LD7592

06/05/2017

9

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7592-DS-00 June 2017

Application Information

Operation Overview

The LD7592 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 demands low power

harmonics distortion. It integrates 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 LD7592 PFC controllers

and further to drive the power MOSFET. As shown in Fig.

8, 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 7.5V, respectively.

Vcc

UVLO(on)

UVLO(off)

t

t

I(Vcc)

startup current

(~µA)

operating current (~ mA)

Fig. 8

Startup Current and Startup Circuit

The typical startup circuit to generate the LD7592 VCC is

shown in Fig. 9. During the startup transient, the VCC is

lower than the UVLO threshold. Thus, there is no gate

pulse produced from LD7592 to drive power MOSFET.

Therefore, the current through R1 will provide the startup

current and charge the capacitor C1. Whenever the VCC

voltage is high enough to turn on the LD7592 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

LD7592 is only 1

A. 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.

LD7592

OUT

CS

GND

COMP

VCC

RAMP

INV

OVP

Vin

R1

C1

Fig. 9

Output Voltage Setting

The LD7592 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.

)

Rb

Ra

1

(

V

5

.

2

V

O





(V)