LD9535T

04/08/2021

13

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD9535T-DS-00 April 2021

Fig. 17

Over Voltage Protection on FB Pin (FB

OVP) – Auto Recovery

An output overvoltage protection is implemented in the

LD9535T. The auxiliary winding voltage can be reflected

from secondary winding, in which the FB pin voltage is

proportional to output voltage during the gate off time.

OVP is worked by sensing the auxiliary voltage via the

divided resistors R2, refereeing to Fig. 18. The equation

of FB OVP is shown as follows.

R2

R1 ∙VFB_OVP

Va VFB_OVP

 

Va

Na

NS

VO VF  

VFB_OVP is the FB pin OVP trip voltage level. Va is the

auxiliary winding voltage which reflects from the forward

voltage VF of Schottky diode and output voltage VO. NS

is turns ration of secondary-side winding.

If VFB overs the FB OVP trip level, the internal counter

starts counting 8 cycles, and then LD9535T goes to

auto-recovery protection mode till the FB OVP status is

defused.

Fig. 18

Under Voltage Protection 1 on FB Pin (FB

UVP 1) – Auto Recovery

The FB UVP 1 function can prevent the damage from

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

immediately. According to the close loop control, COMP

voltage will pull high in the meanwhile. Therefore, as VFB

is lower than FB UVP trip level during gate off region

and VCOMP pulls higher than 2.8V over 8 cycles, then

the FB UVP 1 will be activated to stop the switching of

the power MOSFET until the next UVLO(on). The Fig.

19 shows its operation.

Fig. 19