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1MHz Step-Down Converter/LDO Regulator General Description
The AAT2500 is a member of AnalogicTech's Total Power Management ICTM (TPMICTM) product family. It is a low dropout (LDO) linear regulator and a step-down converter with an input voltage range of 2.7V to 5.5V, making it ideal for applications with single lithium-ion/polymer batteries. The LDO has an independent input and is capable of delivering up to 300mA. The linear regulator has been designed for high-speed turn-on and turn-off performance, fast transient response, and good power supply rejection ratio (PSRR). Other features include low quiescent current and a low dropout voltage. The 400mA step-down converter is designed to minimize external component size and cost while maintaining a low 25A no load quiescent current. Peak current mode control with internal compensation provides a stable converter with a low equivalent series resistance (ESR) ceramic output capacitor for extremely low output ripple. For maximum battery life, the step-down converter increases to 100% duty cycle and has a typical 180mV dropout voltage at 400mA. The output voltage is either fixed or adjustable with an integrated P- and N-channel MOSFET power stage and 1MHz switching frequency. The AAT2500 is available in a 12-pin TDFN33 or STDFN33 package, and is rated over a temperature range of -40C to +85C.
AAT2500
Features
* * * * * * * * * * * * * * * *
SystemPowerTM
VIN Range: 2.7V to 5.5V 300mA LDO Current Output 400mV LDO Dropout Voltage at 300mA High Output Accuracy: 1.5% Fast LDO Line / Load Transient Response 400mA, 96% Efficiency Step-Down Converter 25A No Load Quiescent Current for StepDown Converter Shutdown Current <1A Low RDS(ON) 0.4 Integrated Power Switches Low Dropout 100% Duty Cycle 1MHz Switching Frequency Internal Soft Start Over-Temperature Protection Current Limit Protection Available in TDFN33-12 or STDFN33-12 Package -40C to +85C Temperature Range
Applications
* * * * * * Cellular Phones Digital Cameras Handheld Instruments Microprocessor/DSP Core/IO Power PDAs and Handheld Computers Portable Media Players
Typical Application
VIN = 2.7V to 5.5V
AAT2500 Step-Down Converter Efficiency
VP VLDO ENLDO OUT BYP GND VCC EN LX FB SGND PGND
4 10
C3 10F
3 5
100
2.5V at 400mA
95
2.5V 1.8V
9 6 7 8
2 11 12 1
4.7H
Efficiency (%)
3.3V at 300mA
L1
90 85 80 75 70 65 60 0.1 1 10 100
C4 2.2F
C1 4.7F
C5 10nF
U1 AAT2500
VIN = 3.3V
1000
L1 Sumida CDRH3D16-4R7 C1 Murata GRM219R61A475KE19 C3 Murata GRM21BR60J106KE19
Load Current (mA)
2500.2006.05.1.16
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1MHz Step-Down Converter/LDO Regulator Pin Descriptions
Pin #
1 2 3 4 5 6 7 8 9 10 11
AAT2500
Symbol
PGND LX VP VCC VLDO OUT BYP GND ENLDO EN FB
Function
Step-down converter power ground return pin. Connect to the output and input capacitor return. See section on PCB layout guidelines and evaluation board layout diagram. Power switching node. Output switching node that connects to the output inductor. Step-down converter power stage supply voltage. Must be closely decoupled to PGND. Step-down converter bias supply. Connect to VP. LDO input voltage; should be decoupled with 1F or greater capacitor. 300mA LDO output pin. A 2.2F or greater output low-ESR ceramic capacitor is required for stability. Bypass capacitor for the LDO. To improve AC ripple rejection, connect a 10nF capacitor to GND. This will also provide a soft-start function. LDO ground connection pin. Enable pin for LDO. When connected low, LDO is disabled and consumes less than 1A of current. Step-down converter enable. When connected low, LDO is disabled and consumes less than 1A. Step-down converter feedback input pin. For fixed output voltage versions, this pin is connected to the converter output, forcing the converter to regulate to the specific voltage. For adjustable output versions, an external resistive divider ties to this point and programs the output voltage to the desired value. Step-down converter signal ground. For external feedback, return the feedback resistive divider to this ground. For internal fixed version, tie to the point of load return. See section on PCB layout guidelines and evaluation board layout diagram. Exposed paddle (bottom). Use properly sized vias for thermal coupling to the ground plane. See section on PCB layout guidelines.
