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APW7079

Low-Supply-Current Synchronous Step-up DC-DC Converter

Anpec Electronics 

APW7079

Function Description

Control Scheme

The converter monitors the output voltage. When the in-

ternal feedback voltage falls below the reference voltage,

the main switch turns on and the inductor current ramps

up. The main switch turns off when the current reaches

the peak current limit of typical 600mA. The second crite-

rion that turns off the switch is the maximum on-time of

4µs (typical). As the main switch is turned off, the syn-

chronous switch is turned on and delivers the current to

the output. The main switch remains off for a minimum of

900ns (typical), or until the internal feedback voltage drops

below the reference voltage. By the control scheme with

low quiescent current of 11µA (typical), the converter gets

high efficiency over a wide load range.

Start-Up

A startup oscillator circuit is integrated in the APW7079.

When the power is applied to the device, the circuit pumps

the output voltage high. Once the output voltage reaches

1.4V (typ), the main DC-DC circuitry turns on and boosts

the output voltage to the final regulation point.

Synchronous Rectification

The internal synchronous rectifier eliminates the need

for an external Schottky diode, thus reducing cost and

board space. During the cycle off-time, the P-channel

MOSFET turns on and shunts the MOSFET body diode.

As a rewsult, the synchronous rectifier significantly im-

proves efficiency without the addition of an external

component. Conversion efficiency can be as high as 92%.

Over-Temperature Protection

The over-temperature circuit limits the junction tempera-

ture of the APW7079. When the junction temperature ex-

ceeds 150°C, a thermal sensor turns off the power

MOSFETs, allowing the devices to cool. The thermal sen-

sor allows the converter to start a start-up process and

regulate the output voltage again after the junction tem-

perature cools by 40°C.The OTP is designed with a 40°C

hysteresis to lower the average TJ during continuous ther-

mal overload conditions, increasing lifetime of the device.

Copyright © ANPEC Electronics Corp.

8

Rev. A.3 - Oct., 2008

www.anpec.com.tw

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