LD7577 datasheet(9/18 Pages) LEADTREND

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LD7577

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7577-DS-01 January 2009

Application Information

Operation Overview

As long as the green power requirement becomes a trend

and the power saving is becoming 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

LD7577 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)

Fig. 13

Traditional circuits’ power on the PWM controller through a

startup resistor to constantly provide current from a rectified

voltage to the capacitor connected to Vcc pin. Nevertheless,

this startup resistor was usually of larger resistance, and it

therefore required more power and longer time to start up.

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

the LD7577 is built in with high voltage startup circuit to

optimize the power saving. During the startup sequence, a

high-voltage current source sinks current from CBULK

capacitor to provide the startup current as well as to charge

the Vcc capacitor C1.

During the initialization of the startup,

Vcc voltage is lower than the UVLO(off) threshold thus the

current source is on to supply a current of 1mA.

Meanwhile, the Vcc current consumed by the LD7577 is as

low as 320

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 the same no matter

whether operation condition is under low-line or high-line.

When Vcc voltage reaches UVLO(on) threshold, the

LD7577 is powered on to start issuing the gate drive signal,

the high-voltage current source is then disabled, and the

Vcc supply current will be only provided from the auxiliary

winding of the transformer.

Therefore, the power losses on

the startup circuit beyond the startup period can be

eliminated and the power saving can be easily achieved. In

general application, a 39K

Ω resistor is still recommended to

be placed in high voltage path to limit the current if there is a

negative voltage applying in any case.

An UVLO comparator is included to detect the voltage on

the VCC pin to ensure the supply voltage is high enough to

power on the LD7577 PWM controller and in addition to

drive the power MOSFET.

As shown in Fig. 14, a

Hysteresis is provided to prevent the shutdown caused by

the voltage dip during startup.

The turn-on and turn-off

threshold levels are set at 16V and 10.0V, respectively.

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