APW7074 Ver la hoja de datos (PDF) - Anpec Electronics

Número de pieza

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Fabricante

APW7074

Synchronous Buck PWM Controller

Anpec Electronics 

APW7074

Function Descriptions

Power On Reset (POR)

Voltage

The Power-On Reset (POR) function of APW7074

continually monitors the input supply voltage (VCC),

the enable (EN) pin and OCSET pin. The supply

voltage (VCC) must exceed its rising POR threshold

voltage. The voltage at OCSET pin is equal to V less

IN

a fixed voltage drop (Vocset = V - V ). EN pin can

IN ROCSET

be pulled high with connecting a resistor to VCC. The

POR function initiates soft-start operation after VCC,

EN and OCSET voltages exceed their POR thresholds.

For operation 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 operation at disabled status (EN pin

low). With both input supplies 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 VCC reaches its

Power-On-Reset threshold (9.5V), internal 10uA current

source starts to charge the capacitor. When the SS

reaches the enabled threshold about 1.8V, the inter-

nal 0.8V reference starts to rise and follows the SS;

the error amplifier output (COMP) suddenly raises to 1.

35V, which is the valley of the triangle wave of the

oscillator, leads the V to start up. Until the SS

OUT

reaches about 4.2V, the internal reference completes

the soft-start interval and reaches to 0.8V; 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=10uA

VSS

4.2V

1.8V

VOUT

t0

t1

t2

Figure 1. Soft-Start Internal

Time

Over-Current Protection (monitor upper MOSFET)

The APW7074 provides two manners to protect the

converter from abnormal output load; one monitors

the voltage across the upper MOSFET and use the

OCSET pin to set the over-current trip point, the other

monitors the voltage across the lower MOSFET by

comparing with an internal reference voltage (0.27V).

A resistor (R ) connected between OCSET pin and

OCSET

the drain of the upper MOSFET will determine the over

current limit. An internal 200uA 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

R DS (ON )

For the over-current is never occurred in the normal

operating load range; the variation of all parameters in

the above equation should be determined.

Copyright © ANPEC Electronics Corp.

11

Rev. A.2 - Feb., 2006

www.anpec.com.tw

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