APW8811 datasheet(13/23 Pages) ANPEC

13 / 23 page

Rev. A.3 - Sep., 2012

APW8811

www.anpec.com.tw

Under-Voltage Protection (PWMs)

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

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

ger than the value of current-limit threshold, the output

voltage will fall out of the required regulation range. The

under-voltage continually monitors the setting output 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 for at least 2

µs, the PWM controller starts

a soft-stop process to shut down the output gradually. As

long as either of PWM channels triggers under-voltage,

both of PWM channels active under-voltage protection

and latched off when the soft-stop process is completed.

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

put voltage. Under-voltage protection is ignored for at least

2ms(typ.) after a rising edge on EN. Toggling ENLDO or

ENPWM signal will clear the latch and bring the chip back

to operation.

Function Description (Cont.)

POK is actively held low in shutdown, standby, and soft-

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

output, and it is released with enable delay after ENPWM

goes high (about 2ms typ.). In normal operation, the POK

window is from 90% to its OVP threshold of the converter

reference voltage. Both of V

OUT1 and VOUT2 have to stay

within this window for POK to be high (AND gated). In

order to prevent false POK drop, capacitors need to par-

allel at the output to confine the voltage deviation with

severe load step transient.

Power Good Indicator (PWMs)

Should the output voltage of V

OUT1 and VOUT2 increase over

25% of the setting voltage due to the high-side MOSFET

failure or for other reasons, the over-voltage protection

will active. As long as either of PWM channels triggers

over-voltage, both of PWM channels active overvoltage

protection. Over-voltage protection will force the low-side

MOSFET gate driver fully turn on. This action actively pulls

down the output voltage. When the OVP occurs, the POK

pin will pull down and latch-off the converter. This OVP

scheme only clamps the voltage overshoot, and does not

invert the output voltage when otherwise activated with a

Over-Voltage Protection (OVP)

When the junction temperature increases above the ris-

ing threshold temperature 160°C, the IC will enter the

over -temperature protection (OTP). When the OTP occurs,

REF, LDO and PWM controllers circuitry shuts down. It is

non-latch protection.

Over-Temperature Protection

The current-limit circuit employs a “valley” current-sens-

ing algorithm (See Figure 1). The APW8811 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 PHASE pin is above the current-limit

threshold, the PWM is not allowed to initiate a new cycle.

The actual peak current is greater than the current-limit

threshold by an amount equal to the inductor ripple

current. Therefore, the exact current-limit characteristic

and maximum load capability are a function of the sense

resistance, inductor value, and input voltage.

Current-Limit (PWMs)

Both PWM controllers use the low-side MOSFETs on-

resistance R

DS(ON) to monitor the current for protection

against shorted outputs. The MOSFET’s R

DS(ON)

is varied

by temperature and gate to source voltage, the user

should determine the maximum R

DS(ON)

in manufacture’s

datasheet.

The current-Limit threshold of APW8811 is adjusted with

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 be reset

by toggling ENLDO or ENPWM signal.

Time

PEAK

OUT

LIMIT

∆I

Figure 1. Current-Limit Algorithm

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