LD7831GS datasheet(13/17 Pages) LEADTREND

13 / 17 page

LD7831

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7831-DS-00 April 2014

Output OVP on ZCD

When the LED string open circuit occurs, the reflected

output voltage of aux-winding will cause ZCD voltage up.

If the ZCD voltage runs up to 3.5V, LD7831 will shut the

gate off until the 2nd cycle of Vcc hiccup is tripped, as

shown in Fig. 17. The selection of output OVP level is

subject to ZCD divide resistance as the below equation:

3.5V

VCC

OVP







ZCD

OVP Level

ZCD OVP

Tripped

OUT

Switching

Non-Switching

Vout

Vcc

LED Opened

UVLO(on)

UVLO(off)

Fig. 17.

LED Short Protection (ZCD UVP)

– Auto

Recovery

To protect the circuit from damage due to LED short, an

auto-recovery

type

ZCD

UVP

protection

implemented for it. Fig. 18 shows the waveforms of the

ZCD UVP operation. As LED shorts, the reflected output

voltage of aux-winding will cause ZCD voltage drop. If

the ZCD voltage decreases below 0.5V for over the

delay time of ZCD UVP, the protection will be activated

to turn off the gate until the 4th cycle of Vcc hiccup is

tripped. The ZCD UVP delay time is to prevent the

false-triggering.

Nvcc/Ns

∙V

OUT

ZCD

UVP Level

ZCD UVP

Tripped

OUT

Switching

Non-Switching

Aux-winding

voltage

Vcc

LED Shorted

ZCD UVP

Delay Time

UVLO(on)

UVLO(off)

GND

ZCD UVP

Delay Time

-Nvcc/Np

∙ V

Fig. 18.

OVP (Over Voltage Protection) on Vcc

The maximum rating of the VCC pin is limited below 28V.

To protect VCC from damage due to fault condition, the

LD7831 is implemented with OVP function on Vcc pin.

As soon as the VCC voltage is over OVP threshold

(28V), the output gate drive circuit will be shutdown

simultaneously thus to stop the switching of the power

MOSFET until the second UVLO(on) arrives. The Vcc

OVP function of the LD7831 is an auto-recovery

protection. The Fig. 19 shows its operation. Upon

removal of the OVP condition will resume the VCC level

and the output operation.

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