LD7575GS datasheet(8/17 Pages) LEADTREND

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LD7575

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7575-DS-04b November 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 adaptors,

the traditional PWM controllers are not able to support

such new requirements. Furthermore, the cost and size

limitation force the PWM controllers need to be powerful

to integrate more functions to reduce the external part

counts. The LD7575 is targeted on such application to

provide an easy and cost effective solution; its detail

features are described as below:

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

OUT

VCC

GND

LD7575

Cbulk

Comp

Vin

Fig. 13

Traditional circuit powers up the PWM controller through a

startup resistor to provide the startup current. However,

the startup resistor consumes significant power which is

more and more critical whenever the power saving

requirement is coming tight. Theoretically, this startup

resistor can be very high resistance value. However,

higher resistor value will cause longer startup time.

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

LD7575 implements a high-voltage startup circuit for such

requirement. During the startup, a high-voltage current

source sinks current from the bulk capacitor to provide the

startup current as well as charge the Vcc capacitor C1.

During the startup transient, the Vcc is lower than the

UVLO threshold thus the current source is on to supply a

current with 1mA. Meanwhile, the Vcc supply current is

as low as 100

μA thus most of the HV current is utilized to

charge the Vcc capacitor. By using such configuration,

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

under low-line or high-line conditions.

Whenever the Vcc voltage is higher than UVLO(on) to

power on the LD7575 and further to deliver the gate drive

signal, the high-voltage current source is off and the

supply current is provided from the auxiliary winding of the

transformer. Therefore, the power losses on the startup

circuit can be eliminated and the power saving can be

easily achieved.

An UVLO comparator is included to detect the voltage on

the Vcc pin to ensure the supply voltage enough to power

on the LD7575 PWM controller and in addition to drive the

power MOSFET. As shown in Fig. 14, a hysteresis is

provided to prevent the shutdown from the voltage dip

during startup. The turn-on and turn-off threshold level

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)

LD7575GS Datasheet(PDF) 8 Page - Leadtrend Technology

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