LD7576 datasheet(10/22 Pages) LEADTREND

10 / 22 page

LD7576/76H/76J/76K

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7576-DS-04a

December 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 switching power supplies and switching adaptors, the

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, thereby reducing the external

part count.

The LD7576X series is ideal for these

applications to provide an easy and cost effective solution;

and its detailed features are described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

OUT

VCC

GND

LD7576X

Cbulk

COMP

Vin

Fig. 13

Traditional circuits provide the startup current through a

startup resistor to power up the PWM controller.

Nevertheless, it consumes too significant power to meet

the current power saving requirement. In most cases,

startup resistors carry large resistance, which causes

longer startup time.

To achieve the optimized topology, as shown in figure 13,

LD7576X series is implemented with a high-voltage

startup circuit for such requirement. During startup, a

high-voltage current source sinks current from the bulk

capacitor to provide the startup current as well as to

charge the Vcc capacitor C1.

During the startup

transient when the Vcc is lower than the UVLO threshold,

the high-voltage current source is enabled to supply 1mA

current.

Meanwhile, the Vcc supply current is as low as

100

μA such that most of the HV current is adopted to

charge the Vcc capacitor.

By using such configuration,

the turn-on delay time will be almost the same no matter

under low-line or high-line condition.

As the Vcc voltage rises higher than UVLO(on) to power

on the LD7576X series and further to deliver the gate

drive signal, the high-voltage current source is disabled

and the supply current is solely provided from the auxiliary

winding of the transformer.

Therefore, it eliminates the

power loss on the startup circuit and performs highly

power saving.

An UVLO comparator is embedded to detect the voltage

on the Vcc pin and to ensure the supply voltage high

enough to power on the LD7576X series PWM controller

and to drive the power MOSFET.

As shown in Fig. 14, a

hysteresis is provided to prevent undesired shutdown

from the voltage dip during startup.

The turn-on and

turn-off threshold levels are set at 16V and 10.0V,

respectively.

Vcc

UVLO(on)

UVLO(off)

HV Current

1mA

Startup Current

(<100uA)

Vcc current

~ 0mA (off)

Operating Current

(Supply from Auxiliary Winding)

Fig. 14

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