LD5523K

05/16/2018

12

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5523K-DS-00 May 2018

Over

Load

Protection

(OLP)

-

Auto

Recovery

To protect the circuit from damage due to over-load

condition and short or open-loop condition, the LD5523K

is implemented with smart OLP function. It also features

auto

–recovery function; see Fig. 16 for the waveform. In

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 2.8V 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

2.8V

t

OUT

OLP delay time

Switching

Switching

Non-Switching

OLP trip Level

UVLO(off)

OLP Reset

Fig. 16

Adjustable Over Current Compensation on

FB Pin

For compensating the differential input current from

high/low line conditions on current sensing resistor,

LD5523K mirrors compensation current IOCP from IFB to

CS pin. The relationship of compensation current IOCP and

IFB is expressed by following equation and shown in Fig.

17.

FB

OCP

I

K

I





, where

5

7

.

0

K



K is the mirror current ratio of FB pin, and the IOCP follows

to the input voltage. When the VCOMP ramps up to the IOCP

threshold of 1.5V, the compensation current is added. The

VCOMP hysteresis of IOCP is 0.15V.

The compensation current IOCP supplies an offset voltage

by external resistor ROCP, which is series between the

current sensing resistor RS and CS pin. By selecting a

proper value of the resistor ROCP in series with the CS pin,

the amount of compensation can be adjusted.

IFB

IOCP

105µA

300µA

78.75µA

225µA

Fig. 17

Output Short Circuit Protection (OSCP) -

Auto Recovery

The OSCP function can prevent the damage from output

short circuit. Once the output is shorted, Vo and VCC drop

immediately, which always reflects the auxiliary winding

during the gate off region. Therefore, as VFB is lower than

0.8V during gate off region, then the FB_UVP is triggered,

and skips one cycle. According to the close loop control,

COMP voltage will pull high in the meanwhile. If the

VCOMP pulls higher than 2.8V over 13 ms and Vcc drops

below 9.6V. At this time, the OSCP protection will be

triggered and turn off the gate driving.