LD7591 datasheet(10/20 Pages) LEADTREND

10 / 20 page

LD7591

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7591-DS-00 March 2010

Application Information

Operation Overview

The LD7591 is an excellent voltage mode PFC controller.

It meets the IEC61000-3-2 requirement and is intended

for the use in those pre-regulator that demand low power

harmonics distortion. It integrated more functions to

reduce the external components counts and the size. Its

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 LD7591 PFC controllers

and further to drive the power MOSFET. As shown in

Fig. 12, a hysteresis is built in to prevent the shutdown

from the voltage dip during start up. The turn-on and

turn-off threshold level are set at 12.0V and 8.5V,

respectively.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

startup current

(~uA)

operating current

(~ mA)

Fig. 12

Startup Current and Startup Circuit

The typical startup circuit to generate the LD7591 Vcc is

shown in Fig. 13. During the startup transient, the Vcc is

lower than the UVLO threshold thus there is no gate pulse

produced from LD7591 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 LD7591 and

further to deliver the gate drive signal, the supply current

is provided from the auxiliary winding of the PFC choke.

Lower startup current requirement on the PFC 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

LD7591 is only 30

μA. If a higher resistance value of 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. 13

Output Voltage Setting

The LD7591 monitors the output voltage signal at INV pin

through a resistor divider pair Ra and Rb. A

transconductance amplifier is used instead of the

conventional voltage amplifier. The transconductance

amplifier (voltage controlled current source) aids the

implementation of OVP and disables function. The output

current of the amplifier changes according to the voltage

difference of the inverting and non-inverting input of the

amplifier. The output voltage of the amplifier is compared

with the internal ramp signal to generate the turn-off

signal. The output voltage is determined by the following

relationship.

)

(

VOUT

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