12
SGND
EP
Pin Configuration
TDFN/STDFN33-12 (TopView)
PGND LX VP VCC VLDO OUT
1 2 3 4 5 6
12 11 10 9 8 7
SGND FB EN ENLDO GND BYP
2
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1MHz Step-Down Converter/LDO Regulator Absolute Maximum Ratings1
Symbol
VP, VLDO VLX VFB VEN TJ TLEAD
AAT2500
Description
Input Voltages to GND LX to GND FB to GND EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
6.0 -0.3 to VP + 0.3 -0.3 to VP + 0.3 -0.3 to 6.0 -40 to 150 300
Units
V V V V C C
Thermal Information
Symbol
PD JA
Description
Maximum Power Dissipation Thermal Resistance2
Value
2 50
Units
W C/W
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board with exposed paddle connected to ground plane. 2500.2006.05.1.16
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1MHz Step-Down Converter/LDO Regulator Electrical Characteristics1
Symbol
LDO VOUT VIN VDO VOUT/ VOUT*VIN VOUT(Line) VOUT(Load) IOUT ISC IQLDO ISHDN PSRR TSD THYS eN TC
AAT2500
Description
Conditions
Min
Typ Max
Units
VIN = VLDO = VOUT(NOM) + 1V for VOUT options greater than 1.5V. VIN = VLDO = 2.5V for VOUT 1.5V. IOUT = 1mA, COUT = 2.2F, CIN = 1F, TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. TA = 25C -1.5 1.5 Output Voltage Tolerance IOUT = 1mA to 300mA TA = -40C % -2.5 2.5 to 85C Input Voltage VOUT+VDO2 5.5 V 3, 4 Dropout Voltage IOUT = 300mA 400 600 mV Line Regulation Dynamic Line Regulation Dynamic Load Regulation Output Current Short-Circuit Current LDO Quiescent Current Shutdown Current Power Supply Rejection Ratio Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis Output Noise Output Voltage Temperature Coefficient VIN = VOUT + 1V to 5V IOUT = 300mA, VIN = VOUT + 1V to VOUT + 2V, TR/TF = 2S IOUT = 1mA to 300mA, TR <5S VOUT > 1.3V VOUT < 0.4V VIN = 5V, No Load, ENLDO = VIN VIN = 5V; ENLDO = GND, EN = SGND = PGND 1kHz IOUT = 10mA, CBYP = 10nF 10kHz 1MHz 2.5 60 300 600 70 125 1.0 67 47 45 145 12 eNBW = 300Hz to 50kHz 50 22 0.09 %/V mV mV mA mA A A dB C C VRMS ppm/C
1. The AAT2500 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and 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), as long as VIN 2.5V. 3. For VOUT <2.1V, VDO = 2.5 - VOUT. 4. VDO is defined as VIN - VOUT when VOUT is 98% of nominal.
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1MHz Step-Down Converter/LDO Regulator Electrical Characteristics1
Symbol Description Conditions Min
2.7 100 1.8 -3.0 0.6 0.6 25 600 0.45 0.4 1.0 0.2 597 250 0.7 600 615 0.2 1.0 140 15 0.6 1.5 1.0 1.0 1.5 +3.0 4.0 2.5 50 1.0
AAT2500
Typ
Max
5.5 2.6
Units
V V mV V % V A A mA A %/V mV A k MHz C C V V A
Buck Converter Typical values are TA = 25C, VIN = VCC = Vp = 3.6V. VIN Input Voltage VIN Rising VUVLO UVLO Threshold Hysteresis VIN Falling IOUT = 0 to 400mA, VOUT Output Voltage Tolerance VIN = 2.7V to 5.5V Fixed Output Version VOUT Output Voltage Range Adjustable Output Version2 Step-Down Converter ENLDO = GND, No Load, IQBUCK Quiescent Current 0.6V Adjustable Model ISHDN Shutdown Current EN = SGND = PGND, ENLDO = GND ILIM P-Channel Current Limit High Side Switch On RDS(ON)H Resistance Low Side Switch On RDS(ON)L Resistance VIN = 5.5V, VLX = 0 - VIN ILXLK LX Leakage Current EN = SGND = PGND VLinereg Line Regulation VIN = 2.7V to 5.5V FB Threshold Voltage VFB 0.6V Output, No Load, TA = 25C Accuracy IFB FB Leakage Current 0.6V Output RFB FB Impedance >0.6V Output FOSC Oscillator Frequency TA = 25C Over-Temperature Shutdown TSD Threshold Over-Temperature Shutdown THYS Hysteresis Logic Signals VEN(L) Enable Threshold Low VEN(H) Enable Threshold High IEN(H) Leakage Current
1. The AAT2500 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization, and correlation with statistical process controls. 2. For adjustable version with higher than 2.5V output, please consult your AnalogicTech representative. 2500.2006.05.1.16
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C.
