LD7750EGR データシートの表示（PDF） - Unspecified

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LD7750EGR

High Voltage Green-Mode PWM Controller with Over Temperature Protection

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LD7750E

Application Information

Operation Overview

As green power requirement becomes a trend and the

power saving gets 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

limitations force PWM controllers to be more powerful by

integrating more functions and, thus, reducing the

external part count. LD7750E is designed for such

application 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 powers 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 consumed more power

and required longer time to start up.

To achieve an optimized topology, as shown in Fig. 13,

The LD7750E 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 below UVLO(off) threshold thus

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

Meanwhile, as the Vcc current consumed by the LD7750E

is only 320μA thus most of the HV current is left to charge

the Vcc capacitor. With using such configuration, the

turn-on delay time will be almost the same no matter what

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

Fig. 13

When Vcc voltage reaches UVLO(on) threshold, the

LD7750E is powered on to start issuing the gate drive

signal, disable the high-voltage current source, and the

Vcc supply current will be only provided from the auxiliary

winding of the transformer. Therefore, the power loss for

the startup circuit after the startup period can be

eliminated and the power saving can be easily achieved.

In general application, a 39KΩ 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 of

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

power on the LD7750E PWM controller and in addition to

drive the power MOSFET as well. 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.

Leadtrend Technology Corporation

LD7750E-DS-00 February 2012

9

www.leadtrend.com.tw

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