LD7752 datasheet(8/17 Pages) LEADTREND

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LD7752

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7752-DS-01 June 2013

Application Information

Operation Overview

The LD7752 meets the green-power requirement and is

intended for the use in those modern switching power

suppliers and adaptors which demand higher power

efficiency

and

power-saving.

integrates

functions to reduce the external components counts and

the size. Its major features are described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

OUT

VCC

GND

LD7752

Cbulk

COMP

Vin

Fig. 7

The traditional circuit provides the startup current

through a startup resistor to power up the PWM

controller. However, it consumes significant power. In

most cases, startup resistors carry large resistance. And

larger resistance takes longer startup time.

To achieve optimized topology, as shown in Fig. 7,

LD7752 is designed with high-voltage startup circuit for

it. During startup, the high-voltage current source sinks

through the bulk capacitor to provide the startup current

to charge the Vcc capacitor C1. During initialization, the

Vcc drops below UVLO threshold to enable the current

source to supply 1mA current. Since there

’s only 150

A

required for startup, most of the HV current is reserved

to charge the Vcc capacitor. With such configuration, it

spends almost same time for turn-on delay no matter

under low-line or high-line conditions.

As VCC trips UVLO(ON), HV pin would no longer

charge the capacitor and instead, send a gate drive

signal to draw supply current for VCC from the auxiliary

winding of the transformer. That minimizes the power

loss on the start-up circuit successfully.

An UVLO comparator is embedded to detect the voltage

across the Vcc pin to monitor if the supply voltage is

enough to power on the LD7752 and in addition to drive

the power MOSFET.

As shown in Fig. 8, 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 8V, respectively.

Vcc

UVLO(on)

UVLO(off)

HV Current

1mA

Startup Current

Vcc current

~ 0mA (off)

Operating Current

(Supply from Auxiliary Winding)

Fig. 8

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 and achieve regulation. The LD7752

detects the primary MOSFET current over CS pin for the

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