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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Feature
4.75V to 18V Supply Voltage Operating
Range. 300KHz Fixed Switching Frequency. Voltage-Mode PWM Step-Down Regulator. Built-in Power MOSFET, Rds(on)100m. 2A Output Current, Up to 91.9% Efficiency 1.225V1.5% Internal Reference Shutdown Current less than 25uA. Built-in Soft-Start. Cycle-by-Cycle Current-Limit Protection. Short Circuitry Protection. Under-Voltage Lockout Protection. Output Voltage Adjustable up to 16V. SOP-8 Package.
Description
AT1792 is a voltage-mode PWM step-down DC-DC converter capable of driving up to 2A loads over a wide input supply range with excellent line and load regulation characteristics. High efficiency is obtained through the use of a low Rds(on) P-channel power switch.
It uses a fixed switching frequency up to 300KHz thus allowing smaller sized components. The output voltage level is user-programmable via an external resistive voltage divider. The built-in soft-start can reduce inrush current on the input source at turn on. The device also has built-in cycle-by-cycle current-limit, under-voltage lockout and an ON/OFF logic-control that can power down the regulator to a shutdown mode. In shutdown mode the regulator draws less than 20uA of supply current. When a short circuit is detected on the output , the device immediately change switching frequency to 32 KHz and duty cycle is limited to 5% to protect IC from damage. AT1792 is available in SOP-8 package.
Application
Distributed Power Systems Battery Charger. Palmtop Computers, PDAs. DSL Modems
Block Diagram
EN
Regulator
Under-voltage lockout Current Sense Amplifier Current Sense + Contol Logic P-MOS
VIN
SCP
OSC 32/600KHz PWM Comparator Error Amplifier + 1.225V FB SoftStart
SW
Aimtron reserves the right without notice to change this circuitry and specifications.
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+
GND
COMP
AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Pin Configuration
SOP-8 TOP- VIEW 1 2 3 4 NC IN SW GND NC EN 8 7
COMP 6 FB 5
Ordering Information
Part number AT1792X_GRE
Package SOP-8, Green
Marking 792, Date Code with one bottom line
: Date Code
*For more marking information, contact out sales representative directly
Pin Descriptions
Pin No. Symbol I/O Description
1 2 3 4 5 6 7 8
N/C IN SW GND FB COMP EN N/C P O P I O I
No Connect Power Input. IN supplies power to the IC and Step-down converter switch. IN must be bypassed with a ceramic 0.1uF capacitor. Power Switching Output. Ground. Feedback Input. Connect a resistive divider from switching output to FB to ground. Error Amplifier Output. A series RC network connected to this pin compensates AT1792. Enable input. EN is a digital input that turns the regulator on/off. Drive EN high to turn on the regulator, drive it low to turn it off. No Connect
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Absolute Maximum Ratings *1
Parameter IN voltage EN voltage SW voltage FB, COMP voltage Quiescent current HBM ESD Susceptibility*3 MM Rated Value -0.3 to 20 -0.3 to 5 -1 to IN+1 -0.3 to 6 5 2 200 unit V V V V mA KV V
1. Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2. Measured on approximately 1" square of 1 oz. copper FR4 board. 3. Device are ESD sensitive. Handling precaution recommended. The Human Body model is a 100pF capacitor discharged through a 1.5K resistor into each pin.
Recommended Operation Conditions
Parameter Power supply voltage Operating temperature Symbol Min. IN Top 4.75 -20 Values Typ. 12 +25 Max. 18 +85 Unit V
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Electrical Characteristics
(IN=12V, EN=5V, Ta=+25, unless otherwise noted)
Parameter Input operating voltage Input under-voltage lockout threshold UVLO hysteresis Quiescent Current Shutdown Current Oscillator Frequency Short-Circuit Frequency Feedback Regulation voltage FB Input Bias Current Current limit Maximum Duty Cycle Minimum Duty Cycle SW Switch On-Resistance SW Leakage Current Load Regulation Line Regulation Error Amplifier Trans-conductance Soft-Start Time EN Threshold Voltage EN Pull Up Current
Symbol IN VUVLO
Test Condition IN Rising
Min. 4.75 3.8 -
Typ. 12 4.2 700 3.5 25 300 32
Max. Units 18 4.6 50 330 V V mV mA uA KHz KHz V nA A % % m A % 400 %/V A/V ms V A
Iin ISHDN fSW VFB IFB DMAIN DMIIN RSW-DS(ON) ISW
VFB=1.5V EN =GND Ta=+25 FB=0V Ta=+25 VFB=1.225V Min. On-time>0.5s FB=1.0V FB=1.5V ISW=2A IN=13.5V,VSW=GND , EN=0V 0 A-
1.206 1.225 1.244 -50 2.5 0 100 0.1 0.2 200 1.0 300 10 1.5 -50 100 20
Hysteresis 150mV EN=0V
2
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Typical characteristics
Load Transient Response VIN=12V, VOUT=5.0V, L=22uH, IOUT=500mA to 2A C4=0.1F C4=1F
CH2:VOUT(AC), CH4:IOUT, Time:200s/div
Load Transient Response VIN=18V, VOUT=5.0V, L=22uH, IOUT=500mA to 2A C4=0.1F C4=1F
CH2:VOUT(AC), CH4:IOUT, Time:200s/div
7F, No.9, PARK AVENUE II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C. Tel: 886-3-563-0878 Fax: 886-3-563-0879 WWW: http://www.aimtron.com.tw 11/21/2007 REV:0.5 Email: service@aimtron.com.tw 5
AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator
Load Transient Response VIN=12V, VOUT=3.3V, L=22uH, IOUT=500mA to 2A C4=0.1F C4=1F
