
%
%
μs
V
V
μA
C
C
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Typical Characteristics
LDO Dropout Voltage vs. Temperature
LDO Dropout Characteristics
(VOUT = 3.3V)
3.5
180
IL = 300mA
Output Voltage (V)
Dropout Voltage (mV)
210
150
120
IL = 200mA
90
IL = 100mA
60
30
IL = 50mA
0
-40
-20
0
20
40
60
3.4
3.3
3.2
IOUT = 10mA
IOUT = 50mA
IOUT = 0.1mA
IOUT = 300mA
3.1
3.0
IOUT = 200mA
2.9
IOUT = 100mA
2.8
80
3.0
100
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Input Voltage (V)
Temperature (°C)
LDO Dropout Voltage vs. Output Current
No Load Quiescent Current vs. Input Voltage
(EN_BUCK = EN_LDO = VIN)
150
85°C
200
150
Input Current (µA)
Dropout Voltage (mV)
250
25°C
100
50
-40°C
50
100
150
200
250
300
350
90
70
-40°C
50
3
3.5
4
4.5
5
5.5
LDO Turn-Off Response Time
LDO Turn-On Time From Enable
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
0
3.0
2.0
1.0
0.0
-1.0
Enable Voltage (top) (V)
2
6
4
2
0
3
2
1
0
-1
Output Voltage (bottom)(V)
4
Output Voltage (bottom)(V)
6
Time (50ns/div)
6
Input Voltage (V)
Output Current (mA)
Enable Voltage (top) (V)
25°C
85°C
110
30
2.5
0
0
130
Time (40µs/div)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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5
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
LDO Line Transient Response
LDO Load Transient Response
(VIN = 4V to 5V; VOUT = 3.3V; IOUT = 300mA; COUT = 4.7µF)
(1mA to 300mA; VIN = 5V; VOUT = 3.3V; COUT = 4.7µF)
3.5
3.3
3.1
Output Voltage (top) (V)
4
3.7
3.5
3.3
3.1
2.9
300mA
0.0
-0.2
Time (100µs/div)
LDO VIH and VIL vs. Input Voltage
Step-Down Converter Switching
Frequency vs. Input Voltage
(IOUT = 400mA)
Frequency Variation (%)
1.2
VIH
1.1
VIH and VIL (V)
0.2
1mA
Time (40µs/div)
1.0
0.9
VIL
0.8
0.7
3
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
VOUT = 1.8V
2
1
VOUT = 1.2V
0
-1
-2
-3
0.6
2.7
3.1
3.5
3.9
4.3
4.7
Input Voltage (V)
Input Voltage (V)
Switching Frequency (MHz)
Step-Down Converter Switching
Frequency vs. Temperature
(VIN = 5V; VOUT = 1.8V)
1.9
1.8
1.7
1.6
1.5
-40
-20
0
20
40
60
80
100
Temperature (°C)
6
0.4
Output Current (bottom) (A)
5
Output Voltage (bottom) (V)
Input Voltage (top) (V)
Typical Characteristics
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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5.1
5.5
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Typical Characteristics
Step-Down Converter Efficiency vs. Load
Step-Down Converter DC Regulation
(VOUT = 1.8V; L = 2.2µH)
(VOUT = 1.8V; L = 2.2µH)
1.0
100
VIN = 3.3V
VIN = 2.7V
80
Output Error (%)
Efficiency (%)
90
70
60
50
VIN = 4.2V
40
VIN = 5.5V
30
VIN = 3.3V, 4.2V, 5.5V
0.5
0.0
VIN = 2.7V
-0.5
-1.0
20
0.1
1
10
100
0.1
1000
1
Output Current (mA)
100
1000
Output Current (mA)
Step-Down Converter Efficiency vs. Load
Step-Down Converter DC Regulation
(VOUT = 1.2V; L = 2.2µH)
(VOUT = 1.2V; L = 2.2µH)
1.0
100
VIN = 3.3V
VIN = 2.7V
80
Output Error (%)
90
Efficiency (%)
10
70
VIN = 5V
60
50
VIN = 4.2V
40
VIN = 3.6V to 5.5V
0.5
0.0
VIN = 2.7V
-0.5
30
20
0.1
1
10
100
-1.0
0.1
1000
10
100
Step-Down Converter Output Ripple
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 1mA)
(VOUT = 1.8V; VIN = 5V; IOUT = 400mA)
1.79
0.2
0.1
0.0
Output Voltage (top) (V)
1.80
1.82
1.81
1.80
1.79
0.6
0.4
0.2
0.0
Inductor Current (bottom) (A)
1.81
Time (10µs/div)
1000
Output Current (mA)
Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Current (mA)
1
Time (200ns/div)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
7
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Typical Characteristics
Step-Down Converter Output
Voltage Error vs. Temperature
Step-Down Converter Output
Voltage Error vs. Temperature
(VIN = 5V; VOUT = 1.2V; IOUT = 400mA)
Output Voltage Error (%)
Output Voltage Error (%)
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA)
1.0
0.5
0.0
-0.5
-1.0
-50
-25
0
25
50
75
1.0
0.5
0.0
-0.5
-1.0
100
-50
-25
0