AAT2500
LDO Dropout Voltage vs. Temperature
(EN = GND; ENLDO = VIN)
540 480 420 360 300 240 180 120 60 0 -40 -30 -20 -10 0
3.20
LDO Dropout Characteristics
(EN = GND; ENLDO = VIN)
Dropout Voltage (mV)
IL = 300mA
Output Voltage (V)
3.00 2.80 2.60 2.40 2.20 2.00 2.70
IOUT = 0mA
IL = 150mA
IL = 100mA
IOUT = 300mA IOUT = 150mA IOUT = 10mA
2.80
IL = 50mA
10 20 30 40 50 60 70 80 90 100 110 120
IOUT = 100mA IOUT = 50mA
2.90
3.00
3.10
3.20
3.30
Temperature (C)
Input Voltage (V)
LDO Dropout Voltage vs. Output Current
(EN = GND; ENLDO = VIN)
500 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0 50 100 150 200 250 300 0.00 2
LDO Ground Current vs. Input Voltage
(EN = GND; ENLDO = VIN)
Dropout Voltage (mV)
400 350 300 250 200 150 100 50 0
Ground Current (A)
450
85C 25C -40C
IOUT=300mA IOUT=0mA IOUT=150mA IOUT=50mA
IOUT=10mA
2.5
3
3.5
4
4.5
5
Output Current (mA)
Input Voltage (V)
LDO Output Voltage vs. Temperature
(EN = GND; ENLDO = VIN)
1.203 1.202
LDO Initial Power-Up Response Time
(CBYP = 10nF; EN = GND; ENLDO = VIN) VENLDO (5V/div)
Output Voltage (V)
1.201 1.200 1.199 1.198 1.197 1.196 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
VOUT (1V/div) 400s/div
Temperature (C)
6
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C, VIN = VLDO = VCC = VP.
AAT2500
LDO Turn-Off Response Time
(CBYP = 10nF; EN = GND; ENLDO = VIN)
LDO Turn-On Time From Enable (VIN present)
(CBYP = 10nF; EN = GND; ENLDO = VIN) VENLDO = 5V/div
VENLDO (5V/div)
VOUT (1V/div) 50s/div
VOUT = 1V/div 5s/div
VIN = 4V
LDO Line Transient Response
(CBYP = 10nF; EN = GND; ENLDO = VIN)
6 5 3.04
LDO Load Transient Response
(CBYP = 10nF; EN = GND; ENLDO = VIN)
2.90 2.85 2.80 2.75 2.70 2.65 2.60 500
Input Voltage (V)
VIN
Output Voltage (V)
3.03 3.02 3.01 3.00
VOUT
400 300 200 100 0 -100
Output Current (mA)
Output Voltage (V)
4 3 2 1 0
VOUT
2.99 2.98
IOUT 100S/div
100s/div
LDO Load Transient Response 300mA
(CBYP = 10nF; EN = GND; ENLDO = VIN)
2.90 700
LDO Self Noise
(EN = GND; ENLDO = VIN)
2.80 2.70 2.60 2.50 2.40 2.30 2.20 2.10
VOUT
600 500 400 300 200
Noise Amplitude (V/rtHz)
3.00
800
10
Output Current (mA)
Output Voltage (V)
1
0.1
0.01
IOUT
100 0 -100
Band Power: 300Hz to 50kHz = 44.6Vrms 100Hz to 100kHz = 56.3Vrms 0.1 1 10 100 1000 10000
0.001 0.01
10s/div
Frequency (kHz)
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C.
