connecting R-C series network on the RT pin, as shown
in Figure 19. FSFR-series also has an internal soft-start
for 3ms to reduce the current overshoot during the initial
cycles, which adds 40kHz 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:
5.2kΩ 5.2kΩ
f ISS = (
+
)×100+ 40 (kHz)
(3)
Rmin
RSS
It is typical to set the initial frequency of soft-start two ~
three times the resonant frequency (fO) 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:
Figure 22. Control Pin Configuration for Pulse
Skipping
(4)
TSS = RSS ⋅CSS
Remote On / Off: When an auxiliary power supply is
used for standby, the main power stage using FSFR-
series 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.
fs
f ISS
40kHz
Control loop
take over
time
Figure 20. Frequency Sweeping of Soft-start
4. Control Pin: The FSFR-series has a control pin for
protection, cycle skipping, and remote on/off. Figure 21
shows the internal block diagram for control pin.
Figure 23. Remote On / Off Circuit
Figure 21. Internal Block of Control Pin
5. Protection Circuits: The FSFR-series has several
self-protective functions, such as Overload Protection
(OLP), Over-Current Protection (OCP), Abnormal Over-
Current Protection (AOCP), Over-Voltage Protection
(OVP), and Thermal Shutdown (TSD). OLP, OCP, and
OVP are auto-restart mode protections; while AOCP and
TSD are latch-mode protections, as shown in Figure 24.
Protection: When the control pin voltage exceeds 5V,
protection is triggered. Detailed applications are
described in the protection section.
Pulse Skipping: FSFR-series stops switching when the
control pin voltage drops below 0.4V and resumes
switching when the control pin voltage rises above 0.6V.
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:
Auto-restart Mode Protection: Once a fault condition is
detected, switching is terminated and the MOSFETs
remain off. When LVCC falls to the LVCC stop voltage of
11.3V, the protection is reset. The FPS resumes normal
operation when LVCC reaches the start voltage of 14.5V.
5.2k
4.16k
SKIP
=
+
x100
(
kHz
)
(5)
Rmin
Rmax
© 2007 Fairchild Semiconductor Corporation
FSFR series • 1.0.3
www.fairchildsemi.com
11
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