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AAT2500M
400mA Step-Down Converter and 300mA LDO General Description
The AAT2500M is a high efficiency 400mA stepdown 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. 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 25A no load quiescent current. The LDO is capable of delivering up to 300mA output current. The AAT2500M is available in the Pb-free, spacesaving 12-pin TSOPJW package and is rated over the -40C to +85C operating temperature range.
Features
* *
SystemPowerTM
*
* * * * * * * * *
VIN Range: 2.7V to 5.5V Output Current: -- Step-Down Converter: 400mA -- LDO: 300mA Low Quiescent Current -- 130A 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 -40C to +85C 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
Input Supply 2.7V to 5.5V
4.7F
IN_BUCK
LX FB_BUCK
VOUT_BUCK 2.2H
R1 R2
IN_LDO
1F
OUT_LDO
VOUT(LDO)
C2 4.7F
Enable Buck Enable LDO
EN_BUCK EN_LDO AGND PGND
C1 2.2F
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AAT2500M
400mA Step-Down Converter and 300mA LDO Pin Descriptions
Pin #
1 2 3 4 5 6 7 8, 9, 10, 11 12
Symbol
LX PGND EN_BUCK EN_LDO FB_BUCK OUT_LDO IN_LDO AGND IN_BUCK
Function
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 IN_BUCK AGND AGND AGND AGND IN_LDO
1 2 3 4 5 6
12 11 10 9 8 7
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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
-0.3 to 6.0 -0.3 to +0.3 VIN + 0.3 1000 -40 to 150 -65 to 150 300
Units
V V V mA C C C
Thermal Information
Symbol
JA PD
Description
Thermal Resistance Maximum Power Dissipation
2
Value
110 909
Units
C/W mW
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. 2500M.2007.06.1.0
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AAT2500M
400mA Step-Down Converter and 300mA LDO Electrical Characteristics1
VIN_BUCK = VIN_LDO = 5.0V. TA = -40C to +85C unless noted otherwise. Typical values are at TA = +25C. Symbol Description Conditions Min Typ Max Units
Power Supply VINBUCK, Input Voltage VINLDO VUVLO Under-Voltage Lockout VIN Rising VIN Falling VEN = VIN, No Load VEN = GND No Load, TA = 25C IOUT = 0 to 400mA; VIN = 2.7 to 5.5V VIN = 5.5V, VLX = 0 to VIN, VEN = GND VFB = 1.0 V
2.7
5.5 2.7 2.35 130 1.0
V V V A A V % A A A % % MHz s V % V mA A mV % % s V V A C C
IQ Quiescent Current ISHDN Shutdown Current Step-Down Converter Feedback Voltage Tolerance ILXLEAK LX Reverse Leakage Current IFB Feedback Leakage ILIM P-Channel Current Limit RDS(ON)H High Side Switch On Resistance RDS(ON)L Low Side Switch On Resistance 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 Input Voltage VFB
0.591 -3 -1.0 1.2 0.4 0.25 0.25 0.3 1.8 120 3.24 -3 VOUT + VDO2 300 3.30
0.609 +3 1.0 0.2
ILOAD = 0 to 400mA VIN = 2.7V to 5.5V From Enable to Output Regulation No Load, 25C IOUT = 0 to 300mA
3.36 3 5.5
IOUT Output Current ILIM Current Limit VDO Dropout Voltage3 VOUT/VOUT Load Regulation VOUT/VOUT Line Regulation TS Start-Up Time Logic Signals VEN(L) Enable Threshold Low VEN(H) Enable Threshold High IEN(H) Enable Current Consumption Over-Temperature Shutdown TSD Threshold Over-Temperature Shutdown THYS Hysteresis
IOUT = 300mA ILOAD = 0 to 300mA VIN = 3.7V to 5.5V From Enable to Output Regulation
