LD7538GL datasheet(9/17 Pages) LEADTREND

9 / 17 page

LD7538

Leadtrend Technology Corporation

LD7538-DS-00

November 2012

Application Information

Operation Overview

The LD7538 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 integrated 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 LD7538 PWM controller

and further to drive the power MOSFET.

As shown in Fig.

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

the voltage dip during startup.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

startup current

(~uA)

operating current

(~ mA)

Fig. 13

Startup Current and Startup Circuit

The typical startup circuit to generate VCC of the LD7538 is

shown in Fig. 14.

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 LD7538

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 LD7538 is only 14

A.

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

usually take 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

CS/OVP

VCC

GND

LD7538

input

Cbulk

ig. 14

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. 15, the

LD7538 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 0.85V.

From above, the MOSFET peak current can be obtained

from below.

)

MAX

(

PEAK



LD7538GL Datasheet(PDF) 9 Page - Leadtrend Technology

LD7538GL Datasheet(HTML) 9 Page - Leadtrend Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL