LD7576GS datasheet(11/22 Pages) LEADTREND

11 / 22 page

LD7576/76H/76J/76K

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7576-DS-04a

December 2009

Current Sensing, Leading-Edge Blanking

and the Negative Spike on CS Pin

The typical current mode PWM controller feeds back both

current signal and voltage signal to close the control loop

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:

)

MAX

(

PEAK

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

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

current spike. In 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 output power, circuit design and PCB layout.

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

shown in figure 16) for higher 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 500mA

driving capability, is incorporated to drive a power

MOSFET directly.

The maximum duty-cycle of LD7576X

series is 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,

)

(

)

COMP

PWM

(

COMPARATOR

−

A pull-high resistor is embedded internally. Generally, an

external capacitor in parallel to photo-coupler is required

in application.

VCC

GND

LD7576X

Can be removed if the negative

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

OUT

230ns

blanking

time

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

operations in consideration of the EMI performance,

LD7576GS Datasheet(PDF) 11 Page - Leadtrend Technology

LD7576GS Datasheet(HTML) 11 Page - Leadtrend Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL