APW7108 datasheet(18/28 Pages) ANPEC

18 / 28 page

Rev. A.4 - Jan., 2009

APW7108

www.anpec.com.tw

Function Description (Cont.)

SOFT

RISE

0.6

Power-On-Reset

Soft-Start/Soft-Stop

The time that takes the output voltage to come into

regulation can be obtained from the following equation:

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

the under-voltage protection is prohibited. The over-

voltage and current-limit protection functions are enabled.

If the output capacitor has residue voltage before start-

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

until the soft-start capacitor charges equal to the VSEN

pin voltage. This will ensure the output voltage starts

from its existing voltage level.

In the event of under-voltage or shutdown, the SOFT pin

is used by the soft-stop function. The soft-stop function

discharges the voltage on SOFT pin with the internal

2.2

µA current sink. The channel with soft-stop enabled

gradually ramps down the output voltage, following the

SOFT voltage, by controlling the low-side MOSFET work-

Under-Voltage Protection (UVP)

In the operational process, if a short-circuit occurs, the

output voltage will drop quickly. When load current is bigger

than current-limit threshold value, the output voltage will

fall out of the required regulation range. The under-

voltage continually monitors the V

SEN voltage after soft-

start is completed. If a load step is strong enough to pull

the output voltage lower than the under-voltage

threshold, the offending channel pulls low the PGOOD

immediately and starts a soft-stop process to shut down

the output gradually. The offending channel is latched

off when the soft-stop process is completed.

The under-voltage threshold is 70% of the nominal out-

put voltage. The under voltage comparator has a built-in

2µs noise filter to prevent the chip from wrong UVP shut-

down caused by noise. Toggling both enable pins

to low, or recycling V

CC, will clear the latch and bring the

chip back to operation.

ing as a sinking linear regulator. The soft-stop process is

completed when the falling SOFT voltage reaches about

50mV (typical) threshold. At this moment, the LGATE goes

high level with latch and SOFT pulls low by using the

internal 2k

Ω resistor to the ground. The latch can be re-

set by cycling both of the EN signals or VCC power-on-

reset signal.

Over-Voltage Protection (OVP)

The over-voltage function monitors the output voltage by

VSEN pin. The V

SEN voltage should increase over 115%

of the reference voltage due to the high-side MOSFET

failure or for other reasons, and the over-voltage pro-

tection comparator designed with a 2

µs noise filter will

force the low-side MOSFET gate driver to be high. This

action actively pulls down the output voltage and eventu-

ally attempts to blow the battery fuse. As soon as the out-

put voltage is within regulation, the OVP comparator is

disengaged. The chip will restore its normal operation.

When the OVP occurs, the PGOOD will drop to low as

well.

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-a common problem for OVP schemes with

a latch.

4.5

1.5V

SOFT

set high. When the rising V

CC voltage reaches the ris-

ing POR voltage threshold (4.2V typical), the POR sig-

nal goes high and the chip initiates soft-start opera-

tions for the enabled channels. This voltage should

drop lower than 4V (typical), the POR disables the chip.

When soft-start is initiated, the voltage on the SOFT pin of

the enabled channel starts to ramp up gradually with the

internal 4.5

µA current charging the soft-start capacitor. The

output voltage follows the soft-start voltage with the

converter operating in PWM mode. When the SOFT pin

voltage reaches 0.9V, the output voltage comes into

regulation. When the SOFT voltage reaches 1.5V, the

power-good (PGOOD) is enabled. Even though the SOFT

pin voltage continues to rise after reaching 1.5V, this

voltage does not affect the output voltage. The maxi-

mum SOFT voltage is clamped about 2.4V.

The soft-start time (the time from the moment when EN

becomes high to the moment when PGOOD is reported)

is determined by the following equation:

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