FSL136HR Fairchild Semiconductor, FSL136HR Datasheet - Page 11

• Manufacturer Part Number: FSL136HR
• Description: Power Switch ICs - Power Distribution Green Mode Fairchild Power Switch
• Manufacturer: Fairchild Semiconductor

Specifications of FSL136HR

• On Resistance (max): 4 Ohms
• Maximum Operating Temperature: + 105 C
• Minimum Operating Temperature: - 40 C
• Package / Case: DIP-8
• Maximum Power Dissipation: 1.5 W
• Off Time (max): 20 ns
• On Time (max): 12 ns
• Operating Frequency: 100 KHz
• Supply Current: 1.8 mA
• Supply Voltage (max): 26 V
• Switch Current (typ): 2.5 mA
• Lead Free Status / Rohs Status: Details

© 2011 Fairchild Semiconductor Corporation
FSL136HR • Rev. 1.0.0
OSP detects V
V
is lower than 1.2µs, 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 20ms, 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
below V
output voltages drop at a rate dependent on standby
current load. This causes the feedback voltage to rise.
Once it passes V
FB
is higher than 1.6V and the SenseFET turn-on time
Figure 20. Output Short Waveforms (OSP)
correct
CC
BURL
reaches V
voltage
. At this point, switching stops and the
until
Figure 21. Internal Soft-Start
FB
working
BURH
and SenseFET turn-on time. When the
START
the
drops
, switching resumes. The feedback
conditions
again. An abnormal condition
feedback
below
V
for
BURH
voltage
transformers,
.
Switching
drops
11
voltage then falls and the process repeats. Burst mode
alternately enables and disables switching of the
SenseFET and reduces switching loss in standby mode.
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, FSL136HR 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
||
6kΩ
Figure 22. Burst-Mode Operation

6kΩ
:
XkΩ
LIM
) of 2.15A. I
www.fairchildsemi.com
LIM
PK
pin and
can be
(1)
(2)