LD5532

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5532-DS-00 July 2014

Application Information

Operation Overview

The LD5532 meets the green-power requirement and is

intended for the use in those modern switching power

suppliers and adaptors which demand higher power

efficiency and power-saving. It integrates more functions to

reduce the external components counts and the size. Its

major features are described as below.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage across VCC pin. It would assure the supply voltage

enough to turn on the LD5532 PWM controller and further to

drive the power MOSFET.

As shown in Fig. 17, a

hysteresis is built in to prevent shutdown from the voltage

dip during startup.

The turn-on and turn-off threshold level

are set at 16.0V and 8.0V, respectively.

Vcc

UVLO(on)

UVLO(off)

t

t

I(Vcc)

startup current

(~uA)

operating current

(~ mA)

Fig. 17

Startup Current and Startup Circuit

The typical startup circuit to generate VCC of the LD5532 is

shown in Fig. 18.

At the startup transient, the VCC is below

UVLO threshold. Before it has sufficient voltage to develop

OUT pulse to drive the power MOSFET, R1 will provide the

startup current to charge the capacitor C1.

Once VCC

obtains enough voltage to turn on the LD5532 and further to

deliver the gate drive signal, it will enable the auxiliary

winding of the transformer to provide supply current.

Lower startup current requirement on the PWM controller

will help to increase the value of R1 and then reduce the

power consumption on R1. By using CMOS process and the

special circuit design, the maximum startup current for

LD5532 is only 0.6

A.

If a larger resistor of R1 is chosen, it will usually take more

time to start up. A proper selection of R1 and C1 will

optimize the power consumption and startup time.

EMI

Filter

R1

OUT

CS

VCC

GND

LD5532

AC

input

C1

Cbulk

D1

Fig. 18

Current Sensing and Leading-edge Blanking

The typical current mode of PWM controller feedbacks both

current signal and voltage signal to close the control loop

and achieve regulation. As shown in Fig. 19, the LD5532

detects the primary MOSFET current from the CS pin for the

peak current mode control and also for the pulse-by-pulse

current limit. The maximum voltage threshold of the current