LD7575 Ver la hoja de datos (PDF) - Unspecified

Número de pieza

componentes Descripción

Fabricante

LD7575

Green-Mode PWM Controller

Unspecified 

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:

IPEAK(MAX)

=

0.85V

RS

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,

V+ (PWM COMPARATOR

)

=

1

3

× (VCOMP

−

2VF )

A pull-high resistor is embedded internally thus can be

eliminated on the external circuit.

Leadtrend Technology Corporation

LD7575-DS-04b November 2007

9

www.leadtrend.com.tw

LD7575

Fig. 15

Fig. 16

Share Link: