LD7552B

8

Leadtrend Technology Corporation

LD7552B-DS-01a January 2008

Application Information

Operation Overview

The LD7552B meet the green-power requirement and are

intended for the use in those modern switching power

suppliers and adaptors that demand higher power efficiency

and power-saving. They integrated more functions to reduce

the external component counts and the size. Their major

features are described as below.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage on the VCC pin. It would assure the supply voltage

enough to turn on the LD7552B PWM controller and further

to drive the power MOSFET.

As shown in Fig. 13, a

hysteresis is built in to prevent the shutdown from the

voltage dip during startup.

The turn-on and turn-off

threshold level are set at 16.0V and 10.0V, respectively.

Vcc

UVLO(on)

UVLO(off)

t

t

I(Vcc)

startup current

(~uA)

operating current

(~ mA)

Fig. 13

Startup Current and Startup Circuit

The typical startup circuit to generate the LD7552B Vcc is

shown in Fig. 14.

During the startup transient, the Vcc is

lower than the UVLO threshold thus there is no gate pulse

produced

from

LD7552B

to

drive

power

MOSFET.

Therefore, the current through R1 will provide the startup

current and to charge the capacitor C1.

Whenever the Vcc

voltage is high enough to turn on the LD7552B and further

to deliver the gate drive signal, the supply current is

provided from the auxiliary winding of the transformer.

Lower startup current requirement on the PWM controller

will help to increase the value of R1 and then reduce the

power consumption on R1. By using CMOS process and the

special circuit design, the maximum startup current of

LD7552B is only 20

μA.

If a higher resistance value of the R1 is chosen, it usually

takes more time to start up. To carefully select the value of

R1 and C1 will optimize the power consumption and startup

time.

Fig. 14

Current Sensing and Leading-edge Blanking

The typical current mode of PWM controller feedbacks both

current signal and voltage signal to close the control loop

and achieve regulation. As shown in Fig. 15, the LD7552B

detect 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. From

above, the MOSFET peak current can be obtained from

below.

S

)

MAX

(

PEAK

R

V

85

.

0

I

=