LD5523K

05/16/2018

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5523K-DS-00 May 2018

If VDS voltage falls to resonant valley level from max

plateau value, the QRD comparator will be tripped while

FB pin current is close to 20

A.

However, the QR detection will be influenced by

propagation delay. If inductance of primary winding is less

than 500

H, there is no valley detection in Vds (as shown

in Fig. 11).

L<500μ

H

Vaux.

0V

VQRD(IFB=20µA)

t

TPD

TPD

Fig. 11

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 at

the secondary side through the photo-coupler to the

COMP pin of the LD5523K. Similar to UC3842, the

LD5523K would without voltage offset to feed the voltage

divider at the ratio of RA and RB, that is,

COMP

COMPARATOR

V

RB

RA

RB

)

PWM

(

V









A pull-high resistor is embedded internally and therefore

no external one is required.

The LD5523K integrates the multi-mode PWM controller,

and for enhance the light load efficiency, the comp pin

value corresponding to the frequency is as shown in Fig.

12.

VCOMP

FSW

65kHz

24kHz

0.5V 0.6V

Green Mode + QR PWM/CCM

1.1V

1.45V

Max. frequency depends on LM

Fig. 12

Current Sensing, Leading Edge Blanking

The typical current mode of PWM controller feedbacks

both current signal and voltage signal to close the control

loop and achieve regulation. The LD5523K detect 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 sets at 0.5V. From

above, the MOSFET peak current can be obtained from

below.

CS

)

MAX

(

PEAK

R

V

5

.

0

I



A 275ns leading-edge blanking (LEB) time is included in

the input of CS pin to prevent the false-trigger from the

current spike. In the low power application, if the total

pulse width of the turn-on spike is less than 275ns and the

negative spike on the CS pin doesn

’t exceed -0.3V, it

could remote the R-C filter (as shown in the Fig. 13).

However, the total pulse width of the turn-on spike is

decided by the output power, circuit design and PCB

layout. It is strongly recommended to adopt a smaller R-C

filter (as shown in Fig. 14) for higher power application to

avoid the CS pin being damaged by the negative turn-on

spike.