LD7575GS datasheet(11/17 Pages) LEADTREND

11 / 17 page

LD7575

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7575-DS-04b November 2009

The Vcc OVP function in LD7575 is an auto-recovery type

protection. If the OVP condition, usually caused by the

feedback loop opened, is not released, the Vcc will tripped

the OVP level again and re-shutdown the output. The

Vcc is working as a hiccup mode. Figure 18 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 is

automatically returned to the normal operation.

VCC

UVLO(on)

UVLO(off)

OVP Tripped

OUT

Switching

Non-Switching

OVP Level

Fig. 18

Fault Protection

A lot of protection features have been implemented in the

LD7575 to prevent the power supply or adapter from

being damaged caused by single fault condition on the

open or short condition on the pin of LD7575. Under the

conditions listed below, the gate output will be off

immediately to protect the power circuit ---

RT pin short to ground

RT pin floating

CS pin floating

Pull-Low Resistor on the Gate Pin of

MOSFET

In LD7575, an anti-floating resistor is implemented on the

OUT pin to prevent the output from any uncertain state

which may causes the MOSFET working abnormally or

false triggered-on. However, such design won’t cover

the condition of disconnection of gate resistor Rg thus it is

still strongly recommended to have a resistor connected

on the MOSFET gate terminal (as shown in figure 19) to

provide extra protection for fault condition.

This external pull-low resistor is to prevent the MOSFET

from damage during power-on under the gate resistor is

disconnected. In such single-fault condition, as show in

figure 21, the resistor R8 can provide a discharge path to

avoid the MOSFET from being false-triggered by the

current

through

the

gate-to-drain

capacitor

Cgd.

Therefore, the MOSFET is always pull-low and kept in the

off-state whenever the gate resistor is disconnected or

opened in any case.

Fig. 19

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