Data Sheet

© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSFR2100 • Rev.1.1.0 10
FSFR2100 — Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converters
3 ms to reduce the current overshoot during the initial
cycles, which adds 40 kHz to the initial frequency of the
external soft-start circuit, as shown in Figure 20. The
initial frequency of the soft-start is given as:
min
5.2 5.2
()10040()
ISS
SS
kk
f
kHz
RR


(3)
It is typical to set the initial frequency of soft-start two ~
three times the resonant frequency (f
O
) of the resonant
network.
The soft-start time is three to four times of the RC time
constant. The RC time constant is as follows:
SS SS SS
TRC
(4)
f
s
time
Control loop
take over
40kHz
f
ISS
Figure 20. Frequency Sweeping of Soft-start
4. Control Pin
The FSFR2100 has a control pin for protection, cycle
skipping, and remote on/off. Figure 21 shows the internal
block diagram for control pin.
Figure 21. Internal Block of Control Pin
Protection: When the control pin voltage exceeds 5V,
protection is triggered. Detailed applications are
described in the protection section.
Pulse Skipping: FSFR2100 stops switching when the
control pin voltage drops below 0.4 V and resumes
switching when the control pin voltage rises above 0.6 V.
To use pulse-skipping, the control pin should be
connected to the opto-coupler collector pin. The
frequency that causes pulse skipping is given as:
(5)
Figure 22. Control Pin Configuration for Pulse
Skipping
Remote On / Off: When an auxiliary power supply is
used for standby, the main power stage using FSFR2100
can be shut down by pulling down the control pin voltage,
as shown in Figure 23. R1 and C1 are used to ensure
soft-start when switching resumes.
Figure 23. Remote On / Off Circuit

kHz100x
R
k16.4
R
k2.5
maxmin
SKIP
