FSL117MRIN FAIRCHILD [Fairchild Semiconductor], FSL117MRIN Datasheet - Page 12

• Manufacturer Part Number: FSL117MRIN
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2012 Fairchild Semiconductor Corporation
FSL117MRN • Rev.1.0.0
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 FSL117MRIN 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
be designed to be below 24.5V.
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 creates 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
2.0V and the SenseFET turn-on time is lower than
1.0μs, the FSL117MRIN recognizes this condition as
an abnormal error and shuts down PWM switching
until V
output short is shown in Figure 23.
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 temperature of the
SenseFET. If the temperature exceeds ~140C, the
thermal shutdown is triggered and stops operation.
The FSL117MRIN operates in auto-restart mode until
the temperature decreases to around 75C, when
normal operation resumes.
Over-Voltage
CC
Figure 23. Output-Short Protection
reaches V
CC
is proportional to the output voltage
START
Protection
again. An abnormal condition
FB
CC
climbs up in a similar
CC
instead of directly
(OVP):
FB
exceeds 24.5V,
is higher than
CC
If
should
FB
and
the
12
4.6 Line Over-Voltage Protection (LOVP): If the line
input voltage is increased until unwanted level, high
line input voltage brings high-voltage stress on the
entire system. To protect from this abnormal
condition, LOVP is included. It is comprised of
detecting V
higher than 1.95V, this condition is recognized as an
abnormal error and PWM switching shuts down until
V
5. Soft Burst Mode: To minimize power dissipation in
Standby Mode, the FSL117MRIN enters Burst Mode.
As the load decreases, the feedback voltage
decreases. The device automatically enters Burst
Mode when the feedback voltage drops below V
(300mV), as shown in Figure 25. 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
then falls and the process repeats. Burst Mode
alternately enables and disables switching of the
SenseFET, reducing switching loss in Standby Mode.
IN
BURH
decreases to around 1.89V (60mV hysteresis).
Figure 24. Line Over-Voltage Protection
(450mV), switching resumes. Feedback voltage
Figure 25. Burst-Mode Operation
IN
using divided resistors. When V
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IN
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is