Temperature (°C)
500
85°C
RDS(ON)L (mΩ
Ω)
RDS(ON)H (mΩ
Ω)
600
500
400
300
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
100°C
400
85°C
300
200
25°C
100
2.5
3
3.5
4
4.5
5
Input Voltage (V)
Step-Down Converter Soft Start
6
4
2
0
-2
0.4
0.2
0.0
-0.2
Inductor Current (bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA; CFF = Open)
Time (50µs/div)
8
100
120°C
25°C
2.5
75
Step-Down Converter N-Channel
RDS(ON)L vs. Input Voltage
120°C
100°C
50
Temperature (°C)
Step-Down Converter P-Channel
RDS(ON)H vs. Input Voltage
700
25
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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5.5
6
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Typical Characteristics
Step-Down Converter Load Transient Response
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7µF)
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7µF; CFF = 100pF)
1mA
0.4
0.2
0.0
-0.2
Output Voltage (top) (V)
Output Voltage (top) (V)
400mA
2.0
1.8
400mA
1mA
0.4
0.2
0.0
-0.2
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.8
Output Current (middle) (A)
Inductor Current (bottom) (A)
2.0
Time (100µs/div)
Time (100µs/div)
Step-Down Converter Load Transient Response
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7µF)
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7µF; CFF = 100pF)
0.4
0.2
0.0
-0.2
Output Voltage (top) (V)
Output Voltage (top) (V)
400mA
1mA
1.4
1.2
400mA
1mA
0.4
0.2
0.0
-0.2
Time (100µs/div)
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.2
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.4
Time (100µs/div)
Step-Down Converter Line Transient Response
Step-Down Converter Line Regulation
(VIN = 4V to 5V; VOUT = 1.8V; IOUT = 400mA; COUT = 4.7µF)
(VOUT = 1.2V; L = 2.2µH)
4
1.8
1.7
1.6
1.5
Time (40µs/div)
1.00
Accuracy (%)
Input Voltage (top) (V)
5
Output Voltage (bottom) (V)
6
IOUT = 0.1mA to 400mA
0.50
0.00
-0.50
-1.00
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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9
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Functional Block Diagram
IN_BUCK
Control Circuit
EN_BUCK
EN_LDO
Bias
LX
PGND
FB_BUCK
VCC
VCC
IN_LDO
OUT_LDO
Oscillator
RLDOFB1
RLDOFB2
AGND
Functional Description
The AAT2500M is a high performance power management IC comprised of a buck converter and a linear
regulator. The buck converter is a high efficiency converter capable of delivering up to 400mA. Operating at
1.8MHz, the converter requires only three external
power components (CIN, COUT, and LX) and is stable with
a ceramic output capacitor. The linear regulator delivers
300mA and is also stable with ceramic capacitors.
Linear Regulator
The advanced circuit design of the linear regulator has
been specifically optimized for very fast start-up and
shutdown timing. This proprietary LDO has also been
tailored for superior transient response characteristics.
These traits are particularly important for applications
that require fast power supply timing.
10
The high-speed turn-on capability is enabled through
implementation of a fast-start control circuit, which
accelerates the power-up behavior of fundamental control and feedback circuits within the LDO regulator. Fast
turn-off time response 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 circuit has no adverse effect on normal device
operation. 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.
The regulator comes with complete short-circuit and thermal protection. The combination of these two internal
protection circuits gives a comprehensive safety system
to guard against extreme adverse operating conditions.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
The regulator features an enable/disable function. This
pin (EN_LDO) is active high and is compatible with CMOS
logic. To assure the LDO regulator will switch on, the
EN_LDO turn-on control level must be greater than 1.5V.