AAT2500
Over-Current Protection
(EN = GND; ENLDO = VIN)
1200
1.250 1.225 1.200 1.175 1.150 1.125 1.100 1.075 1.050 2.5 3.0
LDO ENLDO vs. VIN
Output Current (mA)
1000 800 600 400 200 0 -200
VIH
VIL
Time (50ms/div)
3.5
4.0
4.5
5.0
5.5
Input Voltage (V)
Step-Down Converter Efficiency vs. Load
(VOUT = 2.5V; EN = VIN; ENLDO = GND)
100 2.0
Step-Down Converter Load Regulation
(VOUT = 2.5V; EN = VIN; ENLDO = GND)
VIN = 3.0V
Efficiency (%)
90
VIN = 3.3V
Output Error (%)
1.0
80
VIN = 3.6V
VIN = 3.0V
0.0
70
-1.0
VIN = 3.3V VIN = 3.6V
60 0.1
1.0
10
100
1000
-2.0 0.1 1.0 10 100 1000
Output Current (mA)
Output Current (mA)
Step-Down Converter Efficiency vs. Load
(VOUT = 1.8V; EN = VIN; ENLDO = GND)
100 90 2.0
Step-Down Converter DC Regulation
(VOUT = 1.8V; EN = VIN; ENLDO = GND)
Output Error (%)
VIN = 2.7V
VIN = 3.6V
1.0
Efficiency (%)
80 70 60 50
VIN = 4.2V
VIN = 4.2V
0.0
VIN = 2.7V
-1.0
VIN = 3.6V
0.1
1.0
10
100
1000
-2.0 0.1 1.0 10 100 1000
Output Current (mA)
Output Current (mA)
8
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C.
Step-Down Converter Frequency vs. Input Voltage
(VOUT = 1.8V; EN = VIN; ENLDO = GND)
1.0
AAT2500
Step-Down Converter Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.5V; EN = VIN; ENLDO = GND)
2.0
Frequency Variation (%)
Output Error (%)
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
0.5 0.0 -0.5 -1.0 -1.5 -2.0
1.0
0.0
-1.0
-2.0 -40 -20 0 20 40 60 80 100
Input Voltage (V)
Temperature (C)
Step-Down Converter Switching Frequency vs. Temperature
(VIN = 3.6V; VO = 1.5V; EN = VIN; ENLDO = GND)
0.20
Step-Down Converter Input Current vs. Input Voltage
(VO = 1.8V; EN = VIN; ENLDO = GND)
35
Frequency Variation (%)
Input Current (A)
85C
30
0.10
25C
25
0.00
-0.10
20
-0.20 -40
15
-20 0 20 40 60 80 100
-40C
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Temperature (C)
Input Voltage (V)
Step-Down Converter P-Channel RDS(ON) vs. Input Voltage
(EN = VIN; ENLDO = GND)
750 700 650
750 700 650
Step-Down Converter N-Channel RDS(ON) vs. Input Voltage
(EN = VIN; ENLDO = GND)
RDS(ON) (m)
120C
100C
120C
RDS(ON) (m)
600 550 500 450 400 350 300 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 25C 85C
600 550 500 450 400 350 300 2.5 3.0 3.5 25C 85C
100C
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
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Input Voltage (V)
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C.
