AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 1
SystemPower™
Typical Application
2.7V to 5.5V
Input Supply
Enable Buck
Enable LDO
IN_BUCK VOUT_BUC
K
LX
4.7µF
C2
R1
R2
4.7µF
AAT2500M
2.2µH
OUT_LDO
FB_BUCK
VOUT(LDO)
C1
2.2µF
PGNDAGND
IN_LDO
EN_BUCK
EN_LDO
1µF
General Description
The AAT2500M is a high efficiency 400mA step-
down converter and 300mA low dropout (LDO) lin-
ear regulator for applications where power efficien-
cy and solution size are critical. The typical input
power source can be a single-cell Lithium-ion/poly-
mer battery or a 5V or 3.3V power bus.
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 out-
put 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.
Features
•V
IN 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
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
AAT2500M
400mA Step-Down Converter and 300mA LDO
22500M.2007.06.1.0
Pin Descriptions
Pin Configuration
TSOPJW-12
(Top View)
1
2
3
4
5
6
12
11
10
9
8
7
LX
PGND
EN_BUCK
EN_LDO
FB_BUCK
OUT_LDO
IN_BUCK
AGND
AGND
AGND
AGND
IN_LDO
Pin # Symbol Function
1 LX Step-down converter switching node.
2 PGND Power ground for step-down converter.
3 EN_BUCK Enable pin for step-down converter.
4 EN_LDO Enable pin for LDO.
5 FB_BUCK Feedback input pin for step-down converter. Regulated at 0.6V for adjustable version.
6 OUT_LDO LDO power output.
7 IN_LDO Input supply voltage for LDO.
8, 9, 10, 11 AGND Analog signal ground.
12 IN_BUCK Input supply voltage for step-down converter.
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 3
Absolute Maximum Ratings1
Thermal Information
Symbol Description Value Units
θJA Thermal Resistance2110 °C/W
PDMaximum Power Dissipation 909 mW
Symbol Description Value Units
VPInput Voltage -0.3 to 6.0 V
AGND, PGND Ground Pins -0.3 to +0.3 V
VEN, VFB Enable and Feedback Pins VIN + 0.3 V
IOUT Maximum DC Output Current (continuous) 1000 mA
TJOperating Temperature Range -40 to 150 °C
TSStorage Temperature Range -65 to 150 °C
TLEAD Maximum Soldering Temperature (at leads, 10 sec) 300 °C
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at condi-
tions 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.
AAT2500M
400mA Step-Down Converter and 300mA LDO
42500M.2007.06.1.0
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 Min Typ Max Units
Power Supply
VINBUCK, Input Voltage 2.7 5.5 V
VINLDO
VUVLO Under-Voltage Lockout VIN Rising 2.7 V
VIN Falling 2.35 V
IQQuiescent Current VEN = VIN, No Load 130 µA
ISHDN Shutdown Current VEN = GND 1.0 µA
Step-Down Converter
VFB Feedback Voltage Tolerance No Load, TA= 25°C 0.591 0.609 V
`I
OUT = 0 to 400mA; VIN = 2.7 to 5.5V -3 +3 %
ILXLEAK LX Reverse Leakage Current VIN = 5.5V, VLX = 0 to VIN, VEN = GND -1.0 1.0 µA
IFB Feedback Leakage VFB = 1.0 V 0.2 µA
ILIM P-Channel Current Limit 1.2 A
RDS(ON)H High Side Switch On Resistance 0.4 Ω
RDS(ON)L Low Side Switch On Resistance 0.25 Ω
∆VOUT/VOUT Load Regulation ILOAD = 0 to 400mA 0.25 %
∆VOUT/VOUT Line Regulation VIN = 2.7V to 5.5V 0.3 %
FOSC Oscillator Frequency 1.8 MHz
TSStart-Up Time From Enable to Output Regulation 120 µs
LDO (VOUT = 3.3V)
VOUT Output Voltage Tolerance No Load, 25°C 3.24 3.30 3.36 V
VOUT Output Voltage Range IOUT = 0 to 300mA -3 3 %
VIN Input Voltage VOUT + 5.5 V
VDO2
IOUT Output Current 300 mA
ILIM Current Limit 1A
VDO Dropout Voltage3IOUT = 300mA 160 240 mV
∆VOUT/VOUT Load Regulation ILOAD = 0 to 300mA 1.2 %
∆VOUT/VOUT Line Regulation VIN = 3.7V to 5.5V 0.6 %
TSStart-Up Time From Enable to Output Regulation 100 µs
Logic Signals
VEN(L) Enable Threshold Low 0.6 V
VEN(H) Enable Threshold High 1.5 V
IEN(H) Enable Current Consumption -1.0 1.0 µA
TSD
Over-Temperature Shutdown 150 °C
Threshold
THYS
Over-Temperature Shutdown 15 °C
Hysteresis
1. Specification over the -40°C to +85°C operating temperature ranges is assured by design, characterization and correlation with sta-
tistical 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.