1 160 1.2 0.6 100
240
0.6 1.5 -1.0 150 15 1.0
1. Specification over the -40C to +85C 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.
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AAT2500M
400mA Step-Down Converter and 300mA LDO Typical Characteristics
LDO Dropout Voltage vs. Temperature
210 3.5
LDO Dropout Characteristics
(VOUT = 3.3V) IOUT = 50mA
Dropout Voltage (mV)
Output Voltage (V)
180 150 120 90 60 30 0 -40
IL = 300mA IL = 200mA IL = 100mA IL = 50mA
-20 0 20 40 60 80 100
3.4 3.3 3.2 3.1 3.0 2.9 2.8 3.0
IOUT = 10mA IOUT = 0.1mA
IOUT = 300mA IOUT = 200mA IOUT = 100mA
3.1 3.2 3.3 3.4 3.5 3.6 3.7
Temperature (C)
Input Voltage (V)
LDO Dropout Voltage vs. Output Current
250
No Load Quiescent Current vs. Input Voltage
(EN_BUCK = EN_LDO = VIN)
150
Dropout Voltage (mV)
Input Current (A)
200 150 100 50 0 0 50 100
85C 25C
130 110 90 70 50 30 2.5
85C
25C
-40C
-40C
150 200 250 300 350
3
3.5
4
4.5
5
5.5
6
Output Current (mA)
Input Voltage (V)
LDO Turn-Off Response Time
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA)
6 6
LDO Turn-On Time From Enable
(VIN = 5V; VOUT = 3.3V; IOUT = 300mA) Output Voltage (bottom)(V) Output Voltage (bottom)(V) Enable Voltage (top) (V)
4 2 0 3 2 1 0 -1
Enable Voltage (top) (V)
4 2 0 3.0 2.0 1.0 0.0 -1.0
Time (50ns/div)
Time (40s/div)
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AAT2500M
400mA Step-Down Converter and 300mA LDO Typical Characteristics
LDO Line Transient Response
(VIN = 4V to 5V; VOUT = 3.3V; IOUT = 300mA; COUT = 4.7F) Output Voltage (bottom) (V) Output Voltage (top) (V) Input Voltage (top) (V)
3.7 3.5 3.3 3.1 2.9 300mA 1mA 0.4 0.2 0.0 -0.2
LDO Load Transient Response
(1mA to 300mA; VIN = 5V; VOUT = 3.3V; COUT = 4.7F) Output Current (bottom) (A)
5 4
3.5 3.3 3.1
Time (40s/div)
Time (100s/div)
LDO VIH and VIL vs. Input Voltage
1.2 1.1
VIH
VIH and VIL (V)
1.0 0.9 0.8 0.7 0.6 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
VIL
Input Voltage (V)
Step-Down Converter Switching Frequency vs. Input Voltage
Frequency Variation (%)
3 2 1 0 -1 -2 -3
Step-Down Converter Switching Frequency vs. Temperature
Switching Frequency (MHz) (VIN = 5V; VOUT = 1.8V)
1.9
(IOUT = 400mA) VOUT = 1.8V VOUT = 1.2V
1.8
1.7
1.6
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
1.5
-40
-20
0
20
40
60
80
100
Input Voltage (V)
Temperature (C)
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AAT2500M
400mA Step-Down Converter and 300mA LDO Typical Characteristics
Step-Down Converter Efficiency vs. Load
(VOUT = 1.8V; L = 2.2H)
100 90 1.0
Step-Down Converter DC Regulation
(VOUT = 1.8V; L = 2.2H)
Output Error (%)
Efficiency (%)
80 70 60 50 40 30 20 0.1
VIN = 2.7V
VIN = 3.3V
0.5
VIN = 3.3V, 4.2V, 5.5V
0.0
VIN = 4.2V VIN = 5.5V
1 10 100 1000
-0.5
VIN = 2.7V
-1.0 0.1 1 10 100 1000
Output Current (mA)
Output Current (mA)
Step-Down Converter Efficiency vs. Load
(VOUT = 1.2V; L = 2.2H)
100 90 1.0
Step-Down Converter DC Regulation
(VOUT = 1.2V; L = 2.2H)
Output Error (%)
Efficiency (%)
80 70 60 50 40 30 20
VIN = 2.7V
VIN = 3.3V
0.5
VIN = 3.6V to 5.5V
VIN = 5V VIN = 4.2V
0.0
-0.5
VIN = 2.7V
0.1
1
10
100
1000
-1.0 0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 1mA) Inductor Current (bottom) (A)
1.82
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 400mA) Inductor Current (bottom) (A) Output Voltage (top) (V)
1.81 1.80 1.79
Output Voltage (top) (V)
1.81 1.80 1.79 0.2 0.1 0.0
0.6 0.4 0.2 0.0
Time (10s/div)
Time (200ns/div)
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AAT2500M
400mA Step-Down Converter and 300mA LDO Typical Characteristics
Step-Down Converter Output Voltage Error vs. Temperature
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA) Output Voltage Error (%)
1.0
Step-Down Converter Output Voltage Error vs. Temperature
(VIN = 5V; VOUT = 1.2V; IOUT = 400mA) Output Voltage Error (%)
1.0
0.5
0.5
0.0
0.0
-0.5
-0.5
-1.0 -50 -25 0 25 50 75 100
-1.0 -50 -25 0 25 50 75 100
Temperature (C)
Temperature (C)
Step-Down Converter P-Channel RDS(ON)H vs. Input Voltage
700
Step-Down Converter N-Channel RDS(ON)L vs. Input Voltage
500
120C 100C RDS(ON)L (m) 85C
120C 100C 85C
RDS(ON)H (m)
600
400
500
300
400
200
25C
300 2.5 3 3.5 4 4.5 5 5.5 6 100 2.5
25C
3 3.5 4 4.5 5 5.5 6
Input Voltage (V)
Input Voltage (V)
Step-Down Converter Soft Start
(VIN = 5V; VOUT = 1.8V; IOUT = 400mA; CFF = Open) Inductor Current (bottom) (A) Enable Voltage (top) (V) Output Voltage (middle) (V)
6 4 2 0 -2 0.4 0.2 0.0 -0.2
Time (50s/div)
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AAT2500M
400mA Step-Down Converter and 300mA LDO Typical Characteristics
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7F)
2.0
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.8V; COUT = 4.7F; CFF = 100pF)
2.0
Output Current (middle) (A) Inductor Current (bottom) (A)
Output Current (middle) (A) Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Voltage (top) (V)
1.8 400mA 1mA 0.4 0.2 0.0 -0.2
1.8 400mA 1mA 0.4 0.2 0.0 -0.2
Time (100s/div)
Time (100s/div)
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7F)
1.4
Step-Down Converter Load Transient Response
(1mA to 400mA; VIN = 5V; VOUT = 1.2V; COUT = 4.7F; CFF = 100pF)
1.4
Output Current (middle) (A) Inductor Current (bottom) (A)
Output Current (middle) (A) Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Voltage (top) (V)
1.2 400mA 1mA 0.4 0.2 0.0 -0.2
1.2 400mA 1mA 0.4 0.2 0.0 -0.2
Time (100s/div)
Time (100s/div)
Step-Down Converter Line Transient Response
(VIN = 4V to 5V; VOUT = 1.8V; IOUT = 400mA; COUT = 4.7F)
6 1.00
Step-Down Converter Line Regulation
(VOUT = 1.2V; L = 2.2H)
Output Voltage (bottom) (V)
Input Voltage (top) (V)
5 4
Accuracy (%)
0.50
IOUT = 0.1mA to 400mA
0.00
1.8 1.7 1.6 1.5
-0.50
-1.00 2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Time (40s/div)
Input Voltage (V)
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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 startup 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. The high-speed turn-on capability is enabled through implementation of a fast-start control cir-
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AAT2500M
400mA Step-Down Converter and 300mA LDO
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 shutdown circuit has no adverse effect on normal device operation. The LDO regulator output has been specifically optimized to function with lowcost, 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. 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 1.5k 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
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 500A load.
AAT2500M
12 VIN C1 10F 7 IN_LDO 3 EN_BUCK 4 EN_LDO 6 VOUT_LDO 2 C4 4.7F PGND AGND OUT_LDO AGND 8 AGND 9 AGND 10 FB_BUCK 11 VP_BUCK LX 5 1
L1 4.7H
12
AAT2500M
1 VP_BUCK C1 10F 7 IN_LDO 3 EN_BUCK 4 EN_LDO AGND 9 OUT_LDO 2 C4 4.7F PGND AGND AGND 8 AGND 10 FB_BUCK 11 LX 5
L1 4.7uH VOUT_BUCK R1
VOUT _BUCK
VIN
C8 100pF R2 59k C1 4.7F
C1 4.7F
6 VOUT_LDO
Figure 1: AAT2500M Fixed Output.
Figure 2: AAT2500M with Adjustable Step-Down Output and Enhanced Transient Response.
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AAT2500M
400mA Step-Down Converter and 300mA LDO
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 1A.
Applications Information
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 1F or larger capacitor is recommended for C3 in most applications. C3 should be located as closely to the input voltage (IN_LDO) pin as practically possible. An input capacitor greater than 1F 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 1.0F to 10F range are recommended for best performance. Applications utilizing the exceptionally low output noise and optimum power supply ripple rejection should use 2.2F 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 0.47F.
