FSGM0565RB Fairchild Semiconductor, FSGM0565RB Datasheet - Page 11

• Manufacturer Part Number: FSGM0565RB
• Description: The FSGM0565RB is an integrated Pulse Width Modulation (PWM) controller and SenseFET specifically designed for offline Switch-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSGM0565RB • Rev. 1.0.0
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
SenseFET turn-on time. When the V
2V and the SenseFET turn-on time is lower than
1.2μs, the FSGM0565RB recognizes this condition as
an abnormal error and shuts down PWM switching
until V
condition output short is shown in Figure 22.
4.4
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
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
and the FSGM0565RB uses V
monitoring the output voltage. If V
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.
Over-Voltage
Figure 22. Output-Short Protection
CC
reaches V
CC
is proportional to the output voltage
Protection
START
FB
again. An abnormal
CC
climbs up in a similar
CC
instead of directly
(OVP):
FB
exceeds 24.5V,
is higher than
If
FB
and
the
11
5. Soft Burst-Mode Operation: To minimize power
dissipation in standby mode, the FSGM0565RB 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
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
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.
0.60V
0.40V
BURH
V
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
FSGM0565RB operates in auto-restart mode until the
temperature decreases to around 110°C, when
normal operation resumes.
V
I
V
DS
DS
FB
O
(600mV), switching resumes. At this point, the
the
Figure 23. Burst-Mode Operation
FSGM0565RB
t1
Switching
disabled
Soft Burst
t2 t3
stops
BURL
Switching
disabled
(400mV). At this
operation.
t4
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