LD9535

01/08/2019

12

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD9535-DS-00 January 2019

CCM Switching Frequency Clamp

According to the QR operation principle, the switching

frequency is inversely proportional to the output power.

Therefore, as the output power increases, the switching

frequency can become rather low without limiting. The

CCM switching frequency of LD9535 is clamped at 65

kHz internally to provide the optimized operations by

considering the EMI performance, thermal treatment,

component sizes and transformer design

.

Over Voltage Protection on VCC Pin

(VCC OVP)

– Auto Recovery

LD9535 is implemented with an OVP function on VCC.

Whenever the VCC voltage is higher than the OVP

threshold voltage, the LD9535

will be shutdown

simultaneously thus to stop the switching of the power

MOSFET until the next UVLO(ON).The VCC OVP

function in LD9535 is an auto-recovery type protection.

The Fig. 16 shows its operation.

On the other hand, if the OVP condition is removed, the

VCC level will get back to normal level and the output

will automatically return to the normal operation.

VCC

UVLO(on)

UVLO(off)

t

OVP Tripped

t

OUT

Switching

Switching

Non-Switching

OVP Level

Fig. 16

Over

Load

Protection

(OLP)

– Auto

Recovery

To protect the circuit from damage due to over-load

condition and short or open-loop condition, the LD9535

is implemented with smart OLP function. It also features

auto

–recovery function, see Fig. 17 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. 17

Over Voltage Protection on FB Pin (FB

OVP)

– Auto Recovery

An output overvoltage protection is implemented in the

LD9535. 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.