LD7552IN データシートの表示（PDF） - Unspecified

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LD7552IN

Green-Mode PWM Controller

Unspecified 

Application Information

Operation Overview

The LD7552 is optimized to achieve power saving and

minimize the external components counts. The device

incorporated several functions to make it ideal to use in

switching power supplies and switching adaptors.

Under Voltage Lockout (UVLO)

An UVLO comparator is included to detect the voltage on

the Vcc pin to ensure the supply voltage enough to power

on the LD7552 PWM controller and in addition to drive the

power MOSFET. As shown in Fig. 6, a hysteresis is

provided to prevent the shutdown from the voltage dip

during startup. The turn-on and turn-off threshold level are

set at 16V and 11.4V, respectively.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

t

operating current

(~ mA)

special circuit design, the maximum startup current of

LD7552 is only 25 A.

Theoretically, R1 can be very high resistance value.

However, higher R1 will cause longer startup time. By

properly select the value of R1 and C1, it can be optimized

under the consideration of R1 power consumption and the

startup time.

AC

input

EMI

Filter

Cbulk

R1

D1

C1

VCC

OUT

LD7552

CS

GND

Fig. 7

startup current

(~uA)

t

Fig. 6

Startup Current and Startup Circuit

The typical startup circuit as shown in Fig. 7 powers ups the

LD7552. During the startup transient, the Vcc is lower than

the UVLO threshold thus there is no gate pulse generated

from LD7552 to drive power MOSFET. Therefore, the

current through R1 is to provide the startup current as well

as charge the capacitor C1. Whenever the Vcc voltage is

higher enough to power on the LD7552 and further to

deliver the gate drive signal, the supply current is provided

from the auxiliary winding of the transformer. The lower

startup current requirement on the PWM controller will help

to increase the R1 value and then reduce the power

consumption on R1. By using CMOS process and the

Current Sensing and Leading-edge Blanking

The typical current mode PWM controller feedbacks both

current signal and voltage signal to close the control loop

and achieve regulation. As shown in Fig. 8, the LD7552

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 250nS leading-edge blanking time is included in the input

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

spike and further to eliminate the need of R-C filter which is

usually needed in the typical UC384X application (Fig. 9).

6

Leadtrend Technology Corporation

LD7552-DS-00 February, 2005

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