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Advanced Analog Technology, Inc.
October 2007
AAT7301
Product information presented is for internal use within AAT Inc. only. Details are subject to change without notice.
2A 15V STEP-DOWN DC-DC CONVERTER
FEATURES
Buck PWM with Internal PMOS 4V to 15V Input Voltage Range Max. 2A Output Current Under-Voltage Lockout (UVLO) Protection Internal Short Circuit and Thermal Protection Internal Soft-Start Fixed Switching Frequency (380kHz / 47kHz) 2 A Shutdown Current SOP-8 Package
GENERAL DESCRIPTION
The AAT7301 is a 2A 15V step-down converter, which provides an integrated one-channel PWM solution for the power supply of DC-DC system. It offers system engineers the flexibility to tailor-make the power supply circuitry for specific applications. The buck PWM contains enable control signal, error amplifier, PWM comparator, output short driver, circuit under-voltage protection, oscillator,
protection and voltage reference circuit. The AAT7301 contains one buck current mode PWM with internal PMOS. In addition, a digital soft-start is also included to prevent inrush current at startup. AAT7301 comes with a fixed 380kHz oscillator, however, when the feedback voltage is lower than 0.7V, the switching frequency changes to 47kHz and returns to 380kHz after the short-circuit is released.
PIN CONFIGURATION
TYPICAL APPLICATION
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ORDERING INFORMATION
DEVICE TYPE AAT7301 PART NUMBER AAT7301 -S-T PACKAGE S:SOP8 PACKING T: Tape and Reel TEMP. RANGE -40 C to +85 C MARKING AAT7301 XXXXXX MARKING DESCRIPTION Device Type Lot no. (6~9 Digits)
NOTE: All AAT products are lead free and halogen free.
ABSOLUTE MAXIMUM RATINGS
PARAMETER Supply Voltage (VDD, SW) Pin Voltage (IN, ENC, EO) SW Current Operating Temperature Range Storage Temperature Range SYMBOL VDD VI ISW TC TSTORAGE VALUE -0.3 to 15.0 -0.3 to (VDD +0.3) 6.5 -40 C to +85 C -65 C to +150 C UNIT V V A
C C
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ELECTRICAL CHARACTERISTICS
( VDD = 4V~15V, TC = -40 C to 85 C , Tested at TC = 25 C ,VDD = 12V, unless otherwise specified.)
General Item
PARAMETER Input Voltage Range Under Voltage Lockout Under Voltage Lockout Hysteresis Input Current into VDD Shutdown Current into VDD OSC Frequency Short Detection Voltage EN Low Level EN High Level Thermal Shutdown SYMBOL VDD VUVLO VUHYS IDD-OFF IDD-ON ISHDN Normal Operation fOSC VSHORT VIL VIH TSHDN 1.6 160 Short-Circuit VIN Falling 0.67 VIN = 1.5V VIN = 0.85V VDD Falling TEST CONDITION MIN 4 3.33 3.48 150 1 5.5 1.70 380 47.5 0.70 0.73 0.4 10.00 TYP MAX 15 3.63 UNIT V V mV mA mA
A
kHz kHz V V V
C
Buck PWM
PARAMETER IN Regulation Voltage IN to EO Transconductance Maximum Duty Cycle IN Input Leakage Current Current-Sense Amplifier Transresistance SW Leakage Current Switch On-Resistance SW Current Limit Soft-Start Time IL RCS ILSW RON ILIMIT tSS 2.5 VSW = 15V IN = 0V to 1.5V -100.00 SYMBOL VIN Gm TEST CONDITION IN = EO IN = EO MIN 0.88 35 TYP 0.90 80 90 0.01 0.2 0.1 200 3.7 700 5.0 +100.00 MAX 0.92 125 UNIT V
S
%
nA
V/A
A m
A
s
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AAT7301
TYPICAL OPERATING CHARACTERISTICS
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AAT7301
TYPICAL OPERATING CHARACTERISTICS
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AAT7301
PIN DESCRIPTION
PIN NO. 1 2 3 4 5 6 7 8 NAME NC VDD SW GND IN EO ENC NC I/O I I/O I I/O I Not Connected Power Input Switching Node Ground Feedback Input Error Amplifier Compensation Output Enable Control Not Connected FUNCTION
FUNCTION BLOCK DIAGRAM
AAT7301
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AAT7301
TYPICAL APPLICATION CIRCUIT
AAT7301
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AAT7301
DETAILED DESCRIPTION
The AAT7301 is a current-mode buck switch-mode regulator with a fixed switching frequency of 380kHz. It uses an internal P-channel MOSFET switch to step down the input voltage to the regulated output voltage. The converter regulates input voltages from 4V to 15V down to an output voltage as low as 0.9V, and is able to supply up to 2A of load current.
DESIGN PROCEDURE
Programming the Output Voltage
The output voltage for the AAT7301 is programmed using a resistor divider from the output connected to the feedback pins (Figure 1). When setting the output voltage, connect a resistive voltage divider from the output to IN pin and then to GND. Choose the lower-side (IN-to-GND) resistor, then calculate the upper-side (output-to-IN) resistor as follows:
ENC Control Input
The AAT7301 contains built-in pull high logic. The IC is 1.6V). When the voltage is lower than 0.4V, the IC is disabled.