AAT2500
Step-Down Converter Load Transient Response
(30mA - 300mA; VIN = 3.6V; VOUT = 1.8V; C1 = 10F; EN = VIN; ENLDO = GND)
2.0 1.9 1.4 1.2 1.0 300mA 30mA 0.8 0.6 0.4 0.2 0.0 -0.2
Step-Down Converter Load Transient Response
Output Voltage (AC Coupled) (top) (V)
(30mA - 300mA; VIN = 3.6V; VOUT = 1.8V; C1 = 10F; C4 = 100pF; EN = VIN; ENLDO = GND)
0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 300mA 30mA 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2
Load and Inductor Current (200mA/div) (bottom)
Load and Inductor Current (200mA/div) (bottom)
Output Voltage (top) (V)
1.8 1.7 1.6 1.5 1.4 1.3 1.2
Time (25s/div)
Time (25s/div)
Step-Down Converter Load Transient Response
(30mA - 300mA; VIN = 3.6V; VOUT = 1.8V; C1 = 4.7F; EN = VIN; ENLDO = GND)
2.0 1.9 1.4 1.2 1.0 300mA 30mA 0.8 0.6 0.4 0.2 0.0 -0.2
Step-Down Converter Line Transient
(VOUT = 1.8V @ 400mA; EN = VIN; ENLDO = GND)
1.90 1.85 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0
Load and Inductor Current (200mA/div) (bottom)
Output Voltage (top) (V)
1.8 1.7 1.6 1.5 1.4 1.3 1.2
Output Voltage (top) (V)
1.80 1.75 1.70 1.65 1.60 1.55 1.50
Input Voltage (bottom) (V)
Time (25s/div)
Time (25s/div)
Step-Down Converter Line Regulation
(VOUT = 1.8V; EN = VIN; ENLDO = GND)
0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 -0.25 -0.3 -0.35 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0
Step-Down Converter Soft Start
(VIN = 3.6V; VOUT = 1.8V; 400mA; EN = VIN; ENLDO = GND)
IOUT = 100mA
Enable and Output Voltage (top) (V)
3.5 3.0
Accuracy (%)
Inductor Current (bottom) (A)
2.5 2.0 1.5 1.0 0.5 0.0 -0.5
IOUT = 10mA IOUT = 400mA
Input Voltage (V)
250s/div
10
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1MHz Step-Down Converter/LDO Regulator Typical Characteristics
Unless otherwise noted, VIN = 5V, TA = 25C.
Step-Down Converter Output Ripple
(VIN = 3.6V; VOUT = 1.8V; 400mA; EN = VIN; ENLDO = GND)
AAT2500
Output Voltage (AC Coupled) (top) (mV)
40 20 0 -20 -40 -60 -80 -100 -120
0.9 0.8
Inductor Current (bottom) (A)
0.7 0.6 0.5 0.4 0.3 0.2 0.1
Time (250ns/div)
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1MHz Step-Down Converter/LDO Regulator Functional Block Diagram
VCC VP
AAT2500
FB
See Note Voltage Reference
Error Amp. Logic
DH
LX
EN SGND VLDO
Control Logic
DL
PGND
OUT
Over-Current Protection Error Amp.
BYP
Voltage Reference
ENLDO
Fast Start Control
GND
Note: Internal resistor divider included for 1.2V versions. For low voltage versions, the feedback pin is tied directly to the error amplifier input.
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1MHz Step-Down Converter/LDO Regulator Functional Description
The AAT2500 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. Designed to operate at 1.0MHz, the converter requires only three external 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. bination 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 (ENLDO) is active high and is compatible with CMOS logic. To assure the LDO regulator will switch on, the ENLDO 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 pin falls below 0.6V. If the enable function is not needed in a specific application, it may be tied to VIN to keep the LDO regulator in a continuously on state. 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.
AAT2500
Linear Regulator
The advanced circuit design of the linear regulator has been specifically optimized for very fast startup and shutdown timing. This proprietary CMOS LDO has also been tailored for superior transient response characteristics. These traits are particularly important for applications that require fast power supply timing. 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 lowcost, low-ESR ceramic capacitors; however, the design will allow for operation over a wide range of capacitor types. A bypass pin has been provided to allow the addition of an optional voltage reference bypass capacitor to reduce output self noise and increase power supply ripple rejection. Device self noise and PSRR will be improved by the addition of a small ceramic capacitor in this pin. However, increased values of CBYPASS may slow down the LDO regulator turn-on time. The regulator comes with complete short-circuit and thermal protection. The com-
Step-Down Converter
The AAT2500 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 for the internally fixed version, and up to 2.5V for the externally adjustable version. 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 96% efficiency. Light load efficiency exceeds 80% at a 500A load.
Soft Start
The AAT2500 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.
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1MHz Step-Down Converter/LDO Regulator
VIN C3 10F
VIN
3 5 9 4 10 2 11 12
7
AAT2500
VP VLDO ENLDO OUT BYP GND
VCC EN LX FB SGND PGND
L1
VOUTBUCK
C3 10F
3 5 9
VP VLDO ENLDO OUT BYP GND
VCC EN LX FB SGND PGND
4 10 2 11 12 1
VOUTLDO
L1 R1
VOUTBUCK
6 7 8
VOUTLDO
6
1
C4 4.7F
C1 4.7F
8
C5 10nF
U1 AAT2500
C4 4.7F
C8 100pF R2 59k
C5 10nF
U1 AAT2500
C1 4.7F
Figure 1: AAT2500 Fixed Output.
