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LD7577JAGR

High Voltage Green-Mode PWM Controller with Brown-Out Protection

Unspecified 

LD7577JA

1 .6

1 .4

1 .2

1 .0

0 .8

0 .6

0 .4

25 °C

0 .2

0

0

1

2

3

4

5

6

VBNO (V)

Fig. 13 VBNO vs. IBNO

Application Information

Operation Overview

As long as the green power requirement becomes a trend

and the power saving is becoming more and more

important for switching power supplies and switching

adaptors, the traditional PWM controllers are not able to

support such new requirements. Furthermore, the cost

and size limitation forces the PWM controllers to

powerfully integrate more functions, thereby reducing the

external part count. The LD7577JA is ideal for these

applications to provide an easy and cost effective solution;

and its detailed features are described as below.

Internal High-Voltage Startup Circuit and

Under Voltage Lockout (UVLO)

Vin

Cbulk

D1

R1

C1

HV VCC

OUT

LD7577JA

Comp

CS

GND

Rs

Fig. 14

Traditional circuits’ power 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 required more power and

more time to start up.

To achieve the optimized topology, as shown in figure 14,

The LD7577JA 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, the Vcc current consumed by the

LD7577JA 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 whether operation condition is under

low-line or high-line.

When Vcc voltage reaches UVLO(on) threshold, the

LD7577JA 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

Leadtrend Technology Corporation

LD7577JA-DS-00b August 2010

9

www.leadtrend.com.tw

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