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LD7576JPS

Green-Mode PWM Controller

Unspecified 

LD7576/76H/76J/76K

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 LD7576X

series are ideal for these applications to provide an easy

and cost effective solution; its detailed features are

described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

will be enabled to supply 1mA current. Meanwhile, the Vcc

supply current is as low as 100μA 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

same no matter under low-line or high-line conditions.

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 will be disabled and

the supply current is provided from the auxiliary winding of

the transformer. Therefore, it would eliminate the power

loss on the startup circuit and perform highly power saving.

An UVLO comparator is embedded to detect the voltage on

the Vcc pin to ensure the supply voltage enough to power

on the LD7576X series 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

Fig. 13

The traditional circuit provides the startup current through a

startup resistor to power up the PWM controller. However, it

consumes too significant power to meet the current power

saving requirement. In most cases, startup resistors carry

large resistance. And larger resistance takes longer startup

time.

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

LD7576X series are implemented with 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

drops lower than the UVLO threshold so the current source

Vcc current

Startup Current

(<100uA)

~ 0mA (off)

t

Operating Current

(Supply from Auxiliary Winding)

Fig. 14

Current Sensing, Leading-Edge Blanking and

the Negative Spike on CS Pin

The typical current mode PWM controller feedbacks both

current signal and voltage signal to close the control loop

Leadtrend Technology Corporation

LD7576-DS-03 December 2007

10

www.leadtrend.com.tw

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