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LD7750BGS

High Voltage Green-Mode PWM Controller with Over Temperature Protection

Unspecified 

LD7750B

Vcc

UVLO(on)

UVLO(off)

Nevertheless, it is strongly recommended to add a small

R-C filter (as shown in Fig. 16) for higher power

applications to avoid the CS pin being damaged by the

negative turn-on spike.

t

HV Current

1mA

Vcc current

Startup Current

~ 0mA (off)

t

Operating Current

(Supply from Auxiliary Winding)

Fig. 14

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 LD7750B 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 0.45V. Thus the MOSFET

peak current can be calculated as:

I PEAK ( MAX )

=

0.45V

RS

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

LD7750B 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 LD7750B. The input stage of LD7750B,

like the UC384X, is incorporated with 2 diodes voltage

offset circuit and a voltage divider with 1/6 ratio.

Therefore,

V+ (PWM COMPARATOR

)

=

1

6

× (VCOMP

−

2VF )

A pull-high resistor is embedded internally and thus no

external one is required for it.

Switching Frequency

The LD7750B is implemented with frequency swapping

function which helps the power supply designers both

optimize EMI performance and lower system cost. The

switching frequency substantially centers at 100kHz, and

trembles within the range of ±6KHz.

A 250nS leading-edge blanking (LEB) time is included in

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

current spike. For those low power applications, if the total

pulse width of the turn-on spike is less than 250nS and

the negative spike on the CS pin is above -0.3V, the R-C

filter (as shown in Fig.15) is removable.

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

related to the output power, circuit design and PCB layout.

Leadtrend Technology Corporation

LD7750B-DS-01 OCT. 2011

10

www.leadtrend.com.tw

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