LD7765 datasheet(10/19 Pages) LEADTREND

10 / 19 page

LD7765

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7765-DS-00

October 2012

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 LD7765 is 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)

The traditional circuit provides the startup current through

a startup resistor to power up the PWM controller.

However, it consumes much significant power to meet the

current power saving requirement. In most cases, startup

resistors carry larger resistance and spend more time to

start up.

As shown in Fig 9, LD7765 is implemented with a

high-voltage startup circuit with it to minimize power loss

startup

circuit.

During the startup

transient, a

high-voltage current source sinks current from AC Line or

Neutral to provide the startup current and charge the Vcc

capacitor C1 the same time.

See in Fig 10. Once VCC voltage rises up to reach the

UVLO(on) threshold, HV pin will stop charging the VCC

capacitor and BNO will detect the AC line status. In the

meantime, VCC voltage begins to fall and consumes less

current for operation from 0.6mA to 300

A as it’s falling to

UVLO(off)

As VCC trips UVLO(off), HV pin will recharge VCC

capacitor till VCC voltage rises back to UVLO(on) again.

Since then, 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.

By using such configuration, the turn-on delay time will be

almost same no matter under low-line or high-line

conditions.

An UVLO comparator is embedded to detect the voltage

across Vcc pin to ensure the supply voltage enough to

power on the LD7765 and in addition to drive the power

MOSFET.

As shown in Fig 10, 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 17V and 8.5 V, respectively.

VCC

GND

input

Cbulk

current

source

PWM IC

Fig 9.

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