CH2:VOUT(AC), CH4:IOUT, Time:200s/div
Load Transient Response VIN=18V, VOUT=3.3V, L=22uH, IOUT=500mA to 2A C4=0.1F C4=1F
CH2:VOUT(AC), CH4:IOUT, Time:200s/div
7F, No.9, PARK AVENUE II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C. Tel: 886-3-563-0878 Fax: 886-3-563-0879 WWW: http://www.aimtron.com.tw 11/21/2007 REV:0.5 Email: service@aimtron.com.tw 6
AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Typical Application Circuit
VIN 4.75V~15V 15V~18V
C1 220F 470F
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator General Description The AT1792 features a PWM step-down converter operating with a fixed switching frequency of 300 KHz and uses internal power MOSFET to provide maximum efficiency. The output voltage of the converter can be set as low as 1.225V with external resistive divider. When trans-conductance signal is higher than saw-tooth wave, then PWM comparator will produce ON-time signal to set internal flip-flop, which turns on the switching power MOSFET. The external inductor current ramps up linearly, storing energy in a magnetic field. Once peak current of flowing through switching power MOSFET over current-limited threshold or trans-conductance signal level is lower than saw-tooth wave, the switching power MOSFET will turns off, the flip-flop resets, and external schottky diode turns on. This forces the current through the inductor to ramp back down, transferring the energy stored in the magnetic field to the output capacitor and load. To add higher flexibility to the selection of external component values, the device uses external loop compensation. Enable Control
Digital logic of EN provides an electrical ON/OFF control of the power supply. Connecting this pin to ground or to any voltage less than 1.0V will completely turn OFF the regulator. In this state, current drain from the input supply is only 20A, the internal reference, error amplifier, comparators, and biasing circuitry turn off . When EN control is not required that it should be left open circuited.
Soft-Start
AT1792 is built-in soft-start function. When power up, after input voltage is above the under-voltage lockout threshold, then soft-start allows a gradual increase of COMP voltage to get to a steady state operating point. Therefore, reducing input surge currents. During this time, the COMP voltage increases and the output duty cycle is allowed to increase from zero to the value required for regulation. The maximum load current is available after the soft-start cycle is completed. When EN pin is taken low, the soft-start capacitor is discharge to ground.
Short-Circuit Protection If feedback voltage of the regulator falls below 0.85V, the oscillator clock switched operating frequency to 32KHz and duty cycle is limited to 5% to reduce input power delivered to output. If the short circuitry is not removed, the protection circuitry will activate sustainability. When short-circuit problem is to eliminate, oscillator clock will switch back to 300KHz automatically.
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Cycle-by-Cycle Over-Current Protection The AT1792 provides cycle-by-cycle over-current protection. Current limit is accomplished using a separate dedicated comparator. If the current sense amplifier output voltage is larger than current-limited threshold level, it will be immediately turned off power MOS. The current-limit feature protects against a hard short or over current fault at the output.
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator Applications Information External components of step-down converter can be designed by performing simple calculations. It must be to follow regulation by the output voltage and the maximum load current, as well as maximum and minimum input voltages. Begin by selecting an inductor value. Once L is know, choose the diode and capacitors. Inductor Inductor selection depends on input voltage, output voltage, maximum current, switching frequency and availability of inductor values. The following buck circuit equations are useful in choosing the inductor values based on the application. Choose an inductor that does not saturate under the maximum rating load conditions. The magnitude of inductance is selected to maintain a peak to peak ripple current of 30% of the maximum load current. The peak inductor current is given by: I Lpeak = I LAVG + I LAVG = D= VO Vin I in D I L 2
D is the MOSFET turn on ratio where: IL is the inductor peak-to-peak current ripple and is decided by: (V - VO ) x D V - VO D I L = in x L = in L f OSC I L x f OSC fOSC is the switching frequency. The inductor should be chosen to be able to handle this current and inductor saturation current rating should be greater than IPEAK. Diode selection When the power switch turns off, the current through the inductor continues to flow. The path for this current is through the diode connected between the switch output and ground. This forward biased diode must has a minimum voltage drop and recovery times. Schottky diode is recommended and it should be able to handle those current. As usual, the reverse voltage rating of the diode should be at least 1.3 times greater than the maximum input voltage, and current rating is greater than the maximum load current.
