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LD7750B

High Voltage Green-Mode PWM Controller with Over Temperature Protection

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LD7750B

Application Information

Operation Overview

As green power requirements become 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 counts. LD7750B is designed for such

application to provide an easy and cost effective solution.

Its detail features are described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

The traditional circuits power the PWM controller on

through a startup resistor. It will 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 consumes more power

and requires more time to start up.

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

The LD7750B 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, as the Vcc current consumed by the

LD7750B is as low as 320μA 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 what operation condition is, under

low-line or high-line.

When Vcc voltage reaches UVLO(on) threshold, the

LD7750B 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 loss on the startup circuit beyond the startup period

is minimized and the power saving is enhanced. In

general applications, 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

over the VCC pin to ensure the supply voltage is high

enough to power on the LD7750B 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.

Fig. 13

Leadtrend Technology Corporation

LD7750B-DS-01 OCT. 2011

9

www.leadtrend.com.tw

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