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LD7550

Green-Mode PWM Controller

Unspecified 

LD7550

Application Information

Operation Overview

As long as the green power requirement becomes a trend

and the power saving is getting more and more important for

the switching power supplies and switching adaptors, the

traditional PWM controllers are not able to support such new

requirements. Furthermore, the cost and size limitation force

the PWM controllers need to be powerful to integrate more

functions to reduce the external part counts. The LD7550

is targeted on such application to provide an easy and cost

effective solution; its detail features are described as below:

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented to detect the voltage

on the Vcc pin to ensure the supply voltage is enough to

power on the LD7550 PWM controller and further to drive

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

implemented 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)

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

special circuit design, the maximum startup current of

LD7550 is only 25µA.

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

LD7550

CS

GND

Fig. 7

startup current

(~uA)

t

Fig. 6

Startup Current and Startup Circuit

The typical startup circuit to power up the LD7550 is shown

in Fig. 7. During the startup transient, the Vcc is lower than

the UVLO threshold thus there is no gate pulse generated

from LD7550 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 LD7550 and further to

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 LD7550

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

6

Leadtrend Technology Corporation

LD7550-DS-00 August 2004

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