FSL126HR Fairchild Semiconductor, FSL126HR Datasheet - Page 11

• Manufacturer Part Number: FSL126HR
• Description: Power Switch ICs - POE / LAN Green Mode Fairchild Power Switch (FPS)
• Manufacturer: Fairchild Semiconductor

Specifications of FSL126HR

• Number Of Switches: Single
• On Resistance (max): 6.2 Ohms
• On Time (max): 23 ns
• Off Time (max): 17.2 ns
• Supply Voltage (max): 26 V
• Maximum Operating Temperature: + 105 C
• Minimum Operating Temperature: - 40 C
• Package / Case: DIP-8
• Maximum Power Dissipation: 1.5 W
• Mounting Style: Through Hole
• Operating Frequency: 100 KHz
• Switch Configuration: Single
• Switch Current (typ): 1.5 A
• Lead Free Status / Rohs Status: Details

© 2011 Fairchild Semiconductor Corporation
FSL126HR • Rev. 1.0.0
Output-Short Protection (OSP)
If the output is shorted, steep current with extremely
high di/dt can flow through the SenseFET during the
LEB time. Such a steep current brings high-voltage
stress on the drain of SenseFET when turned off. To
protect the device from such an abnormal condition,
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 to 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 voltage drops below V
FB
is higher than 1.6V and the SenseFET turn-on time
Figure 20. Output Short Waveforms (OSP)
correct
CC
reaches V
Figure 21.
FB
working
and SenseFET turn-on time. When the
START
conditions
again. An abnormal condition
Internal Soft-Start
BURH
.
for
transformers,
Switching continues until the feedback voltage drops
below V
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 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, FSL126HR has a typical
SenseFET peak current limit (I
adjusted to 1A by inserting Rx between the I
the ground. The value of the Rx can be estimated by the
following equations:
1.5A
X 
where X is the resistance of the parallel network.
11
Rx
:
1A
Figure 23. Peak Current Limit Adjustment
||
BURL
6kΩ

6kΩ
Figure 22. Burst-Mode Operation
. At this point, switching stops and the
:
XkΩ
BURH
, switching resumes. The
LIM
) of 1.5A. I
www.fairchildsemi.com
LIM
PK
pin and
can be
(1)
(2)