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 5
Typical Characteristics
No Load Quiescent Current vs. Input Voltage
(EN_BUCK = EN_LDO = VIN)
Input Voltage (V)
Input Current (µA)
30
50
70
90
110
130
150
2.5 3 3.5 4 4.5 5 5.5 6
25°C
85°C
-40°C
LDO Turn-On Time From Enable
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
Time (40µs/div)
Enable Voltage (top) (V)
Output Voltage (bottom)(V)
0
2
4
6
-1
0
1
2
3
LDO Turn-Off Response Time
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
Time (50ns/div)
Enable Voltage (top) (V)
Output Voltage (bottom)(V)
0
2
4
6
-1.0
0.0
1.0
2.0
3.0
LDO Dropout Voltage vs. Output Current
Output Current (mA)
Dropout Voltage (mV)
0
50
100
150
200
250
0 50 100 150 200 250 300 35
0
85°C
25°C
-40°C
LDO Dropout Characteristics
(VOUT = 3.3V)
Input Voltage (V)
Output Voltage (V)
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7
IOUT = 100mA
IOUT = 200mA
IOUT = 300mA
IOUT = 0.1mA
IOUT = 10mA
IOUT = 50mA
LDO Dropout Voltage vs. Temperature
Temperature (°C)
Dropout Voltage (mV)
0
30
60
90
120
150
180
210
-40 -20 0 20 40 60 80 100
IL = 300mA
IL = 200mA
IL = 100mA
IL = 50mA
AAT2500M
400mA Step-Down Converter and 300mA LDO
62500M.2007.06.1.0
Typical Characteristics
Step-Down Converter Switching
Frequency vs. Input Voltage
(IOUT = 400mA)
Input Voltage (V)
Frequency Variation (%)
-3
-2
-1
0
1
2
3
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.
5
VOUT = 1.2V
VOUT = 1.8V
Step-Down Converter Switching
Frequency vs. Temperature
(VIN = 5V; VOUT = 1.8V)
Temperature (°C)
Switching Frequency (MHz)
1.5
1.6
1.7
1.8
1.9
-40 -20 0 20 40 60 80 10
0
LDO VIH and VIL vs. Input Voltage
Input Voltage (V)
VIH and VIL (V)
0.6
0.7
0.8
0.9
1.0
1.1
1.2
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
VIH
VIL
LDO Load Transient Response
(1mA to 300mA; VIN = 5V; VOUT = 3.3V; COUT = 4.7µF)
Time (100µs/div)
Output Voltage (top) (V)
Output Current (bottom) (A)
2.9
3.1
3.3
3.5
3.7
-0.2
0.0
0.2
0.4
300mA
1mA
LDO Line Transient Response
(VIN = 4V to 5V; VOUT = 3.3V; IOUT = 300mA; COUT = 4.7µF)
Time (40µs/div)
Input Voltage (top) (V)
Output Voltage (bottom) (V)
4
5
3.1
3.3
3.5
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 7
Typical Characteristics
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 1mA)
Time (10µs/div)
Output Voltage (top) (V)
Inductor Current (bottom) (A)
1.79
1.80
1.81
0.0
0.1
0.2
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 400mA)
Time (200ns/div)
Output Voltage (top) (V)
Inductor Current (bottom) (A)
1.79
1.80
1.81
1.82
0.0
0.2
0.4
0.6
Step-Down Converter Efficiency vs. Load
(VOUT = 1.2V; L = 2.2µH)
Output Current (mA)
Efficiency (%)
20
30
40
50
60
70
80
90
100
0.1 1 10 100 1000
VIN = 2.7V VIN = 3.3V
VIN = 4.2V
VIN = 5V
Step-Down Converter DC Regulation
(VOUT = 1.2V; L = 2.2µH)
Output Current (mA)
Output Error (%)
-1.0
-0.5
0.0
0.5
1.0
0.1 1 10 100 1000
VIN = 3.6V to 5.5V
VIN = 2.7V
Step-Down Converter Efficiency vs. Load
(VOUT = 1.8V; L = 2.2µH)
Output Current (mA)
Efficiency (%)
20
30
40
50
60
70
