LD7523GS datasheet(10/18 Pages) LEADTREND

10 / 18 page

LD7523

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7523-DS-00

June 2009

Application Information

Operation Overview

As long as the green power requirement becomes a trend

and the power saving is getting more and more important

for the switching power supplies and switching adapters,

traditional PWM controllers are not able to support such

new requirements. Furthermore, the cost and size

limitation forces the PWM controllers to powerfully

integrate more functions for reducing the external part

counts.

The LD7523 is targeted on such applications

and provides an easy and cost effective solution; its

detailed features are described as below.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage on the VCC pin, for assuring the supply voltage

high enough to turn on the LD7523 PWM controller and to

drive the power MOSFET.

As shown in Fig. 19, a

hysteresis is built in to prevent the shutdown due to the

voltage dip during startup.

The turn-on and turn-off

threshold

levels

are

set

12.75V

and

9.75V,

respectively.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

startup current

(~uA)

operating current

(~ mA)

Fig. 19

Startup Current and Startup Circuit

The typical startup circuit for the LD7523 is shown in Fig.

20.

During the startup transient, the Vcc is lower than

UVLO threshold and thus there is no gate pulse produced

from the LD7523 to drive power MOSFET. Therefore, the

current through R1 will provide the startup current and to

charge the capacitor C1.

Once upon the Vcc voltage is

high enough to turn on the LD7523 and further to deliver

the gate drive signal, the supply current is provided from

the auxiliary winding of the transformer. Lower startup

current requirement for the PWM controller will help to

increase the maximum value on R1 and then reduce the

power consumption on R1. By using CMOS process and

the special circuit design, the maximum startup current of

LD7523 is only 35

μA.

If a higher resistance value of the R1 is chosen, it usually

takes more time to start up. To carefully select the value of

R1 and C1 will optimize the power consumption and

startup time.

EMI

Filter

OUT

VCC

GND

LD7523

input

Cbulk

Fig. 20

Output Stage and Maximum Duty-Cycle

An output stage of a CMOS buffer, with typical 500mA

driving capability, is incorporated to drive a power

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