APW8713 datasheet(15/24 Pages) ANPEC

15 / 24 page

Rev. A.3 - Sep., 2013

APW8713

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Function Description (Cont.)

The APW8713 integrates soft-start circuits to ramp up the

output voltage of the converter to the programmed regu-

lation set point at a predictable slew rate. The slew

rate of output voltage is internally controlled to limit the

inrush current through the output capacitors during soft-

start process. When the EN pin is pulled above the rising

EN threshold voltage, the device initiates a soft-start pro-

cess to ramp up the output voltage.

During soft-start stage before the PGOOD pin is ready,

the under voltage protection is prohibited. The over volt-

age and current limit protection functions are enabled. If

the output capacitor has residue voltage before startup,

both low-side and high-side MOSFETs are in off-state

until the soft start voltage equal the V

FB voltage. This will

ensure the output voltage starts from its existing voltage

level.

In the event of under-voltage, over-voltage, over-tempera-

ture or shutdown, the chip enables the soft-stop function.

The soft-stop function discharges the output voltages by

low side turns MOSFET on linearly.

Power Good Indicator

POK is actively held low in shutdown and soft-start status.

In the soft-start process, the POK is an open-drain. When

the soft-start is finished, the POK is released. In normal

operation, the POK window is from 90% to 125% of the

converter reference voltage. When the output voltage has

to stay within this window, POK signal will become high.

When the output voltage outruns 90% or 125% of the

target voltage, POK signal will be pulled low immediately.

In order to prevent false POK drop, capacitors need to

parallel at the output to confine the voltage deviation with

severe load step transient.

Under-Voltage Protection (UVP)

In the process of operation, if a short circuit occurs, the

output voltage will drop quickly. When load current is big-

ger than current limit threshold value, the output voltage

will fall out of the required regulation range. The under-

voltage protection circuit continually monitors the FB volt-

age after soft-start is completed. If a load step is strong

enough to pull the output voltage lower than the under

voltage threshold, the under voltage threshold is 70% of

the nominal output voltage, the internal UVP delay counter

starts to count. After 16ms de-bounce time, the device

turns off both high side and low-side MOSEFET with

latched. Toggling enable pin to low, or recycling VIN, will

clear the latch and bring the chip back to operation.

Soft-Start

The APW8713 provides the programmed soft-start func-

tion to limit the inrush current. The soft-start time can be

programmed by the external capacitor between SS and

GND. Typical charge current is 10uA, and the soft-start

time can be calculated by the following formula:

Over-Voltage Protection (OVP)

The over voltage function monitors the output voltage by

FB pin. Should the FB voltage increase over 125% of the

reference voltage due to the high-side MOSFET failure or

for other reasons, the over voltage protection comparator

designed with a 3

µs noise filter will force the low-side

MOSFET gate driver fully turn on and latch high. This ac-

tion actively pulls down the output voltage.

This OVP scheme only clamps the voltage overshoot,

and does not invert the output voltage when otherwise

activated with a continuously high output from low-side

MOSFET driver. It’s a common problem for OVP schemes

with a latch. Once an over-voltage fault condition is set, it

can only be reset by toggling EN or VIN power-on-reset

signal.

(nF)

330

(

Current Limit

The current limit circuit employs a "valley" current-sens-

ing algorithm (See Figure 1). The APW8713 uses the

low-side MOSFET’s R

DS(ON) of the synchronous rectifier

as a current-sensing element. If the magnitude of the

current-sense signal at LX pin is above the current-limit

threshold 11A(minimum), the PWM is not allowed to ini-

tiate a new cycle. The actual peak current is greater than

the current-limit threshold by an amount equal to the in-

ductor ripple current. Therefore, the exact current-limit char-

acteristic and maximum load capability are a function of

the sense resistance, inductor value, and input voltage.

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