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APW7073A

Synchronous Buck PWM Controller

Anpec Electronics 

APW7073A

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW7073A continu-

ally monitors the input supply voltage (V ), the enable

CC

(EN) pin, and the OCSET pin. The supply voltage (VCC)

must exceed its rising POR threshold voltage. The volt-

age at OCSET pin is equal to V less a fixed voltage

IN

drop (VOCSET = VIN- VROCSET). The EN pin can be pulled

high with connecting a resistor to the VCC. The POR

function initiates soft-start operation after VCC, EN, and

OCSET voltages exceed their POR thresholds. For op-

eration with a single +12V power source, VIN and Vcc are

equivalent and the +12V power source must exceed the

rising VCC threshold. The POR function inhibits opera-

tion at disabled status (EN pin low). With both input sup-

plies above their POR thresholds, the device initiates a

soft-start interval.

Soft-Start/EN

The SS/EN pins control the soft-start and enable or

disable the controller. Connect a soft-start capacitor

from SS pin to GND to set the soft-start interval. Figure1.

shows the soft-start interval. When V reaches its Power-

CC

On-Reset threshold (9.5V), internal 30µA current source

starts to charge the capacitor. When the VSS reaches the

enabled threshold about 1.8V, the internal 0.6V reference

starts to rise and follows the VSS; the error amplifier

output (VCOMP) suddenly raises to 1.35V, which is the

valley of the triangle wave of the oscillator, leads the V

OUT

to start-up. Until the V reaches about 4.2V, the internal

SS

reference completes the soft-start interval and reaches

to 0.6V, and then VOUT is in regulation. The SS still rises

to 5.5V and then stops.

TSoft −Start

=

t2

− t1

=

CSS

ISS

⋅ 2.4V

Where:

CSS = external Soft-Start capacitor

ISS = Soft-Start current=30µA

Voltage

V SS

4.2V

1.8V

V OUT

t0

t1

t2

Time

Figure 1. Soft-Start Internal

Over-Current Protection (monitor upper MOSFET)

The APW7073A monitors the voltage across the upper

MOSFET and uses the OCSET pin to set the over-current

trip point.

A resistor (ROCSET) connected between OCSET pin and

the drain of the upper MOSFET will determine the over

current limit. An internal 200µA current source will flow

through this resistor, creating a voltage drop, which will

be compared with the voltage across the upper MOSFET.

When the voltage across the upper MOSFET exceeds the

voltage drop across the ROCSET, an over-current will be

detected. The threshold of the over current limit is

therefore given by:

ILIMIT

=

IOCSET × ROCSET

RDS (ON )

For the over-current, it is never occurred in the normal

operating load range; the variation of all parameters in

the above equation should be determined.

- The MOSFET’s R is varied by temperature and

DS(ON)

gate to source voltage, the user should determine

the maximum RDS(ON) in manufacturer’s datasheet.

- The minimum IOCSET (170µA) and minimum ROCSET

should be used in the above equation.

- Note that the ILIMIT is the current flow through the

upper MOSFET; I must be greater than maximum

LIMIT

output current add the half of inductor ripple current.

Copyright © ANPEC Electronics Corp.

10

Rev. A.5 - Nov., 2012

www.anpec.com.tw

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