Figure 2: AAT2500 with Adjustable Step-Down Output and Enhanced Transient Response.
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 1MHz 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).
Applications Information
Linear Regulator
Input and Output Capacitors: An input capacitor is not required for basic operation of the linear regulator. However, if the AAT2500 is physically located more than three centimeters from an input power source, a CIN capacitor will be needed for stable operation. Typically, a 1F or larger capacitor is recommended for CIN in most applications. CIN should be located as closely to the device VIN 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 CIN. There is no specific capacitor ESR requirement for CIN. However, for 300mA LDO regulator output operation, ceramic capacitors are recommended for CIN 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 OUT and GND. The COUT capacitor 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
2500.2006.05.1.16
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 140C with 15C of hysteresis.
14
1MHz Step-Down Converter/LDO Regulator
the exceptionally low output noise and optimum power supply ripple rejection should use 2.2F or greater for COUT. In low output current applications, where output load is less than 10mA, the minimum value for COUT can be as low as 0.47F. 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.
AAT2500
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 AAT2500 is 0.24A/sec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.5V output and 4.7H inductor.
m=
Bypass Capacitor and Low Noise Applications
A bypass capacitor pin is provided to enhance the low noise characteristics of the LDO. The bypass capacitor is not necessary for operation; however, for best device performance, a small ceramic capacitor in the range of 470pF to 10nF should be placed between the bypass pin (BYP) and the device ground pin (GND). To practically realize the highest power supply ripple rejection and lowest output noise performance, it is critical that the capacitor connection between the BYP pin and GND pin be direct and PCB traces should be as short as possible. DC leakage on this pin can affect the LDO regulator output noise and voltage regulation performance. For this reason, the use of a low leakage, high quality ceramic (NPO or C0G type) or film capacitor is highly recommended.
0.75 VO 0.75 1.5V A = = 0.24 L 4.7H sec
This is the internal slope compensation for the adjustable (0.6V) version or low-voltage fixed versions. When externally programming the 0.6V version to 2.5V, the calculated inductance is 7.5H.
L=
0.75 VO = m
sec 0.75 VO 3 A VO A 0.24A sec
=3
sec 2.5V = 7.5H A
In this case, a standard 10H value is selected. For high-voltage fixed versions (2.5V and above), m = 0.48A/sec. Table 1 displays inductor values for the AAT2500 fixed and adjustable options.
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
4.7H 10H 4.7H 4.7H
Slope Compensation
0.24A/sec 0.24A/sec 0.24A/sec 0.48A/sec
Table 1: Inductor Values.
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1MHz Step-Down Converter/LDO Regulator
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 4.7H CDRH3D16 series inductor selected from Sumida has a 105m DCR and a 900mA DC current rating. At full load, the inductor DC loss is 17mW which gives a 2.8% loss in efficiency for a 400mA, 1.5V output. The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current.
VOBUCK VOBUCK * 1= VIN VIN 1 2
AAT2500
D * (1 - D) =
0.52 =
for VIN = 2 x VOBUCK
IOBUCK 2
VOBUCK
IRMS(MAX) =
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 C. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage.
* 1The term appears in both the VIN VIN input voltage ripple and input capacitor RMS current equations and is a maximum when VOBUCK is twice VIN. This is why the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle.
VOBUCK
CIN =
VOBUCK VOBUCK * 1 VIN VIN
VPP - ESR * FS IOBUCK
The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2500. 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.
1 VOBUCK V * 1 - OBUCK = for VIN = 2 x VOBUCK VIN VIN 4
CIN(MIN) =
1
VPP - ESR * 4 * FS IOBUCK
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:
VOBUCK V * 1 - OBUCK VIN VIN
IRMS = IOBUCK *
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1MHz Step-Down Converter/LDO Regulator
AAT2500
Figure 3: AAT2500 Evaluation Board Top Side.
Figure 4: AAT2500 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:
1 2* 3 VOUT * (VIN(MAX) - VOUT) L * F * VIN(MAX)
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. This dampens the high Q network and stabilizes the system.