80
90
100
0.1 1 10 100 1000
VIN = 2.7V
VIN = 3.3V
VIN = 5.5V
VIN = 4.2V
Step-Down Converter DC Regulation
(VOUT = 1.8V; L = 2.2µH)
Output Current (mA)
Output Error (%)
-1.0
-0.5
0.0
0.5
1.0
0.1 1 10 100 1000
VIN = 3.3V, 4.2V, 5.5V
VIN = 2.7V
AAT2500M
400mA Step-Down Converter and 300mA LDO
82500M.2007.06.1.0
Typical Characteristics
Step-Down Converter Soft Start
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA; CFF = Open)
Time (50µs/div)
Inductor Current (bottom) (A)
Enable Voltage (top) (V)
Output Voltage (middle) (V)
-2
0
2
4
6
-0.2
0.0
0.2
0.4
Step-Down Converter P-Channel
RDS(ON)H vs. Input Voltage
Input Voltage (V)
RDS(ON)H (mΩ
Ω
)
300
400
500
600
700
2.5 3 3.5 4 4.5 5 5.5 6
120°C
85°C
100°C
25°C
Step-Down Converter N-Channel
RDS(ON)L vs. Input Voltage
Input Voltage (V)
RDS(ON)L (mΩ
Ω
)
100
200
300
400
500
2.5 3 3.5 4 4.5 5 5.5 6
120°C
85°C
100°C
25°C
Step-Down Converter Output
Voltage Error vs. Temperature
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA)
Temperature (°C)
Output Voltage Error (%)
-1.0
-0.5
0.0
0.5
1.0
-50 -25 0 25 50 75 100
Step-Down Converter Output
Voltage Error vs. Temperature
(VIN = 5V; VOUT = 1.2V; IOUT = 400mA)
Temperature (°C)
Output Voltage Error (%)
-1.0
-0.5
0.0
0.5
1.0
-50 -25 0 25 50 75 100
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 9
Typical Characteristics
Step-Down Converter Line Transient Response
(VIN = 4V to 5V; VOUT = 1.8V; IOUT = 400mA; COUT = 4.7µF)
Time (40µs/div)
Input Voltage (top) (V)
Output Voltage (bottom) (V)
4
5
6
1.5
1.6
1.7
1.8
Step-Down Converter Line Regulation
(VOUT = 1.2V; L = 2.2µH)
Input Voltage (V)
Accuracy (%)
-1.00
-0.50
0.00
0.50
1.00
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
IOUT = 0.1mA to 400mA
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7µF)
Time (100µs/div)
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.2
1.4
-0.2
0.0
0.2
0.4
400mA
1mA
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7µF; CFF = 100pF)
Time (100µs/div)
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.2
1.4
-0.2
0.0
0.2
0.4
400mA
1mA
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7µF)
Time (100µs/div)
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.8
2.0
-0.2
0.0
0.2
0.4
400mA
1mA
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7µF; CFF = 100pF)
Time (100µs/div)
Output Voltage (top) (V)
Output Current (middle) (A)
Inductor Current (bottom) (A)
1.8
2.0
-0.2
0.0
0.2
0.4
400mA
1mA
AAT2500M
400mA Step-Down Converter and 300mA LDO
10 2500M.2007.06.1.0
Functional Block Diagram
EN_BUCK
IN_BUCK
LX
PGND
Bias
Oscillator
EN_LDO
AGND
RLDOFB1
VCC
RLDOFB2
IN_LDO
OUT_LDO
FB_BUCK
Control Circuit
VCC
Functional Description
The AAT2500M is a high performance power man-
agement IC comprised of a buck converter and a lin-
ear regulator. The buck converter is a high efficien-
cy 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 sta-
ble 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 impor-
tant for applications that require fast power supply
timing.
