APW8713 datasheet(16/24 Pages) ANPEC

16 / 24 page

Rev. A.3 - Sep., 2013

APW8713

www.anpec.com.tw

Function Description (Cont.)

Figure 1. Current Limit algorithm

Time

IPEAK

IOUT

ILIMIT

ΔI

Current Limit( Cont.)

The PWM controller uses the low-side MOSFETs on-re-

sistance 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 PCB layout guidelines should ensure that noise and

DC errors do not corrupt the current-sense signals at LX.

Place the hottest power MOSEFTs as close to the IC as

possible for best thermal coupling. When combined with

the under-voltage protection circuit, this current-limit

method is effective in almost every circumstance.

Over-Temperature Protection (OTP)

When the junction temperature increases above the ris-

ing threshold temperature T

OTR, the IC will enter the over

temperature protection state that suspends the PWM,

which forces the UG and LG gate drivers output low. The

thermal sensor allows the converters to start a start-up

process and regulate the output voltage again after the

junction temperature cools by 45oC. The OTP designed

with a 45oC hysteresis lowers the average T

J during con-

tinuous thermal overload conditions, which increases life-

time of the APW8713.

Programming the On-Time Control and PWM Switch-

ing Frequency

The APW8713 does not use a clock signal to produce

PWM. The device uses the constant on-time control ar-

chitecture to produce pseudo-fixed frequency with input

voltage feed-forward. The on-time pulse width is propor-

tional to output voltage V

OUT and inverse proportional to

input voltage V

IN. In PWM, the on-time calculation is writ-

ten as below equation.

(V)

)

(

TON

-12

Ω

Where:

TON is the resistor connected from TON pin to VIN pin.

Furthermore, The approximate PWM switching frequency

is written as:

OUT

，

Where:

SW is the PWM switching frequency.

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