Output Capacitor
The 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:
3 * ILOAD VDROOP * FS
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 Resistor Selection
For applications requiring an adjustable output voltage, 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 17
COUT =
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1MHz Step-Down Converter/LDO Regulator
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.
VOUT 1.5V R1 = V -1 * R2 = 0.6V - 1 * 59k = 88.5k REF
AAT2500
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 AAT2500, 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.
LX1 VOUTBUCK C7 0.01F C1 4.7F1 L1 Table 3
1 2 3 4 3 2 1 5 6
U1 AAT2500
PGND LX VP VCC IN OUT SGND FB EN ENLDO GND BYP
12 11 10 9 8 7
C9 n/a
R1 Table 3 C81
C2 10F VIN1
R2 59k
3 2 1
Buck Enable
3 2 1
LDO Input
C3 10F GND
C4 4.7F
C5 10nF
LDO Enable GND
VOUTLDO
Figure 5: AAT2500 Evaluation Board Schematic.
1. For step-down converter, enhanced transient configuration C8 = 100pF and C1 = 10uF.
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1MHz Step-Down Converter/LDO Regulator
Thermal Calculations
There are three types of losses associated with the AAT2500 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
AAT2500
Given the total losses, the maximum junction temperature can be derived from the JA for the TDFN/STDFN33-12 package which is 50C/W.
TJ(MAX) = PTOTAL * JA + TAMB
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 VP and PGND, as shown in Figure 4. 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 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. 5. For good thermal coupling, PCB vias are required from the pad for the TDFN/STDFN paddle to the ground plane. The via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid. 6. LDO bypass capacitor (C5) should be connected directly between pins 7 (BYP) and 8 (GND)
PTOTAL =
+ (tsw * F * 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.
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1MHz Step-Down Converter/LDO Regulator Step-Down Converter Design Example
Specifications
VOBUCK = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA VOLDO = 3.3V @ 300mA VIN FS TAMB = 2.7V to 4.2V (3.6V nominal) = 1.0MHz = 85C
AAT2500
1.8V Buck Output Inductor
L1 = 3 sec sec VO2 = 3 1.8V = 5.4H A A
(see Table 1)
For Sumida inductor CDRH3D16, 4.7H, DCR = 105m.
1.8V VOBUCK VOBUCK 1.8V 1= 1= 218mA VIN 4.7H 1.0MHz 4.2V L1 F
IL1 =
IPKL1 = IOBUCK +
IL1 = 0.4A + 0.11A = 0.51A 2
PL1 = IOBUCK2 DCR = 0.4A2 105m = 17mW
1.8V Output Capacitor
VDROOP = 0.2V
3 * ILOAD 3 * 0.3A = = 4.5F 0.2V * 1MHz VDROOP * FS (VOBUCK) * (VIN(MAX) - VOBUCK) 1 1.8V * (4.2V - 1.8V) * = 63mArms = L1 * F * VIN(MAX) 2 * 3 4.7H * 1.0MHz * 4.2V 2* 3 1 *
COUT = IRMS =
Pesr = esr * IRMS2 = 5m * (63mA)2 = 20W
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1MHz Step-Down Converter/LDO Regulator
Input Capacitor
Input Ripple VPP = 25mV
AAT2500
CIN =
1
VPP - ESR * 4 * FS IOBUCK
=
1 = 4.75F 25mV - 5m * 4 * 1MHz 0.4A
IRMS =
IOBUCK = 0.2Arms 2
P = esr * IRMS2 = 5m * (0.2A)2 = 0.2mW
AAT2500 Losses
PTOTAL = IOBUCK2 * (RDSON(HS) * VOBUCK + RDSON(LS) * [VIN - VOBUCK]) VIN
+ (tsw * F * IOBUCK + IQBUCK + IQLDO) * VIN + (VIN - VLDO) * ILDO
=
0.42 * (0.725 * 1.8V + 0.7 * [4.2V - 1.8V])
4.2V
+ (5ns * 1.0MHz * 0.4A + 50A +125A) * 4.2V + (4.2V - 3.3V) * 0.3A = 392mW
TJ(MAX) = TAMB + JA * PLOSS = 85C + (50C/W) * 392mW = 105C
