FSD156MRBN Fairchild Semiconductor, FSD156MRBN Datasheet - Page 11

• Manufacturer Part Number: FSD156MRBN
• Description: The FSD156MRBN 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

© 2011 Fairchild Semiconductor Corporation
FSD156MRBN • 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 FSD156MRBN 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 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
2.0V and the SenseFET turn-on time is lower than
1.0μs, this condition is recognized as an abnormal
error and PWM switching shuts down until V
reaches V
short is shown in Figure 22.
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 ~135C, the
thermal shutdown is triggered and stops operation.
The FSD156MRBN operates in Auto-Restart Mode
until the temperature decreases to around 75C,
when normal operation resumes.
Over-Voltage
Figure 22. Output-Short Protection
START
CC
again. An abnormal condition output
is proportional to the output voltage
Protection
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
CC
11
5. Soft Burst-Mode Operation: To minimize power
dissipation in Standby Mode, the FSD156MRBN 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
voltage then falls and the process repeats. Burst Mode
alternately enables and disables SenseFET switching,
reducing switching loss in Standby Mode.
6. Random Frequency Fluctuation (RFF): Fluctuating
switching frequency of an SMPS can reduce EMI by
spreading the energy over a wide frequency range. The
amount of EMI reduction is directly related to the
switching frequency variation, which is limited internally.
The switching frequency is determined randomly by
external feedback voltage and internal free-running
oscillator at every switching instant. RFF effectively
scatters EMI noise around typical switching frequency
(67kHz) and can reduce the cost of the input filter
included to meet the EMI requirements (e.g. EN55022).
BURH
Figure 23. Figure 23. Burst-Mode Operation
Figure 24. Random Frequency Fluctuation
(500mV), switching resumes. The feedback
BURL
(350mV). At this
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