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator
Feedback Resistor Network An external resistor divider is required to divide the output voltage down to the nominal reference voltage. Current drawn by the resistor network should be limited to maintain the overall converter efficiency. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise being coupled into the feedback pin. A resistor network in the order of 100k is recommended. The buck converter output voltage is determined by the following relationship:
R VO = VREF x 1 + 3 R 2
where VREF =1.225V as specified. Input capacitor An input capacitor helps to provide additional current to the power supply as well as smooth input voltage variations in high current switching regulators. When selecting an input capacitor, a low ESR capacitor is required to keep the noise at the IC to a minimum. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose an input capacitor with maximum voltage rating is 1.3 times greater than the maximum input voltage, and RMS current rating is equal to one-half of the maximum dc load current. It may be necessary in some designs to add a small valued ceramic type capacitor in parallel with the input capacitor to prevent any ring.
Output Capacitor The output capacitor acts to smooth the dc output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple is estimated to be: (1 - D) x VO Vripple = 2 8 x L x C OUT x f OSC For low ESR ceramic capacitors, the output ripple is dominated by the charging or discharging of the output capacitor.
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator In the case of low-ESR electrolytic capacitors, Output ripple voltage drop caused by the switching current through the ESR of the output capacitor. The output ripple is estimated as: V RIPPLE I L x RESR Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero. These frequency response effects together with the internal frequency compensation circuitry of AT1792 modify the gain and phase shift of the closed loop system.
Compensation The step-down loop can be compensated by adjusting the external components connected to the COMP pin. The COMP pin is connected to the output of the internal trans-conductance error amplifier. The compensation capacitor adjusts the low frequency gain , and the series resistor value adjusts the high frequency gain. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The system has 2 poles and one zero of importance, 2 poles consist of compensation capacitor and output capacitor. One zero is produced by the compensation capacitor and the compensation resistor. If electrolytic capacitor with relatively high ESR is used, the zero due to the capacitance and ESR of the output capacitor can be compensated by a third pole set by the compensation resistor and another compensation capacitor connected form COMP pin to ground.
PCB layout guidelines
Careful printed circuit layout is extremely important to avoid causing parasitical capacitance and line inductance. The following layout guidelines are recommended to achieve optimum performance. Please the buck converter diode and inductor close to the SW pin and no via. Please ceramic bypass capacitors near the input pin. Locate all feedback resistive dividers as close to their respective feedback pins as possible. Use wide traces and trace length is short as possible.
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator
Outline 8-pin SOP
SYMBOL A B C D E F H J K M e1
INCHES MIN MAX 0.188 0.197 0.149 0.158 0.228 0.244 0.050 BSC 0.013 0.020 0.004 0.010 0.053 0.069 0.011 0.019 0.007 0.010 0.016 0.050 45O 80 00
MILLIMETERS MIN MAX 4.80 5.00 3.80 4.00 5.80 6.20 1.27 BSC 0.33 0.51 0.10 0.25 1.35 1.75 0.28 0.48 0.19 0.25 0.40 1.27 45O 0 0 80
NOTES -
-
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AT1792
Preliminary Product Information 2A/18V/300KHz Step-Down Voltage Regulator
Reflow Profiles
Profile Feature
Average ramp-up rate (TL to TP) Preheat -Temperature Min(Tsmin) -Temperature Max (Tsmax) -Time (min to max)(ts) Tsmax to TL -Ramp-up Rate Time maintained above: -Temperature (TL) -Time (tL) Peak Temperature(TP) Time within 5C of actual Peak Temperature (tP) Ramp-down Rate Time 25C to Peak Temperature
Sn-Pb Eutectic Assembly Large Body Small Body Pkg. thickness Pkg. thickness <2.5mm or Pkg. 2.5mm or Pkg. 3 volume 350mm3 volume <350mm 3C/second max.
Pb-Free Assembly Large Body Small Body Pkg. thickness Pkg. thickness <2.5mm or Pkg. 2.5mm or Pkg. 3 volume 350mm3 volume <350mm 3C/second max.
100C 150C 60-120 seconds
150C 200C 60-180 seconds 3C/second max.
183C 60-150 seconds 225+0/-5C 10-30 seconds 240+0/-5C 10-30 seconds
217C 60-150 seconds 245+0/-5C 10-30 seconds 250+0/-5C 20-40 seconds
6C/second max. 6 minutes max.
6C/second max. 8 minutes max.
*All temperatures refer to topside of the package, measured on the package body surface.
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