FSD176MRT Fairchild Semiconductor, FSD176MRT Datasheet - Page 11

• Manufacturer Part Number: FSD176MRT
• Description: The FSD176MRT 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
FSD176MRT • 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 FSD176MRT 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.
4.5 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 stops operation.
The FSD176MRT operates in auto-restart mode until
the temperature decreases to around 75C, when
normal operation resumes.
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, the FSD176MRT recognizes this condition as
an abnormal error and shuts down PWM switching
until V
output short is shown in Figure 22.
Over-Voltage
CC
Figure 22. 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
instead of directly
CC
(OVP):
FB
exceeds 24.5V,
is higher than
CC
If
should
FB
and
the
11
5. Soft Burst-Mode Operation: To minimize power
dissipation in Standby Mode, the FSD176MRT enters
Burst-Mode operation. As the load decreases, the
feedback voltage decreases. 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 switching of the
SenseFET, 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 an internal free-running
oscillator at every switching instant. This random
frequency fluctuation scatters the EMI noise around
typical switching frequency (67kHz) effectively and can
reduce the cost of the input filter included to meet the
EMI requirements (e.g. EN55022).
BURH
Figure 24. Random Frequency Fluctuation
(450mV), switching resumes. The feedback
BURL
Figure 23. Burst-Mode Operation
(300mV), as shown in Figure 23. At this
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