The high-speed turn-on capability is enabled
through implementation of a fast-start control cir-
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 11
cuit, 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 shut-
down 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 comprehen-
sive safety system to guard against extreme
adverse operating conditions.
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.
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.
When the regulator is in shutdown mode, an internal
1.5kΩresistor is connected between OUT and GND.
This is intended to discharge COUT when the LDO
regulator is disabled. The internal 1.5KΩresistor
has no adverse impact on device turn-on time.
Step-Down Converter
The AAT2500M buck is a constant frequency peak
current mode PWM converter with internal com-
pensation. 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.
Figure 1: AAT2500M Fixed Output. Figure 2: AAT2500M with Adjustable Step-Down
Output and Enhanced Transient Response.
L1
4. 7µH
C1
4.7µF
C4
4. 7µF
C1
10 µF
AGND
AGND
AGND
AGND
PGND
FB_BUCK
LXVP _BUC K
IN_ LDO
OUT_LDO
EN_ LDO
EN_ BU CK
AAT2500M
1
5
11
10
9
82
6
4
3
7
12
VOUT _BUCK
VOUT_LDO
VIN
L1
4. 7uH
C1
4. 7µF
R1
C4
4. 7µF
C1
10 µF
R2
59 k
C8
100 pF
AGND
AGND
AGND
AGND
PGND
FB_BUCK
LXVP _BUC K
IN_ LDO
OUT_LDO
EN_ LDO
EN_ BU CK
AAT2500M
1
5
11
10
9
82
6
4
3
7
12
VOUT_ BUC
K
VOUT_LDO
VIN
AAT2500M
400mA Step-Down Converter and 300mA LDO
12 2500M.2007.06.1.0
Soft Start
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, non-switching state
with a bias current of less than 1µA.
Low Dropout Operation
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 out-
put 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).
Low Supply
The under-voltage lockout (UVLO) guarantees suf-
ficient VIN bias and proper operation of all internal
circuitry prior to activation.
Fault Protection
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.
Applications Information
LDO Regulator
Input and Output Capacitors: An input capacitor
is not required for basic operation of the linear reg-
ulator. However, if the AAT2500M is physically
located at a reasonable distance from an input
power source, an input capacitor (C3) will be need-
ed for stable operation. Typically, a 1µF or larger
capacitor is recommended for C3 in most applica-
tions. C3 should be located as closely to the input
voltage (IN_LDO) pin as practically possible.
An input capacitor greater than 1µF will offer supe-
rior input line transient response and maximize
power supply ripple rejection. Ceramic, tantalum,
or aluminum electrolytic capacitors may be select-
ed for C3. There is no specific capacitor ESR
requirement for C3. However, for 300mA LDO reg-
ulator output operation, ceramic capacitors are rec-
ommended for C3 due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries 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 regu-
lator has been designed to function with very low
ESR capacitors, ceramic capacitors in the 1.0µF to
10µF range are recommended for best perform-
ance. 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.
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 13
Equivalent Series Resistance: ESR is a very
important characteristic to consider when selecting a
capacitor. ESR is the internal series resistance asso-
ciated with a capacitor that includes lead resistance,
internal connections, size and area, material compo-
sition, and ambient temperature. Typically, capacitor
ESR is measured in milliohms for ceramic capacitors
and can range to more than several ohms for tanta-
lum or aluminum electrolytic capacitors.
Step-Down Converter
Inductor Selection: The step-down converter
uses peak current mode control with slope com-
pensation 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 require-
ments. The internal slope compensation for the
adjustable and low-voltage fixed versions of the
AAT2500M is 0.24A/µsec. This equates to a slope
compensation that is 35% of the inductor current
down slope for a 1.5V output and 2.2µH inductor.
This is the internal slope compensation for the
adjustable (VO= 0.6V) version or low output volt-
age fixed versions. When externally programming
the 0.6V version to 2.5V, the calculated inductance
is 3.75µH.
