FSFR1700L 查看數據表（PDF） - Fairchild Semiconductor
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FSFR1700L Datasheet PDF : 18 Pages
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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:
f ISS = (5.2kΩ + 5.2kΩ) ×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:
TSS = RSS ⋅ CSS
(4)
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 22. Control Pin Configuration for
Pulse Skipping
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.
R1
C1
FPS
RT
Rmin
OP1
Main
Output
Main Off
Aux
Output
CON
OP1
Figure 23. Remote On / Off Circuit
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: 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:
SKIP = 5.2k + 4.16k x100(kHz)
(5)
R min R max
4. Current Sensing Method
Current Sensing Using Resistor: FSFR-series senses
drain current as a negative voltage, as shown in Figure
24 and Figure 25. Half-wave sensing allows low power
dissipation in the sensing resistor, while full-wave
sensing has less switching noise in the sensing signal.
© 2007 Fairchild Semiconductor Corporation
FSFR series Rev.1.0.9
11
www.fairchildsemi.com
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