LD7752GS datasheet(9/17 Pages) LEADTREND

9 / 17 page

LD7752

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7752-DS-01 June 2013

peak

current

mode

control

and

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 is calculated as:

PEAK(MAX)

0.85V



250nS

leading-edge

blanking

(LEB)

time

programed

the

input

prevent

false-triggering from the current spike. For most low

power applications, the R-C filter (as shown in Fig.9) is

eliminable if the total pulse width of the turn-on spikes is

less than 250nS and the negative spike on the CS pin

does not exceed -0.3V.

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

subject to the output power, circuit design and PCB

layout.

It is strongly recommended to add a small R-C

filter (as shown in Fig. 10) for larger power applications

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 250mA

source and 500mA sink driving capability is incorporated

to drive a power MOSFET directly.

And the maximum

duty-cycle of LD7752 is limited to 75% to avoid the

transformer saturation.

Voltage Feedback Loop

The voltage feedback signal is delivered from the TL431

on the secondary side through the photo-coupler to the

COMP pin. The input stage of LD7752, as UC384X,

consists of 2 diodes voltage offset to feed the voltage

divider with 1/3 ratio, that is,

)

COMP

(PWM

COMPARATOR









A pull-high resistor is embedded internally to optimize

the external circuit.

VCC

GND

LD7752

Removable if the negative spike

is not over spec. (-0.3V).

OUT

250ns

blanking

time

Fig. 9

VCC

GND

LD7752

An R-C filter is required once if the negative

spike is over -0.3V or the total spike width over

250nS LEB period.

OUT

Fig. 10

Oscillator and Switching Frequency

The switching frequency of LD7752 is fixed at 65kHz

internally to optimize the operation in consideration of

the EMI performance, thermal treatment, component

sizes and transformer design.

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