FSL136MR Fairchild Semiconductor, FSL136MR Datasheet - Page 11

• Manufacturer Part Number: FSL136MR
• Description: IC PWM/SEMSEFET SMPS 3A 8-DIP
• Manufacturer: Fairchild Semiconductor
• Series: FPS™r

Specifications of FSL136MR

• Output Isolation: Isolated
• Frequency Range: 61kHz ~ 73kHz
• Voltage - Input: 9 V ~ 26 V
• Voltage - Output: 650V
• Power (watts): 26W
• Operating Temperature: -40°C ~ 105°C
• Package / Case: 8-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2009 Fairchild Semiconductor Corporation
FSL136MR • Rev. 1.0.7
is lower than 1.0µs, the FPS recognizes this condition
as an abnormal error and shuts down PWM switching
until V
output is shown in Figure 20.
Soft-Start
The FPS has an internal soft-start circuit that slowly
increases the feedback voltage, together with the
SenseFET current, after it starts. The typical soft-start
time is 15ms, as shown in Figure 21, 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
inductors, and capacitors. The voltage on the output
capacitors is progressively increased with the intention
of smoothly establishing the required output voltage.
Soft-start helps prevent transformer saturation and
reduce the stress on the secondary diode.
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 22,
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
drop at a rate dependent on standby current load. This
causes the feedback voltage to rise. Once it passes
V
falls and the process repeats. Burst mode alternately
enables and disables switching of the SenseFET and
reduces switching loss in standby mode.
BURH
Figure 20.
correct
, switching resumes. The feedback voltage then
CC
reaches V
voltage
Figure 21.
working
Output Short Waveforms (OSP)
START
drops
conditions
Internal Soft-Start
again. An abnormal condition
below
for
V
BURH
transformers,
.
Switching
BURL
.
11
Adjusting Peak Current Limit
As shown in Figure 23, a combined 6kΩ internal
resistance is connected to the non-inverting lead on the
PWM comparator. An external resistance of Rx on the
current limit pin forms a parallel resistance with the 6kΩ
when the internal diodes are biased by the main current
source of 400µA. For example, FSL136MR has a typical
SenseFET peak current limit (I
adjusted to 1.5A by inserting Rx between the I
the ground. The value of the Rx can be estimated by
the following equations:
where X is the resistance of the parallel network.
2.15A
X =
Figure 23. Peak Current Limit Adjustment
Rx
:
1.5A
Figure 22. Burst-Mode Operation
||
6kΩ
=
6kΩ
:
XkΩ
LIM
) of 2.15A. I
www.fairchildsemi.com
LIM
PK
pin and
can be
(1)
(2)