Soft-Start
The AAT7301 has built-in 700s soft-start time. Upon power turn-on, if ENC pin is not pulled low; and VDD is above VUVLO (typ. 3.48V), IN pin will climb from 0V to 0.9V during 700s soft-start time to reduce inrush current.
Short Circuit Protection
When short circuit happens, and the feedback voltage (IN) is less than 0.7V, the switching frequency will change to 47kHz to reduce the power supply from input to output to protect the system. The frequency will switch back to 380kHz after the short circuit is released. Figure 1. Feedback Network
Thermal Shutdown
The AAT7301 includes a thermal-limit circuit that shuts down the IC at approximately +160 C . The part turns on after the IC cools by approximately 20 C .
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enabled even ENC left floating (ENC pin voltage
V RUPPER = RLOWER OUT - 1 V IN
Where VIN is the feedback regulation voltage, 0.9V (typ). Typical values for RLOWER are in the range of 10k 100k . to
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AAT7301
Inductor Selection
A good rule of thumb when choosing the inductance is to allow the peak inductor current in the inductor to be approximately 115% of the maximum load current. Also, make sure that the peak inductor current is below the 2.5A minimum current limit. The inductance value can be calculated by the equation:
V VOUT (1 - OUT ) VI L 0.3 IOUT fSW
Input Capacitor
The input capacitor in a DC-to-DC converter reduces current peaks drawn from the input power source and reduces switching noise in the controller. The impedance of the input capacitor at the switching frequency should be less than that of the input source so high-frequency switching currents do not pass through the input source. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also satisfy. For insuring stable operation the IC must be bypassed with a 0.22 F ceramic capacitor placed close to the VDD.
Where VOUT is the output voltage, VI is the input voltage, fSW is the switching frequency (380kHz), and IOUT is the maximum load current. When the inductance value is determined, then the peak inductor current can be calculated by the equation:
V VOUT (1 - OUT ) VI IL(peak) = IOUT + 2 fSW Lsetting
Output Capacitor
The output capacitor keeps output ripple small and ensures control-loop stability. The output capacitor must also have low impedance at the switching frequency. Ceramic, polymer, and tantalum or low-ESR electrolytic capacitors are suitable. Output ripple with a ceramic output capacitor is approximately as follows:
V VOUT (1 - OUT ) VI 8 L setting COUT fSW 2
Vripple
Where IOUT is maximum load current, VOUT is the output voltage, VI is the input voltage, fSW is the switching frequency (380kHz), and Lsetting is the inductance value. Choose an inductor that does not saturate under the peak inductor current.
Where VOUT is the output voltage, VI is the input voltage, fSW is the switching frequency (380kHz), COUT is the output capacitance, and Lsetting is the inductance value. If the capacitor has significant ESR, the output ripple component due to capacitor ESR is as follows:
Vripple 2 (IL(peak) - IOUT ) x RESR
Schottky Diode Selection
Choose a Schottky diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the peak inductor current.
Where IL(peak) is the peak inductor current, IOUT is maximum load current, and RESR is the capacitor's ESR.
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Compensation
The AAT7301 employs current-mode control, thereby simplifying the control-loop compensation. When the AAT7301 operates with continuous inductor current (typically the case), a RLOAD COUT pole appears in the loop-gain frequency response. To ensure stability, set the compensation RCCC to zero to compensate for the RLOAD COUT pole. Set the loop crossover below the lower of 1/10 the switching frequency (380kHz). The compensation resistor and capacitor are then chosen to optimize control-loop stability. Choose the compensation resistor RC to set the desired crossover frequency fC. Determine the value by the following equation:
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AAT7301
from EO to GND:
CP =
C OUT x RESR RC
If CP is calculated to be < 10pF, it can be omitted.
LAYOUT CONSIDERATIONS
Conductors carrying discontinuous currents and any high-current path should be made as short and wide as possible. The compensation network should be very close to the EO pin and avoid through VIA. The IC must be bypassed with a 0.22 F ceramic capacitor placed close to the VDD. Tie the feedback resistor divider to be very close to output capacitor and far away from the inductor or Schottky diode. Keep the feedback network IN close to the IC. Switching nodes (SW) should be kept as small as possible and should be routed away from high-impedance nodes such as IN.
RC =
2 x COUT x VOUT x RCS x fC Gm x VIN
Where COUT is the output capacitance, VOUT is the output voltage, RCS is the current-sense amplifier transresistance (0.2V/A), Gm is the error amplifier transconductance (80 S ) and VIN is the feedback threshold voltage (0.9V). If the value calculated for RC is greater than 100 k , recommend 100 k . Choose the compensation capacitor CC to set the zero to 1/4 of the crossover frequency.
CC <
COUT x VOUT 4 x RC
If the output filter capacitor COUT has significant ESR, a zero occurs at the following:
ZESR =
1 2 x C OUT x RESR
If ZESR > fC / 4, it can be ignored, as is typically the case with ceramic output capacitors. If ZESR < fC / 4, it should be cancelled with a pole set by capacitor CP connected
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PACKAGE DIMENSION
SOP-8
Symbol A A1 A2 b C D E E1 e L y
Dimensions In Millimeters MIN TYP MAX 1.35 1.60 1.73 0.05 -----0.15 -----1.45 -----0.33 0.41 0.51 0.19 0.20 0.25 4.80 4.85 4.95 5.79 5.99 6.20 3.81 3.91 3.99 1.27 BSC 0.406 0.710 1.270 ----------0.076 0 -----8
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