The LDO regulator will go into the disable shutdown
mode when the voltage on the EN_LDO pin falls below
0.6V. If the enable function is not needed in a specific
application, it may be tied to VIN_LDO to keep the LDO
regulator in a continuously on state.
Soft Start
The IN_LDO input powers the internal reference, oscillator, and bias control blocks. For this reason, the IN_LDO
input must be connected to the input power source to
provide power to both the LDO and step-down converter
functions.
For conditions where the input voltage drops to the output
voltage level, the converter duty cycle increases to 100%.
As 100% duty cycle is approached, the minimum off-time
initially forces the high side on-time to exceed the 1.8MHz
clock cycle and reduce the effective switching frequency.
Once the input drops below the level where the output
can be regulated, the high side P-channel MOSFET is
turned on continuously for 100% duty cycle. At 100%
duty cycle, the output voltage tracks the input voltage
minus the IR drop of the high side P-channel MOSFET
RDS(ON).
The AAT2500M soft-start control prevents output voltage
overshoot and limits inrush current when either the input
power or the enable input is applied. When pulled low, the
enable input forces the converter into a low-power, nonswitching state with a bias current of less than 1μA.
Low Dropout Operation
When the regulator is in shutdown mode, an internal
1.5k resistor is connected between OUT and GND. This
is intended to discharge COUT when the LDO regulator is
disabled. The internal 1.5K resistor has no adverse
impact on device turn-on time.
Step-Down Converter
Low Supply
The AAT2500M buck is a constant frequency peak current mode PWM converter with internal compensation. It
is designed to operate with an input voltage range of
2.7V to 5.5V. The output voltage ranges from 0.6V to the
input voltage. The 0.6V fixed model shown in Figure 1 is
also the adjustable version and is externally programmable with a resistive divider, as shown in Figure 2. The
converter MOSFET power stage is sized for 400mA load
capability with up to 92% efficiency. Light load efficiency
is close to 80% at a 500μA load.
The under-voltage lockout (UVLO) guarantees sufficient
VIN bias and proper operation of all internal circuitry prior
to activation.
AAT2500M
12
VIN
1
VP_BUCK
C1
10μF
VOUT _BUCK
VIN
11
EN_BUCK
AGND
EN_LDO
AGND
OUT_LDO
AGND
PGND
AGND
4
9
2
FB_BUCK
11
EN_BUCK
AGND
EN_LDO
AGND
OUT_LDO
AGND
PGND
AGND
C8
100pF
10
6
9
2
C4
4.7μF
VOUT_BUCK
R1
5
IN_LDO
VOUT_LDO
8
Figure 1: AAT2500M Fixed Output.
7
4
C1
4.7μF
L1
4.7uH
LX
3
10
6
1
VP_BUCK
C1
10μF
FB_BUCK
3
C4
4.7μF
AAT2500M
12
5
IN_LDO
VOUT_LDO
For overload conditions, the peak inductor current is limited. Thermal protection disables switching when the
internal dissipation or ambient temperature becomes
excessive. The junction over-temperature threshold is
150°C with 15°C of hysteresis.
L1
4.7μH
LX
7
Fault Protection
R2
59k
C1
4.7μF
8
Figure 2: AAT2500M with Adjustable Step-Down
Output and Enhanced Transient Response.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202015A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 4, 2012
11
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Applications Information
than several ohms for tantalum or aluminum electrolytic
capacitors.
LDO Regulator
Input and Output Capacitors: An input capacitor is not
required for basic operation of the linear regulator.
However, if the AAT2500M is physically located at a reasonable distance from an input power source, an input
capacitor (C3) will be needed for stable operation.
Typically, a 1μF or larger capacitor is recommended for C3
in most applications. C3 should be located as closely to
the input voltage (VIN_LDO) pin as practically possible.
An input capacitor greater than 1μF will offer superior
input line transient response and maximize power supply ripple rejection. Ceramic, tantalum, or aluminum
electrolytic capacitors may be selected for C3. There is
no specific capacitor ESR requirement for C3. However,
for 300mA LDO regulator output operation, ceramic
capacitors are recommended for C3 due to their inherent
capability over tantalum capacitors to withstand input
current surges from low impedance sources such as batteries in portable devices.
For proper load voltage regulation and operational stability, a capacitor is required between the OUT_LDO and
AGND pins. The output capacitor (C4) connection to the
LDO regulator ground pin should be made as directly as
practically possible for maximum device performance.
