LD5537

09/07/2015

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5537-DS-01 September 2015

Application Information

Operation Overview

The LD5537 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 on the VCC pin. It would assure the supply

voltage enough to turn on the LD5537 PWM controller

and further to drive the power MOSFET.

As shown in Fig.

14, a hysteresis is built in to prevent the shutdown from

the voltage dip during startup.

The turn-on and turn-off

threshold level are set at 16.0V and 7.5V, respectively.

VCC

UVLO(ON)

UVLO(OFF)

t

t

I(VCC)

startup current

(~uA)

operating current

(~ mA)

Fig. 14

Startup Current and Startup Circuit

The typical startup circuit to generate VCC of the LD5537

is shown in Fig. 15.

During 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 obtain enough voltage to turn

on the LD5537 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 LD5537 is only

12

A.

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

usually spend more time to start up. To carefully select the

value of R1 and C1 will optimize the power consumption

and startup time.

R1

OUT

CS

VCC

GND

LD5537

AC

input

C1

Cbulk

D1

EMI

Filter

Fig. 15

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. 16, the

LD5537 detects the primary MOSFET current from the CS

pin, which is not only for the peak current mode control

but also for the pulse-by-pulse current limit. The maximum

voltage threshold of the current sensing pin is set at 0.75V.

From above, the MOSFET peak current is concluded as

below.