APW8723 datasheet(13/23 Pages) ANPEC

13 / 23 page

Rev. A.2 - Apr., 2013

APW8723

www.anpec.com.tw

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW8723 con-

tinually monitors the input supply voltage (VCC) and en-

sures that the IC has sufficient supply voltage and can

work well. The POR function initiates a soft-start process

while the VCC voltage just exceeds the POR threshold;

the POR function also inhibits the operations of the IC

while the VCC voltage falls below the POR threshold.

Soft-Start

The APW8723 builds in a soft-start function about 2ms

(Typ.) interval, which controls the output voltage rising as

well as limiting the current surge at the start-up. During

soft-start, an internal ramp voltage connected to the one

of the positive inputs of the error amplifier replaces the

reference voltage (0.8V typical) until the ramp voltage

reaches the reference voltage. The soft-start circuit inter-

val is shown as figure 1.

Figure 1. Soft-Start Interval

Over-Current Protection of the PWM Converter

The over-current function protects the switching converter

against over-current or short-circuit conditions. The con-

troller senses the inductor current by detecting the drain-

to-source voltage which is the product of the inductor’s

current and the on-resistance of the low-side MOSFET

during it’s on-state. This method enhances the converter’s

efficiency and reduces cost by eliminating a current sens-

ing resistor required.

A resistor (R

OCSET), connected from the LGATE/OCSET to

GND, programs the over-current trip level. Before the IC

initiates a soft-start process, an internal current source,

OCSET (10µA typical), flowing through the ROCSET develops

a voltage (V

ROCSET) across the ROCSET. The device holds

ROCSET and stops the current source IOCSET during normal

operation. When the voltage across the low-side MOSFET

exceeds the V

ROCSET, the APW8723 turns off the high side

and low-side MOSFET, and the device will enter hiccup

mode until the over-current phenomenon is released.

The APW8723 has an internal OCP voltage, V

OCP_MAX, and

the value is 0.621V(minmum). When the R

OCSET x IOCSET

exceed 0.621V or the R

OCSET is floating or not connected,

the V

ROCSET will be the default value 0.621V. The over cur-

rent threshold would be 0.621V across low-side MOSFET.

The threshold of the valley inductor current limit is there-

fore given by:

)

side

low

(

)

(

OCSET

LIMIT

−

For the over-current is never occurred in the normal oper-

ating load range, the variation of all parameters in the

above equation should be considered:

- The R

DS(ON) of low-side MOSFET is varied by tempera-

ture and gate to source voltage. Users should deter-

mine the maximum R

DS(ON) by using the manufacturer’s

datasheet.

- The minimum I

OCSET (9µA) and minimum ROCSET should

be used in the above equation.

- Note that the I

LIMIT is the current flow through the low-

side MOSFET; I

LIMIT must be greater than valley inductor

current which is output current minus the half of induc-

tor ripple current.

)

MAX

(

OUT

LIMIT

∆

−

Where

∆I = output inductor ripple current

- The overshoot and transient peak current also should

be considered.

t1 t2

VVCC

VOUT

Time

Voltage(V)

VPOK

POK Delay

Time

OCSET count start

(OCSET duratiom, t2- t1, less than 0.474ms)

0.9 xVREF

OCSET count completed

Soft Start

Time

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