Since the regulator has been designed to function with
very low ESR capacitors, ceramic capacitors in the 1.0μF
to 10μF range are recommended for best performance.
Applications utilizing the exceptionally low output noise
and optimum power supply ripple rejection should use
2.2μF or greater for C4. In low output current applications, where output load is less than 10mA, the minimum value for C4 can be as low as 0.47μF.
Equivalent Series Resistance: ESR is a very important characteristic to consider when selecting a capacitor. ESR is the internal series resistance associated with
a capacitor that includes lead resistance, internal connections, size and area, material composition, and ambient temperature. Typically, capacitor ESR is measured in
milliohms for ceramic capacitors and can range to more
Step-Down Converter
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 internal slope compensation for
the adjustable and low-voltage fixed versions of the
AAT2500M is 0.24A/μs. This equates to a slope compensation that is 35% of the inductor current down slope for
a 1.5V output and 2.2μH inductor.
m=
0.35 ⋅ VO 0.35 ⋅ 1.5V
A
=
= 0.24
L
2.2µH
µsec
This is the internal slope compensation for the adjustable (VO = 0.6V) version or low output voltage fixed
versions. When externally programming the 0.6V version to 2.5V, the calculated inductance is 3.75μH.
L=
0.35 ⋅ VO
=
m
= 1.5
µsec
0.35 ⋅ VO
≈ 1.5 A ⋅ VO
A
0.24A µsec
µsec
⋅ 2.5V = 3.75µH
A
In this case, a standard 4.7μH value is selected.
For high output voltage fixed versions (2.5V and above),
m = 0.48A/μs. Table 1 displays inductor values for the
AAT2500M fixed and adjustable options.
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.
Configuration
Output Voltage
Inductor
Slope Compensation
0.6V Adjustable With External
Resistive Divider
0.6V to 2.0V
2.5V
0.6V to 2.0V
2.5V to 3.3V
2.2μH
4.7μH
2.2μH
2.2μH
0.24A/μs
0.24A/μs
0.24A/μs
0.48A/μs
Fixed Output
Table 1: Inductor Values.
12
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
The 2.2μH CDRH3D16 series inductor selected from
Sumida has a 59m DCR and a 1.3A DC current rating.
At full load, the inductor DC loss is 9.4mW which gives a
1.5% loss in efficiency for a 400mA, 1.5V output.
Input Capacitor
Select a 4.7μF to 10μF 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 C2. The calculated value varies with input voltage and is a maximum when VIN_BUCK is double the output
voltage (VO).
CIN =
IRMS = IO ·
⎛ VPP
⎞
- ESR · FOSC
⎝ IO
⎠
1
⎛ VPP
⎞
- ESR · 4 · FOSC
⎝ IO
⎠
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.
Figure 3: AAT2500M Evaluation Board
Top Side Layout.
VO ⎛
V ⎞
· 1- O
VIN ⎝
VIN ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load (output) current.
VO ⎛
V ⎞
· 1- O =
VIN ⎝
VIN ⎠
D · (1 - D) =
for VIN = 2 · VO
IRMS(MAX) =
V ⎞
VO ⎛
· 1- O
VIN ⎝
VIN ⎠
VO ⎛
V ⎞ 1
· 1 - O = for VIN = 2 · VO
VIN ⎝
VIN ⎠ 4
CIN(MIN) =
The maximum input capacitor RMS current is:
VO
0.52 =
1
2
IO
2
⎛
V ⎞
· 1- O
The term VIN ⎝ VIN ⎠ appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when VIN_BUCK is twice VOUT_BUCK. 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 AAT2500M. 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 (C2) can be
seen in the evaluation board layout in Figure 3.
Figure 4: AAT2500M Evaluation Board
Bottom Side Layout.
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evaluation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain
measurements can also result.
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead inductance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the
low ESR, ESL bypass ceramic capacitor. This dampens
the high Q network and stabilizes the system.
Output Capacitor
The step-down converter 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 =
14
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.
The maximum output capacitor RMS ripple current is
given by:
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
L · FOSC · VIN(MAX)
2· 3
·
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
Adjustable Output Voltage
Resistor Selection
For applications requiring an adjustable output voltage
(VO or VOUT), the 0.6V version can be externally programmed. Resistors R1 and R2 of Figure 5 program the
output to regulate at a voltage higher than 0.6V. To limit
the bias current required for the external feedback resistor string while maintaining good noise immunity, the
minimum suggested value for R2 is 59k. Although a
larger value will further reduce quiescent current, it will
also increase the impedance of the feedback node, making it more sensitive to external noise and interference.
