FSL206MR Fairchild Semiconductor, FSL206MR Datasheet - Page 11

• Manufacturer Part Number: FSL206MR
• Description: The FSL206MR integrated Pulse Width Modulator (PWM) and SenseFET is specifically designed for highperformance offline Switch Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSL206MR • Rev. 1.0.2
Line Under-Voltage Protection (LUVP)
If the input voltage of the converter is lower than the
minimum operating voltage, the converter input current
increases too much, causing components failure. If the
input voltage is low, the converter should be protected.
In the FSL206MR, the LUVP circuit senses the input
voltage using the LS pin and, if this voltage is lower than
1.5V, the LUVP signal is generated. The comparator
has 0.5V hysteresis. If the LUVP signal is generated, the
output drive block is shut down and the output voltage
feedback loop is saturated.
Soft-Start
The FSL206MR has an internal soft-start circuit that
slowly increases the feedback voltage, together with the
SenseFET current, after it starts up. The typical soft-
start time is 15ms, as shown in Figure 22, where
progressive increments of the SenseFET current are
allowed during the startup phase. The pulse width to the
power switching device is progressively increased to
establish the correct working conditions for transformers,
inductors, and capacitors. The voltage on the output
capacitors is progressively increased with the intention
of smoothly establishing the required output voltage. It
also helps prevent transformer saturation and reduce
the stress on the secondary diode.
Figure 21.
Figure 22.
Line UVP Circuit


Internal Soft-Start
11
Burst Operation
To minimize power dissipation in standby mode, the
FPS enters burst mode. As the load decreases, the
feedback voltage decreases. As shown in Figure 23, the
device automatically enters burst mode when the
feedback
continues until the feedback voltage drops below V
At this 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
then falls and the process repeats. Burst mode
alternately enables and disables switching of the
SenseFET and reduces switching loss in standby mode.
BURH
Figure 23.
voltage
, switching resumes. The feedback voltage
drops
Burst-Mode Operation
below
V
BURH
.
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Switching
BURL
.