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 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).
Low Supply
The under-voltage lockout (UVLO) guarantees sufficient 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 150C with 15C of hysteresis.
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400mA Step-Down Converter and 300mA LDO
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 than several ohms for tantalum or aluminum electrolytic capacitors.
L=
0.35 VO = m
sec 0.35 VO 1.5 A VO A 0.24A sec
= 1.5
sec 2.5V = 3.75H A
In this case, a standard 4.7H value is selected. For high output voltage fixed versions (2.5V and above), m = 0.48A/sec. 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. The 2.2H 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.
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/sec. This equates to a slope compensation that is 35% of the inductor current down slope for a 1.5V output and 2.2H inductor.
m=
0.35 VO 0.35 1.5V A = = 0.24 L 2.2H sec
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 3.75H.
Configuration
0.6V Adjustable With External Resistive Divider Fixed Output
Output Voltage
0.6V to 2.0V 2.5V 0.6V to 2.0V 2.5V to 3.3V
Inductor
2.2H 4.7H 2.2H 2.2H
Slope Compensation
0.24A/sec 0.24A/sec 0.24A/sec 0.48A/sec
Table 1: Inductor Values.
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AAT2500M
400mA Step-Down Converter and 300mA LDO
Input Capacitor
Select a 4.7F to 10F 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). V VO * 1- O VIN VIN
VPP - ESR * FOSC IO
for VIN = 2 * VO
IRMS(MAX) =
VO
IO 2
CIN =
The term VIN * 1 - 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. 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.
VO
VO V 1 * 1 - O = for VIN = 2 * VO VIN VIN 4 CIN(MIN) = 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 10F, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6F. The maximum input capacitor RMS current is: VO V * 1- O VIN VIN
IRMS = IO *
The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load (output) current. VO V * 1- O = VIN VIN 1 2
D * (1 - D) =
0.52 =
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400mA Step-Down Converter and 300mA LDO
Figure 3: AAT2500M Evaluation Board Top Side.
Figure 4: AAT2500M Evaluation Board Bottom Side. 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 4.7F. 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:
VOUT * (VIN(MAX) - VOUT) L * FOSC * VIN(MAX) 2* 3 * 1
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 4.7F to 10F 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 =
IRMS(MAX) =
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 15
3 * ILOAD VDROOP * FOSC
2500M.2007.06.1.0
AAT2500M
400mA Step-Down Converter and 300mA LDO
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.
VOUT 1.5V R1 = V -1 * R2 = 0.6V - 1 * 59k = 88.5k REF
R2 = 59k VOUT (V)
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5
R2 = 221k R1 (k)
75 113 150 187 221 261 301 332 442 464 523 715
R1 (k)
19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187
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.
Table 2: Adjustable Resistor Values For Use With 0.6V Step-Down Converter.
VIN1
3 2 1
LDO Input VOUT BUCK C1 4.7F
LX1
3 2 1
LDO Enable L1 4.7H U1 AAT2500M
1
C7 0.01F
3 2 1
Buck Enable
12 11 10 9 8 7
R1 Table 2
C8 n/a
LX PGND EN_BUCK EN_LDO FB_BUCK OUT_LDO
IN_BUCK AGND AGND AGND AGND IN_LDO
2 3 4 5
C2 10F
R2 59k
C9 n/a
6
C4 4.7F GND
C3 10F GND
VOUT LDO
Figure 5: AAT2500M Evaluation Board Schematic.
16
2500M.2007.06.1.0
AAT2500M
400mA Step-Down Converter and 300mA LDO
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 converter and LDO losses is given by:
IOBUCK2 * (RDSON(HS) * VOBUCK + RDSON(LS) * [VIN - VOBUCK]) VIN
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 connection 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 possible 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 signal ground and the power ground.
PTOTAL =
+ (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. 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. Given the total losses, the maximum junction temperature can be derived from the JA for the TSOPJW-12 package which is 110C/W.
