LD9535C

09/21/2020

13

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD9535C-DS-00 September 2020

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. 17

Over Voltage Protection on FB Pin (FB

OVP)

– Auto Recovery

An output overvoltage protection is implemented in the

LD9535C. 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 LD9535C goes to

auto-recovery protection mode till the FB OVP status is

defused.

Fig. 18

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

15 ms and Vcc drops below 9.6V. At this time, the OSCP

protection will be triggered and turn off the gate driving.

Oscillator and Switching Frequency

The LD9535C is implemented with frequency swapping

function which helps the power supply designers to both

optimize EMI performance and lower system cost.

Green Mode Operation

By

using

the

green-mode

control,

the

switching

frequency can be reduced under the light load condition.

This feature helps to improve the efficiency in light load

conditions.

The

green-mode

control

is

Leadtrend

Technology

’s own property.

Fault Protection

There are several critical protections integrated in the

LD9535C to prevent from damage to the power supply.

Those damages usually come from open or short

conditions on the pins of LD9535C.

LD9535C

R1

R2

VCC

FB

GND

Va

VFB