In this case, a standard 4.7µH value is selected.
For high output voltage fixed versions (2.5V and
above), m = 0.48A/µsec. 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 loss-
es 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 2.2µH CDRH3D16 series inductor selected
from Sumida has a 59mΩDCR and a 1.3A DC cur-
rent 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.
0.35 ⋅ V
O
L = =
≈
1.5
⋅ V
O
= 1.5 ⋅ 2.5V = 3.75µH
m
0.35
⋅
V
O
0.24A
µsec
A
µsec
A
A
µsec
0.35 ⋅ V
O
m = = = 0.24
L
0.35 ⋅ 1.5V
2.2µH
A
µsec
Table 1: Inductor Values.
Configuration Output Voltage Inductor Slope Compensation
0.6V Adjustable With 0.6V to 2.0V 2.2µH 0.24A/µsec
External Resistive Divider 2.5V 4.7µH 0.24A/µsec
Fixed Output 0.6V to 2.0V 2.2µH 0.24A/µsec
2.5V to 3.3V 2.2µH 0.48A/µsec
AAT2500M
400mA Step-Down Converter and 300mA LDO
14 2500M.2007.06.1.0
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple 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).
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er 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:
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.
for VIN = 2 · VO
The term appears in both the input volt-
age ripple and input capacitor RMS current equa-
tions 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 maxi-
mum 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.
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 induc-
tance of these wires, along with the low-ESR
ceramic input capacitor, can create a high Q net-
work 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 meas-
urements 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.
⎛⎞
· 1 -
⎝⎠
VO
VIN
VO
VIN
IO
RMS(MAX)
I2
=
⎛⎞
· 1 - = D · (1 - D) = 0.52 =
⎝⎠
VO
VIN
VO
VIN
1
2
⎛⎞
IRMS = IO · · 1 -
⎝⎠
VO
VIN
VO
VIN
CIN(MIN) = 1
⎛⎞
- ESR · 4 · FOSC
⎝⎠
VPP
IO
⎛⎞
· 1 - = for VIN = 2 · VO
⎝⎠
VO
VIN
VO
VIN
1
4
⎛⎞
· 1 -
⎝⎠
VO
VIN
CIN =
VO
VIN
⎛⎞
- ESR · FOSC
⎝⎠
VPP
IO
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 15
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 capaci-
tance to stabilize the output during large load tran-
sitions 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 out-
put capacitor. During a step increase in load cur-
rent, 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:
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:
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 volt-
age (VOor VOUT), the 0.6V version can be externally
programmed. Resistors R1 and R2 of Figure 5 pro-
gram the output to regulate at a voltage higher than
0.6V. To limit the bias current required for the exter-
nal feedback resistor string while maintaining good
noise immunity, the minimum suggested value for R2
1
23
VOUT · (VIN(MAX) - VOUT)
RMS(MAX)
IL · FOSC · VIN(MAX)
=·
·
COUT =
3 · ∆ILOAD
VDROOP · FOSC
Figure 3: AAT2500M Evaluation Board Top Side. Figure 4: AAT2500M Evaluation Board
Bottom Side.
AAT2500M
400mA Step-Down Converter and 300mA LDO
16 2500M.2007.06.1.0
is 59kΩ. Although a larger value will further reduce
quiescent current, it will also increase the impedance
of the feedback node, making it more sensitive to
external noise and interference. Table 2 summarizes
the resistor values for various output voltages with
R2 set to either 59kΩfor good noise immunity or
221kΩfor reduced no load input current.
The adjustable version of the AAT2500M, com-
bined 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. Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
R2 = 59kΩR2 = 221kΩ
VOUT (V) R1 (kΩ) 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
⎛⎞
⎝⎠
R1 = -1 · R2 = - 1 · 59kΩ = 88.5kΩ
VOUT
VREF
⎛⎞
⎝⎠
1.5V
0.6V
Figure 5: AAT2500M Evaluation Board Schematic.
