LD7576PN 데이터 시트보기 (PDF) - Unspecified

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LD7576PN

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

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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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