Table 2 summarizes the resistor values for various output voltages with R2 set to either 59k for good noise
immunity or 221k for reduced no load input current.
3 · ΔILOAD
VDROOP · FOSC
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
VIN1
3
2
1
3
2
1
LX1
LDO Input
LDO Enable
VOUT BUCK
C1
4.7μF
C7
0.01μF
L1
4.7μH
3
2
1
U1
AAT2500M
1
R1
Table 2
C8
n/a
2
3
4
5
C9
n/a
R2
59k
6
LX
Buck Enable
IN_BUCK
PGND
AGND
EN_BUCK
AGND
EN_LDO
AGND
FB_BUCK
AGND
OUT_LDO
IN_LDO
C4
4.7μF
12
11
10
C2
10μF
9
8
7
C3
10μF
GND
GND
VOUT LDO
Figure 5: AAT2500M Evaluation Board Schematic.
VOUT (V)
R2 = 59k
R1 (k)
R2 = 221k
R1 (k)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
75
113
150
187
221
261
301
332
442
464
523
715
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
⎛ VOUT ⎞
⎛ 1.5V ⎞
R1 = V
-1 · R2 = 0.6V - 1 · 59kΩ = 88.5kΩ
⎝ REF ⎠
⎝
⎠
The adjustable version of the AAT2500M, combined with
an external feedforward capacitor (C8 in Figures 2 and
5), delivers enhanced transient response for extreme
pulsed load applications. The addition of the feedforward
capacitor typically requires a larger output capacitor C1
for stability.
Thermal Calculations
There are three types of losses associated with the
AAT2500M 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
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the step-down
converter and LDO losses is given by:
PTOTAL =
IOBUCK2 · (RDSON(HS) · VOBUCK + RDSON(LS) · [VIN - VOBUCK])
PCB Layout
The following guidelines should be used to ensure a
proper layout.
1.
VIN
+ (tsw · FOSC · IOBUCK + IQBUCK + IQLDO) · VIN + IOLDO · (VIN - VOLDO)
IQBUCK is the step-down converter quiescent current and
IQLDO is the LDO quiescent current. The term tsw is used
to estimate the full load step-down converter switching
losses.
2.
3.
For the condition where the buck converter is in dropout
at 100% duty cycle, the total device dissipation reduces
to:
PTOTAL = IOBUCK2 · RDSON(HS) + IOLDO · (VIN - VOLDO)
+ (IQBUCK + IQLDO) · 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.
4.
The input capacitor C2 should connect as closely as
possible to IN_BUCK and PGND, as shown in Figure
5.
The output capacitor and inductor should be connected as closely as possible. The connection of the
inductor to the LX pin should also be as short as possible.
The feedback trace 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. If external feedback
resistors are used, they should be placed as closely
as possible to the FB_BUCK pin. This prevents noise
from being coupled into the high impedance feedback node.
The resistance of the trace from the load return to
GND 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.
Given the total losses, the maximum junction temperature can be derived from the JA for the TSOPJW-12
package which is 110°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
16
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Step-Down Converter Design Example
Specifications
VOBUCK = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA
VOLDO = 3.3V @ 300mA
VIN = 2.7V to 4.2V (3.6V nominal)
FOSC = 1.8MHz
TAMB = 85°C
1.8V Buck Output Inductor
L1 = 1.5
µsec
µsec
⋅ VOBUCK = 1.5
⋅ 1.8V = 2.7µH (see Table 1)
A
A
For Sumida inductor CDRH3D16, 2.2μH, DCR = 59m.
