LD7765GS datasheet(13/19 Pages) LEADTREND

13 / 19 page

LD7765

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7765-DS-00

October 2012

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

IP.

Maximum Duty-Cycle

The maximum duty-cycle of LD7765 is limited to 75% to

avoid the transformer saturation.

On/Off Control

The gate driver of LD7765 can be disabled immediately

by pulling COMP pin voltage level lower than Zero Duty

Trip Level. The disable-mode can be released when

Comp pin voltage level is pulled high above Zero Duty

Trip Level.

Over Load Protection (OLP)- Auto Recovery

To protect the circuit from being damaged at over load

condition, short or open loop condition, the LD7765 is

implemented with smart OLP function. LD7765 features

auto recovery function, see Fig 14 for the waveform. In the

example of the fault condition, the feedback system will

force the voltage loop enter toward saturation and then

pull the voltage high across COMP pin (VCOMP).

When

the VCOMP ramps up to the OLP tripped level (4.6V) for

more than the OLP delay time, the protection will be

activated to turn off the gate output and to stop the

switching of power circuit. The OLP delay time is set by

internal high frequency counter. It is to prevent the false

triggering during the power-on and turn-off transient.

A divide-2 counter is implemented to reduce the average

power under OLP behavior.

Whenever OLP is activated,

the output is latched off and the divide-2 counter starts to

count the number of UVLO(off).

The latch will not be

released and the output will not be resumed until the 3rd

UVLO(off) level is tripped. With the protection mechanism,

the average input power will be less than ever, so that the

component temperature and stress can be controlled

within the safe operating area.

VCC

UVLO(on)

UVLO(off)

COMP

OLP

VOLP

OUT

OLP Delay Time

Switching

Non-Switching

OLP trip Level

2nd UVLO(off)

OLP Counter Reset

BNO

Detecting

Fig 14.

OVP (Over Voltage Protection) on Vcc -

Auto Recovery

The maximum Vcc ratings of the LD7765 are mostly for

30V. To protect the controller in over-voltage condition,

LD7765 is implemented with OVP function on VCC. Once

the VCC voltage ramps over the OVP threshold, it will

shut off the output gate drive circuit right away and disable

the power MOSFET until the 2nd UVLO(ON) is tripped.

The Vcc OVP function is auto-recoverable.

If the OVP

condition, usually caused by open-loop of feedback, is not

released, the Vcc will trip the OVP level again and

re-shutdown the output to resume.

That makes the Vcc

work in hiccup mode. Fig 15 shows its operation.

After the OVP condition is removed, the Vcc will resume

its operation level and the output in the normal operation.

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