LD7575PS データシートの表示（PDF） - Unspecified

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LD7575PS

Green-Mode PWM Controller

Unspecified 

LD7575

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)

Vin

Cbulk

D1

R1

C1

HV VCC

OUT

LD7575

Comp

CS

GND

Rs

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

100μA 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)

t

HV Current

1mA

Vcc current

Startup Current

(<100uA)

~ 0mA (off)

t

Operating Current

(Supply from Auxiliary Winding)

Fig. 14

Leadtrend Technology Corporation

LD7575-DS-04b November 2007

8

www.leadtrend.com.tw

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