ΔIL1 =
⎛ 1.8V ⎞
VOBUCK ⎛ VOBUCK⎞
1.8V
⋅ 1=
⋅ 1= 260mA
L1 ⋅ F ⎝
VIN ⎠ 2.2µH ⋅ 1.8MHz ⎝ 4.2V⎠
IPKL1 = IOBUCK +
ΔIL1
= 0.4A + 0.130A = 0.53A
2
PL1 = IOBUCK2 ⋅ DCR = (0.4A)2 ⋅ 59mΩ = 9.4mW
1.8V Output Capacitor
VDROOP = 0.2V
COUT =
IRMS =
3 · ΔILOAD
3 · 0.3A
=
= 2.5µF
VDROOP · FOSC 0.2V · 1.8MHz
1
2· 3
·
(VOBUCK) · (VIN(MAX) - VOBUCK)
1
1.8V · (4.2V - 1.8V)
·
= 75mARMS
=
L1 · FOSC · VIN(MAX)
2 · 3 2.2µH · 1.8MHz · 4.2V
Pesr = esr · IRMS2 = 5mΩ · (75mA)2 = 28.1µW
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Input Capacitor
Input Ripple VPP = 25mV
CIN =
IRMS =
1
⎛ VPP
⎞
- ESR · 4 · FOSC
⎝ IOBUCK
⎠
=
1
= 2.42µF
⎛ 25mV
⎞
- 5mΩ · 4 · 1.8MHz
⎝ 0.4A
⎠
IOBUCK
= 0.2ARMS
2
P = esr · IRMS2 = 5mΩ · (0.2A)2 = 0.2mW
AAT2500M Losses
PTOTAL =
IOBUCK2 · (RDSON(HS) · VOBUCK + RDSON(LS) · [VIN - VOBUCK])
VIN
+ (tsw · FOSC · IOBUCK + IQBUCK + IQLDO) · VIN + (VIN - VLDO) · ILDO
=
(0.4A)2 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V])
4.2V
+ (5ns · 1.8MHz · 0.4A + 50μA +125μA) · 4.2V + (4.2V - 3.3V) · 0.3A = 399mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (110°C/W) · 399mW = 129°C
18
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
VOUT (V)
Adjustable Version
(0.6V device)
R1 (kΩ)
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ1
L1 (μH)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
75.0
113
150
187
221
261
301
332
442
464
523
715
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2 or 3.3
4.7
VOUT (V)
Fixed Version
R1 (kΩ)
R2 Not Used
L1 (μH)
0.6-3.3V
0
2.2
Table 3: Evaluation Board Component Values.
Manufacturer
Part Number
Inductance (μH)
Max DC
Current (A)
DCR
(Ω)
Size (mm)
LxWxH
Type
Sumida
Sumida
Murata
Murata
Coilcraft
Coilcraft
Coiltronics
CDRH3D16-4R7
CDRH3D161HP-2R2
LQH32CN4R7M23
LQH32CN2R2M23
LPO3310-222
LPO3310-472
SD3118-4R7
4.7
2.2
4.7
2.2
2.2
4.7
4.7
0.90
1.30
0.45
0.60
1.10
0.80
0.98
0.11
0.059
0.20
0.13
0.15
0.27
0.122
3.8x3.8x1.8
4.0x4.0x1.8
2.5x3.2x2.0
2.5x3.2x2.0
3.3x3.3x1.0
3.3x3.3x1.0
3.1x3.1x1.85
Shielded
Shielded
Non-Shielded
Non-Shielded
Non-Shielded
Non-Shielded
Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
Murata
Murata
Murata
Murata
GRM21BR61A475KA73L
GRM18BR60J475KE19D
GRM21BR60J106KE19
GRM21BR60J226ME39
4.7μF
4.7μF
10μF
22μF
10V
6.3V
6.3V
6.3V
X5R
X5R
X5R
X5R
0805
0603
0805
0805
Table 5: Surface Mount Capacitors.
1. For reduced quiescent current R2 = 221k.
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19
DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Ordering Information
Voltage
Package
Buck Converter
LDO
Marking1
Part Number (Tape and Reel)2
TSOPJW-12
Adj ≥ 0.6V
3.3V
XLXYY
AAT2500MITP-AW-T1
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.
Legend
Voltage
Adjustable
(0.6V)
0.9
1.2
1.5
1.8
1.9
2.5
2.6
2.7
2.8
2.85
2.9
3.0
3.3
4.2
Code
A
B
E
G
I
Y
N
O
P
Q
R
S
T
W
C
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. Contact Sales for availability.
20
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DATA SHEET
AAT2500M
400mA Step-Down Converter and 300mA LDO
Package Information
TSOPJW-12
2.85 ± 0.20
2.40 ± 0.10
0.20 + 0.10
- 0.05
0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC
7° NOM
0.04 REF
0.055 ± 0.045
0.15 ± 0.05
+ 0.10
1.00 - 0.065
0.9625 ± 0.0375
3.00 ± 0.10
4° ± 4°
0.45 ± 0.15
0.010
2.75 ± 0.25
All dimensions in millimeters.
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