LD7575GS datasheet(9/17 Pages) LEADTREND

9 / 17 page

LD7575

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7575-DS-04b November 2009

Fig. 14

Current Sensing, Leading-edge Blanking

and the Negative Spike on CS Pin

The typical current mode PWM controller feedbacks both

current signal and voltage signal to close the control loop

and achieve regulation. The LD7575 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 as 0.85V. Thus the MOSFET

peak current can be calculated as:

)

MAX

(

PEAK

A 350nS leading-edge blanking (LEB) time is included in

the input of CS pin to prevent the false-trigger caused by

the current spike. In the low power application, if the total

pulse width of the turn-on spikes is less than 350nS and

the negative spike on the CS pin is 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 the small R-C filter (as

shown in figure 16) for higher power application to avoid

the CS pin 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

LD7575 is limited to 75% to avoid the transformer

saturation.

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 in

the secondary side through the photo-coupler to the

COMP pin of LD7575. The input stage of LD7575, like

the UC384X, is with 2 diodes voltage offset then feeding

into the voltage divider with 1/3 ratio, that is,

)

(

)

COMP

PWM

(

COMPARATOR

−

A pull-high resistor is embedded internally thus can be

eliminated on the external circuit.

VCC

GND

LD7575

Can be removed if the negative

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

OUT

350ns

blanking

time

Fig. 15

Fig. 16

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