TJ(MAX) = PTOTAL * JA + TAMB
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17
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 FOSC TAMB = 2.7V to 4.2V (3.6V nominal) = 1.8MHz = 85C
1.8V Buck Output Inductor
L1 = 1.5 sec sec VOBUCK = 1.5 1.8V = 2.7H A A
(see Table 1)
For Sumida inductor CDRH3D16, 2.2H, DCR = 59m. 1.8V VOBUCK VOBUCK 1.8V 1= 1= 260mA L1 F VIN 2.2H 1.8MHz 4.2V
IL1 =
IPKL1 = IOBUCK +
IL1 = 0.4A + 0.130A = 0.53A 2
PL1 = IOBUCK2 DCR = (0.4A)2 59m = 9.4mW
1.8V Output Capacitor
VDROOP = 0.2V
3 * ILOAD 3 * 0.3A = = 2.5F VDROOP * FOSC 0.2V * 1.8MHz (VOBUCK) * (VIN(MAX) - VOBUCK) 1 1.8V * (4.2V - 1.8V) * = 75mARMS = L1 * FOSC * VIN(MAX) 2 * 3 2.2H * 1.8MHz * 4.2V 2* 3 1 *
COUT = IRMS =
Pesr = esr * IRMS2 = 5m * (75mA)2 = 28.1W
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2500M.2007.06.1.0
AAT2500M
400mA Step-Down Converter and 300mA LDO
Input Capacitor
Input Ripple VPP = 25mV CIN = 1
VPP - ESR * 4 * FOSC IOBUCK
=
1 = 2.42F 25mV - 5m * 4 * 1.8MHz 0.4A
IRMS =
IOBUCK = 0.2ARMS 2
P = esr * IRMS2 = 5m * (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 * (0.725 * 1.8V + 0.7 * [4.2V - 1.8V])
4.2V
+ (5ns * 1.8MHz * 0.4A + 50A +125A) * 4.2V + (4.2V - 3.3V) * 0.3A = 399mW
TJ(MAX) = TAMB + JA * PLOSS = 85C + (110C/W) * 399mW = 129C
2500M.2007.06.1.0
19
AAT2500M
400mA Step-Down Converter and 300mA LDO
VOUT (V)
Adjustable Version (0.6V device)
R1 (k)
R2 = 59k
R1 (k)
R2 = 221k1
L1 (H)
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 (k)
R2 Not Used
L1 (H)
2.2
0.6-3.3V
0
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida Sumida MuRata MuRata Coilcraft Coilcraft Coiltronics
Part Number
CDRH3D16-4R7 CDRH3D161HP-2R2 LQH32CN4R7M23 LQH32CN2R2M23 LPO3310-222 LPO3310-472 SD3118-4R7
Inductance (H)
4.7 2.2 4.7 2.2 2.2 4.7 4.7
Max DC Current (A)
0.90 1.30 0.45 0.60 1.10 0.80 0.98
DCR ()
0.11 0.059 0.20 0.13 0.15 0.27 0.122
Size (mm) LxWxH
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
Type
Shielded Shielded Non-Shielded Non-Shielded Non-Shielded Non-Shielded Shielded
Table 4: Typical Surface Mount Inductors.
Manufacturer
MuRata MuRata MuRata MuRata
Part Number
GRM21BR61A475KA73L GRM18BR60J475KE19D GRM21BR60J106KE19 GRM21BR60J226ME39
Value
4.7F 4.7F 10F 22F
Voltage
10V 6.3V 6.3V 6.3V
Temp. Co.
X5R X5R X5R X5R
Case
0805 0603 0805 0805
Table 5: Surface Mount Capacitors.
1. For reduced quiescent current R2 = 221k.
20
2500M.2007.06.1.0
AAT2500M
400mA Step-Down Converter and 300mA LDO Ordering Information
Voltage Package
TSOPJW-12
Buck Converter LDO
Marking1
XLXYY
Part Number (Tape and Reel)2
AAT2500MITP-AW-T1
Adj 0.6V
3.3V
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.
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. 2500M.2007.06.1.0
21
AAT2500M
400mA Step-Down Converter and 300mA LDO Package Information
TSOPJW-12
0.10 0.20 + 0.05 -
2.40 0.10
0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC
2.85 0.20
7 NOM 3.00 0.10
0.9625 0.0375 + 0.10 1.00 - 0.065
0.04 REF 0.15 0.05 4 4
0.010
0.055 0.045
0.45 0.15 2.75 0.25
All dimensions in millimeters.
(c) 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 without notice. Except as provided in AnalogicTech's terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to 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.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737- 4600 Fax (408) 737- 4611 22
2500M.2007.06.1.0

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