AN-6754A

APPLICATION NOTE

© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 10/8/10

6

Figure 15.

Brownout Circuit

Figure 16.

Brownout Protection Behavior

Overload / Open-Loop Protection (OLP)

When output is overloaded, the drain current reaches its

pulse-by-pulse current limit level, limiting the input power.

Then, the output voltage drops and no current flows through

the opto-diode, which causes the feedback voltage to

increase above the OLP protection threshold (4.6V). A

similar response occurs when the feedback loop is open and

no current flows through the opto-diode.

When the feedback voltage is higher than 4.6V for longer

than the OLP delay time, the OLP protection is triggered, as

shown in Figure 17.

Figure 17.

OLP Behavior

Sense Pin Short-Circuit Protection (SSCP)

This protection is used for Limited Power Source (LPS)

ground, thus increasing the output power. If the output

voltage rating is set to 19V, this protection should be

triggered before the output wattage increases to 100W.

shorted to ground, the FAN6754A detects the voltage on the

SENSE pin during the gate’s on time. After debounce time,

The sense resistor short-circuit protection is triggered if all

of the following occur:

gate is in a HIGH state; and

50mV; and

during the debounce time of 120µs, if all pulse

conditions are as above or the SSCP counter is reset.

Because of the fixed gate’s on-time checking, the peak

current of the inductor is different between high line and

input voltage ranges.

Figure 18.

SSCP Detection Method

To prevent miss-triggered protection in normal mode

input voltages and light-load conditions, the transformer’s

inductance should not be too high in inductance.

The following is a design example;

datasheet.

the datasheet.