L1
4.7µH
C1
4.7µF
C2
10µF
VOUT BUCK
VOUT LDO
GND
VIN1
1
2
3
Buck Enable
LX1
GND
Table 2
R1
59k
R2
LX
1
PGND
2
EN_BUCK
3
EN_LDO
4AGND 9
AGND 10
AGND 11
IN_BUCK 12
FB_BUCK
5
OUT_LDO
6
AGND 8
IN_LDO 7
U1
AAT2500M
C4
4.7µF
C3
10µF
1
2
3
LDO Enable
1
2
3
LDO Input
C7
0.01µF
C8
n/a
C9
n/a
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 17
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 switching devices.
At full load, assuming continuous conduction mode
(CCM), a simplified form of the step-down convert-
er and LDO losses is given by:
IQBUCK is the step-down converter quiescent cur-
rent and IQLDO is the LDO quiescent current. The
term tsw is used to estimate the full load step-down
converter switching losses.
For the condition where the buck converter is in
dropout at 100% duty cycle, the total device dissi-
pation reduces to:
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
TSOPJW-12 package which is 110°C/W.
PCB Layout
The following guidelines should be used to ensure
a proper layout.
1. The input capacitor C2 should connect as
closely as possible to IN_BUCK and PGND, as
shown in Figure 5.
2. The output capacitor and inductor should be
connected as closely as possible. The connec-
tion of the inductor to the LX pin should also be
as short as possible.
3. 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 pos-
sible to the FB_BUCK pin. This prevents noise
from being coupled into the high impedance
feedback node.
4. 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 sig-
nal ground and the power ground.
TJ(MAX) = PTOTAL · ΘJA + TAMB
PTOTAL = IOBUCK
2 · RDSON(HS) + IOLDO · (VIN - VOLDO)
+ (IQBUCK + IQLDO) · VIN
PTOTAL
IOBUCK
2 · (RDSON(HS) · VOBUCK + RDSON(LS) · [VIN - VOBUCK])
VIN
=
+ (tsw · FOSC · IOBUCK + IQBUCK + IQLDO) · VIN + IOLDO · (VIN - VOLDO)
AAT2500M
400mA Step-Down Converter and 300mA LDO
18 2500M.2007.06.1.0
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
(see Table 1)
For Sumida inductor CDRH3D16, 2.2µH, DCR = 59mΩ.
1.8V Output Capacitor
VDROOP = 0.2V
1
23
1 1.8V · (4.2V - 1.8V)
2.2µH · 1.8MHz · 4.2V
23
RMS
IL1 · FOSC · VIN(MAX)
= ·
·
3 · ∆ILOAD
VDROOP · FOSC
3 · 0.3A
0.2V · 1.8MHz
COUT = = = 2.5µF
· = 75mARM
·
(VOBUCK) · (VIN(MAX) - VOBUCK)=
Pesr = esr · IRMS2 = 5mΩ · (75mA)2 = 28.1µW
V
OBUCK
V
OBUCK
1.8
V
1.8V
∆I
L1
=
⋅ 1 - = ⋅ 1 - = 260m
A
L1 ⋅ F
V
IN
2.2µH ⋅ 1.8MHz
4.2V
I
PKL1
= I
OBUCK
+ ∆I
L1
= 0.4A + 0.130A = 0.53A
2
P
L1
= I
OBUCK
2
⋅ DCR = (0.4A)
2
⋅ 59mΩ = 9.4mW
⎛
⎝⎞
⎠
⎛
⎝⎞
⎠
L1 = 1.5 ⋅ V
OBUCK
= 1.5 ⋅ 1.8V = 2.7µH
µsec
A
µsec
A
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 19
Input Capacitor
Input Ripple VPP = 25mV
AAT2500M Losses
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (110°C/W) · 399mW = 129°C
PTOTAL
+ (tsw · FOSC · IOBUCK + IQBUCK + IQLDO) · VIN + (VIN - VLDO) · ILDO
IOBUCK
2 · (RDSON(HS) · VOBUCK + RDSON(LS) · [VIN - VOBUCK])
VIN
=
=
+ (5ns · 1.8MHz · 0.4A + 50µA +125µA) · 4.2V + (4.2V - 3.3V) · 0.3A = 399mW
(0.4A)2 · (0.725Ω · 1.8V + 0.7Ω · [4.2V - 1.8V])
4.2V
IOBUCK
RMS
I
P = esr · IRMS
2 = 5mΩ · (0.2A)2 = 0.2mW
2
= = 0.2ARMS
CIN = = = 2.42µF
1
⎛⎞
- ESR · 4 · FOSC
⎝⎠
VPP
IOBUCK
1
⎛⎞
- 5mΩ · 4 · 1.8MHz
⎝⎠
25mV
0.4A
AAT2500M
400mA Step-Down Converter and 300mA LDO
20 2500M.2007.06.1.0
Table 3: Evaluation Board Component Values.
