LD7576J Ver la hoja de datos (PDF) - Unspecified

Número de pieza

componentes Descripción

Fabricante

LD7576J

Green-Mode PWM Controller with High-Voltage Start-Up Circuit and Adjustable OLP Delay Time

Unspecified 

and achieve regulation. The LD7576X series detects the

primary MOSFET current from the CS pin, which is not only

for the peak current mode control but also for the

pulse-by-pulse current limit. The maximum voltage

threshold of the current sensing pin is set at 0.85V. Thus the

MOSFET peak current can be calculated as:

IPEAK(MAX)

=

0.85V

RS

A 230nS leading-edge blanking (LEB) time is provided in the

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

spike. In the low power applications, if the total pulse width

of the turn-on spikes is less than 230nS and the negative

spike on the CS pin does not exceed -0.3V, the R-C filter (as

shown in figure15) can be eliminated.

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

to the output power, circuit design and PCB layout. It is

strongly recommended to add a small R-C filter (as shown in

figure 16) for higher power application to avoid the CS pin

from 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 LD7576X series

are limited to 75% to avoid the transformer saturation.

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 on

the secondary side through the photo-coupler to the COMP

pin of LD7576X series. The input stage of LD7576X series,

like the UC384X, is with 2 diodes voltage offset to feed the

voltage divider with 1/3 ratio, that is,

V+ (PWM COMPARATOR

)

=

1

3

× (VCOMP

−

2VF )

A pull-high resistor is embedded internally to optimize the

external circuit. Generally, an external capacitor in parallel

to photo-coupler is required in application.

LD7576/76H/76J/76K

VCC

OUT

LD7576X

CS

GND

230ns

blanking

time

Can be removed if the negative

spike is not over spec. (-0.3V).

Fig. 15

Fig. 16

Oscillator and Switching Frequency

The switching frequency of LD7576X series are fixed at

65KHz and 100KHz internally to provide the optimized

Leadtrend Technology Corporation

LD7576-DS-03 December 2007

11

www.leadtrend.com.tw

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