APW8720B datasheet(11/22 Pages) ANPEC

11 / 22 page

Rev. A.5 - Mar., 2012

APW8720B

www.anpec.com.tw

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW8720B 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 APW8720B builds in a soft-start function about

1.5ms (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 re-

places the reference voltage (0.8V typical) until the ramp

voltage reaches the reference voltage. The soft-start cir-

cuit interval is shown as figure 1. The UVP function en-

able delay is from t2 to t3.

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 APW8720B turns off the high-

side and low-side MOSFET,and the device will enters

hiccup mode until the over-current phenomenon is

released.

The APW8720B has an internal OCP voltage, V

OCP_MAX,

and the value is 0.35V (minimum). When the R

OCSET x

OCSET exceed 0.35V or the ROCSET is floating or not

connected, the V

ROCSET will be the default value 0.35V. The

over current threshold would be 0.35V across low-side

MOSFET. The threshold of the valley inductor current-limit

is therefore 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.

For avoid large inductor current occurring in short circuit

before power on, the controller reduces internal current

source, I

ocset, to half during soft start time.

It means that when APW8720B is in soft start interval, the

internal current source, I

ocset, is only 5µA (typical).

VOUT

(OCSET duratiom, t2-t1, less than 1.3ms)

t1 t2

VVCC

Time

Voltage(V)

VPOK

POK Delay Time

OCSET count start

0.9xVREF

OCSET count completed

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