LD7531 datasheet(9/18 Pages) LEADTREND

Part #	LD7531

Description	Green-Mode PWM Controller with Frequency Trembling and Integrated Protections

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Manufacturer	LEADTREND [Leadtrend Technology]

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LD7531

9

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7531-DS-00 June 2008

S

)

MAX

(

PEAK

R

V

85

.

0

I

=

R1

OUT

CS

VCC

GND

LD7531

C1

Cbulk

D1

Rs

Comp

Vin

Fig. 15

A 190nS leading-edge blanking (LEB) time is included in the

input of CS pin to prevent the false-trigger from the current

spike. In the low power application, if the total pulse width of

the turn-on spikes is less than 190nS and the negative spike

on the CS pin doesn’t exceed -0.3V, it could eliminated the

R-C filter (as shown in the figure16).

However, the total pulse width of the turn-on spike is

decided by the output power, circuit design and PCB layout.

It is strongly recommended to adopt a smaller R-C filter (as

shown in figure 17) for higher power application to avoid the

CS pin being damaged by the negative turn-on spike.

Output Stage and Maximum Duty-Cycle

An output stage of a CMOS buffer, with typical 500mA

driving capability, is incorporated to drive a power MOSFET

directly. And the maximum duty-cycle of LD7531 is limited

to 75% to avoid the transformer saturation.

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 at

the secondary side through the photo-coupler to the COMP

pin of the LD7531. Similar to UC3842, the LD7531 would

carry 2 diodes voltage offset at the stage to feed the voltage

divider at the ratio of 1/3, that is,

)

V

2

V

(

3

1

V

F

COMP

)

PWM

(

COMPARATOR

−

×

=

−

A pull-high resistor is embedded internally and can be

eliminated externally.

Fig. 16

Fig. 17

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