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1MHz Step-Down Converter/LDO Regulator
VOUT (V)
Adjustable Version (0.6V device)
AAT2500
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
4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 or 6.8 10
VOUT (V)
Fixed Version
R1 (k)
R2 Not Used
L1 (H)
4.7
0.6-3.3V
0
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida Sumida MuRata MuRata MuRata Coilcraft Coilcraft Coiltronics Coiltronics Coiltronics Coiltronics
Part Number
CDRH3D16-4R7 CDRH3D16-100 LQH32CN4R7M23 LQH32CN4R7M33 LQH32CN4R7M53 LPO6610-472 LPO3310-472 SDRC10-4R7 SDR10-4R7 SD3118-4R7 SD18-4R7
Inductance (H)
4.7 10 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7 4.7
Max DC Current (A)
0.90 0.55 0.45 0.65 0.65 1.10 0.80 1.53 1.30 0.98 1.77
DCR ()
0.11 0.21 0.20 0.15 0.15 0.20 0.27 0.117 0.122 0.122 0.082
Size (mm) LxWxH
3.8x3.8x1.8 3.8x3.8x1.8 2.5x3.2x2.0 2.5x3.2x2.0 2.5x3.2x1.55 5.5x6.6x1.0 3.3x3.3x1.0 4.5x3.6x1.0 5.7x4.4x1.0 3.1x3.1x1.85 5.2x5.2x1.8
Type
Shielded Shielded Non-Shielded Non-Shielded Non-Shielded 1mm 1mm 1mm Shielded 1mm Shielded Shielded Shielded
Table 4: Typical Surface Mount Inductors.
1. For reduced quiescent current R2 = 221k.
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1MHz Step-Down Converter/LDO Regulator
Manufacturer
MuRata MuRata MuRata MuRata
AAT2500
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.
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1MHz Step-Down Converter/LDO Regulator Ordering Information
Package
STDFN33-123 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12 TDFN33-12
AAT2500
Voltage
Buck Converter LDO
Marking1
OAXYY NZXYY NYXYY OAXYY ORXYY OOXYY ONXYY OMXYY OWXYY PFXYY SHXYY
Part Number (Tape and Reel)2
AAT2500IFP-AQ-T1 AAT2500IWP-AW-T1 AAT2500IWP-AT-T1 AAT2500IWP-AQ-T1 AAT2500IWP-AP-T1 AAT2500IWP-AN-T1 AAT2500IWP-AI-T1 AAT2500IWP-AG-T1 AAT2500IWP-ET-T1 AAT2500IWP-IP-T1 AAT2500IWP-IW T1
Adj - 0.6V Adj - 0.6V Adj - 0.6V Adj - 0.6V Adj - 0.6V Adj - 0.6V Adj - 0.6V Adj - 0.6V 1.2V 1.8V 1.8V
2.8V 3.3V 3.0V 2.8V 2.7V 2.5V 1.8V 1.5V 3.0V 2.7V 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.
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2500.2006.05.1.16
1MHz Step-Down Converter/LDO Regulator Package Information
TDFN33-12
Index Area (D/2 x E/2)
AAT2500
Detail "B"
3.00 0.05
2.40 0.05
0.3 0.10 0.16 0.375 0.125 0.075 0.075 0.1 REF
Top View
Bottom View
Pin 1 Indicator (optional)
7.5 7.5
+ 0.05 0.8 -0.20
0.229 0.051
0.05 0.05
Option A: C0.30 (4x) max Chamfered corner
Option B: R0.30 (4x) max Round corner
Detail "B"
Side View
Detail "A"
All dimensions in millimeters.
0.23 0.05
0.45 0.05
Detail "A"
3.00 0.05
1.70 0.05
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1MHz Step-Down Converter/LDO Regulator
STDFN33-12
Detail "B"
AAT2500
Index Area (D/2 x E2)
3.00 0.05
2.40 0.05
0.35 0.10 0.16 MIN Detail "A" 3.00 0.05 1.70 0.05 0.075 0.075
Top View
Bottom View
0.45 0.05
Pin 1 Indicator (Optional)
0.1 REF (Optional) 7.5 7.5
0.21 0.05 0.152
0.60 MAX
0.05 0.05
C0.30 (4x) Max Chamfered Corner (Optional)
Detail "B" Detail "A"
Side View
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. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech's standard warranty. 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.
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830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 26
2500.2006.05.1.16

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