Table 4: Typical Surface Mount Inductors.
Table 5: Surface Mount Capacitors.
Manufacturer Part Number Value Voltage Temp. Co. Case
MuRata GRM21BR61A475KA73L 4.7µF 10V X5R 0805
MuRata GRM18BR60J475KE19D 4.7µF 6.3V X5R 0603
MuRata GRM21BR60J106KE19 10µF 6.3V X5R 0805
MuRata GRM21BR60J226ME39 22µF 6.3V X5R 0805
Inductance Max DC DCR Size (mm)
Manufacturer Part Number (µH) Current (A) (Ω) LxWxH Type
Sumida CDRH3D16-4R7 4.7 0.90 0.11 3.8x3.8x1.8 Shielded
Sumida CDRH3D161HP-2R2 2.2 1.30 0.059 4.0x4.0x1.8 Shielded
MuRata LQH32CN4R7M23 4.7 0.45 0.20 2.5x3.2x2.0 Non-Shielded
MuRata LQH32CN2R2M23 2.2 0.60 0.13 2.5x3.2x2.0 Non-Shielded
Coilcraft LPO3310-222 2.2 1.10 0.15 3.3x3.3x1.0 Non-Shielded
Coilcraft LPO3310-472 4.7 0.80 0.27 3.3x3.3x1.0 Non-Shielded
Coiltronics SD3118-4R7 4.7 0.98 0.122 3.1x3.1x1.85 Shielded
VOUT (V) R1 (kΩ) R1 (kΩ) L1 (µH)
Adjustable Version R2 = 59kΩR2 = 221kΩ1
(0.6V device)
0.8 19.6 75.0 2.2
0.9 29.4 113 2.2
1.0 39.2 150 2.2
1.1 49.9 187 2.2
1.2 59.0 221 2.2
1.3 68.1 261 2.2
1.4 78.7 301 2.2
1.5 88.7 332 2.2
1.8 118 442 2.2
1.85 124 464 2.2
2.0 137 523 2.2 or 3.3
2.5 187 715 4.7
VOUT (V) R1 (kΩ) L1 (µH)
Fixed Version R2 Not Used
0.6-3.3V 0 2.2
1. For reduced quiescent current R2 = 221kΩ.
AAT2500M
400mA Step-Down Converter and 300mA LDO
2500M.2007.06.1.0 21
Ordering Information
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/pbfree.
Voltage
Package Buck Converter LDO Marking1Part Number (Tape and Reel)2
TSOPJW-12 Adj ≥0.6V 3.3V XLXYY AAT2500MITP-AW-T1
Legend
Voltage Code
Adjustable A
(0.6V)
0.9 B
1.2 E
1.5 G
1.8 I
1.9 Y
2.5 N
2.6 O
2.7 P
2.8 Q
2.85 R
2.9 S
3.0 T
3.3 W
4.2 C
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. Contact Sales for availability.
AAT2500M
400mA Step-Down Converter and 300mA LDO
22 2500M.2007.06.1.0
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085
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 specifications or to discontinue any product or service with-
out notice. Except as provided in AnalogicTech’s terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied war-
ranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent,
copyright or other intellectual property right. In order to minimize risks associated with the customer’s applications, adequate design and operating safeguards must be provided by the
customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty.
Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated.
All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.
Package Information
TSOPJW-12
All dimensions in millimeters.
0.20 + 0.10
- 0.05
0.055 ± 0.045 0.45 ± 0.15
7° NOM
4° ± 4°
3.00 ± 0.10
2.40 ± 0.10
2.85 ± 0.20
0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC
0.15 ± 0.05
0.9625
±
0.0375
1.00 + 0.10
- 0.065
0.04 REF
0.010
2.75 ± 0.25