LD7535A データシートの表示（PDF） - Unspecified

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LD7535A

Green-Mode PWM Controller

Unspecified 

LD7535/LD7535A

Application Information

Operation Overview

The LD7535/LD7535A meet the green-power requirement

and are intended for the use in those modern switching

power suppliers and adaptors which 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 LD7535/LD7535A 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.

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 LD7535/LD7535A is only 20μA.

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.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

t

operating current

(~ mA)

Fig. 14

Current Sensing and Leading-edge Blanking

startup current

(~uA)

t

Fig. 13

Startup Current and Startup Circuit

The typical startup circuit to generate the LD7535/LD7535A

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 LD7535/LD7535A 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

LD7535/LD7535A 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

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

LD7535/LD7535A 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.

IPEAK(MAX)

=

0.85V

RS

9

Leadtrend Technology Corporation www.leadtrend.com.tw

LD7535&LD7535A-DS-01c December 2007

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