Data Sheet
© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSGM0765R • Rev. 1.0.2 11
FSGM0765R — Green-Mode Fairchild Power Switch (FPS™)
4.3. Output-Short Protection (OSP): If the output is
shorted, steep current with extremely high di/dt can
flow through the SenseFET during the minimum turn-
on time. Such a steep current brings high-voltage
stress on the drain of the SenseFET when turned off.
To protect the device from this abnormal condition,
OSP is included. It is comprised of detecting V
FB
and
SenseFET turn-on time. When the V
FB
is higher than
2V and the SenseFET turn-on time is lower than
1.2μs, the FSGM0765R recognizes this condition as
an abnormal error and shuts down PWM switching
until V
CC
reaches V
START
again. An abnormal
condition output short is shown in Figure 22.
Figure 22. Output-Short Protection
4.4 Over-Voltage Protection (OVP): If the
secondary-side feedback circuit malfunctions or a
solder defect causes an opening in the feedback
path, the current through the opto-coupler transistor
becomes almost zero. Then V
FB
climbs up in a similar
manner to the overload situation, forcing the preset
maximum current to be supplied to the SMPS until
the overload protection is triggered. Because more
energy than required is provided to the output, the
output voltage may exceed the rated voltage before
the overload protection is triggered, resulting in the
breakdown of the devices in the secondary side. To
prevent this situation, an OVP circuit is employed. In
general, the V
CC
is proportional to the output voltage
and the FSGM0765R uses V
CC
instead of directly
monitoring the output voltage. If V
CC
exceeds 24.5V,
an OVP circuit is triggered, resulting in the
termination of the switching operation. To avoid
undesired activation of OVP during normal operation,
V
CC
should be designed to be below 24.5V.
4.5 Thermal Shutdown (TSD): The SenseFET and
the control IC on a die in one package makes it
easier for the control IC to detect the over
temperature of the SenseFET. If the temperature
exceeds ~140°C, the thermal shutdown is triggered
and the FSGM0765R stops operation. The
FSGM0765R operates in auto-restart mode until the
temperature decreases to around 110°C, when
normal operation resumes.
5. Soft Burst-Mode Operation: To minimize power
dissipation in standby mode, the FSGM0765R enters
burst-mode operation. As the load decreases, the
feedback voltage decreases. As shown in Figure 23,
the device automatically enters burst mode when the
feedback voltage drops below V
BURL
(400mV). At this
point, switching stops and the output voltages start to
drop at a rate dependent on standby current load. This
causes the feedback voltage to rise. Once it passes
V
BURH
(600mV), switching resumes. At this point, the
drain current peak increases gradually. This soft burst-
mode can reduce audible noise during burst-mode
operation. The feedback voltage then falls and the
process repeats. Burst-mode operation alternately
enables and disables switching of the SenseFET,
thereby reducing switching loss in standby mode.
Figure 23. Burst-Mode Operation