FSD146MRBN Fairchild Semiconductor, FSD146MRBN Datasheet - Page 11

• Manufacturer Part Number: FSD146MRBN
• Description: The FSD146MRBN is an integrated Pulse Width Modulation (PWM) controller and SenseFET designed for offline Switch-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSD146MRBN • 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 FSD146MRBN 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.
OSP protects the device from this abnormal condition.
It is comprised of detecting V
on time. When the V
SenseFET turn-on time is lower than 1.0μs, this
condition is recognized as an abnormal error and
PWM switching shuts down until V
again. An abnormal condition output short is shown in
Figure 22.
4.5 Thermal Shutdown (TSD): The SenseFET and
the control IC on a die in one package make 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 FSD146MRBN operates in auto-restart mode
until the temperature decreases to around 75C,
when normal operation resumes.
Over-Voltage
Figure 22. Output-Short Protection
CC
is proportional to the output voltage
FB
Protection
is higher than 2.0V and the
FB
FB
CC
climbs up in a similar
and SenseFET turn-
CC
instead of directly
CC
(OVP):
exceeds 24.5V,
reaches V
CC
If
should
START
the
11
5. Soft Burst-Mode Operation: To minimize power
dissipation in Standby Mode, the FSD146MRBN 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 the standby current load.
This causes the feedback voltage to rise. Once it
passes V
feedback voltage then falls and the process repeats.
Burst Mode alternately enables and disables switching
of the SenseFET, thereby 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 the EMI noise around typical switching
frequency (67kHz) and can reduce the cost of the input
filter used to meet EMI requirements (e.g. EN55022).
Figure 24. Random Frequency Fluctuation
Figure 23. Burst Mode Operation
BURH
(500mV), switching resumes. The
BURL
(350mV). At this
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