LD7538

12

Leadtrend Technology Corporation

LD7538-DS-00

November 2012

case of fault condition, the feedback system will force the

voltage loop toward the saturation and then pull the

voltage high on COMP pin (VCOMP).

When the VCOMP

ramps up to the OLP threshold of 4.4V and continues over

OLP delay time, the protection will be activated and then

turn off the gate output to stop the switching of power

circuit.

With the protection mechanism, the average input power

will be minimized to remain the component temperature

and stress within the safe operating area.

VCC

UVLO(on)

UVLO(off)

t

t

COMP

OLP

OLP

t

OUT

OLP delay time

Switching

Switching

Non-Switching

OLP trip Level

UVLO(off)

OLP Reset

Fig. 20

OVP (Over Voltage Protection) on Vcc -

Latch mode

The Vcc OVP function of LD7538 is in latch mode. As

soon as the voltage of the Vcc pin rises above OVP

threshold, the output gate drive circuit will be shutdown

simultaneous to latch off the power MOSFET. On the

contrast, if the voltage on Vcc pin drops below OVP

threshold and starts AC-recycling again, it will soon

resume to normal operation. Fig. 21 shows its operation.

UVLO(on)

UVLO(off)

t

AC input Voltage

t

OUT

Switching

Switching

Non- Switching

t

VCC

AC Off

AC On( Recycle)

Latch Released

OVP

Level

Fig. 21

OTP Pin --- Latched Mode Protection

The OTP circuit is implemented to sense whether there is

any hot-spot of power circuit like power MOSFET or

output rectifier.

Once an over-temperature condition is

detected, the OTP is enabled to shut down the controller

to protect the controller. Typically, a NTC is recommended

to connect with OTP pin.

The NTC resistance will

decrease as the device or ambient in high temperature.

The relationship is as below.

NTC

OTP

R

A

μ

100

V





When the VOTP is below the defined voltage threshold (typ.

0.95V), LD7538 will shutdown the gate output and latch

off the power supply.

There are 2 conditions required to

restart it successfully. First, cool down the circuit so that

NTC resistance will increase and raise VOTP up above

1.05V. Then, remove the AC power cord and re-plug AC

power.

MOSFET Characteristic

The MOSFET is divided into three operation regions,

ohmic region, saturation region, and the cut-off region,

shown as Fig. 22.

For switching power supply applications, it shall operate in

ohmic and cut-off region. Never reach the region of