FSFM260N View Datasheet(PDF) - Fairchild Semiconductor
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FSFM260N Datasheet PDF : 18 Pages
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where, Ilim_spec is the limit current stated on the
specification; Na and Np are the number of turns for VCC
and primary side, respectively; Ifb is the internal current
source at feedback pin with a typical value of 0.9mA; and
ΔIlim_comp is the current difference that must be
eliminated. In case of capacitor in the circuit 1µF, 100V is
good choice for all applications.
VDC
Np
L
Vfb Drain
Na
Vcc
I_lim GND
RLIM
RY
FSFM260 Rev. 00
compensation
network
CY
-
+
Vy
= VDC
× Na
Np
Figure 18. Constant Power Limit Circuit
Vds Power
on
Fault
occurs
Fault
removed
Vcc
12V
8V
components, the reliability is improved without
increasing cost. Once the fault condition occurs,
switching is terminated and the SenseFET remains off.
This causes VCC to fall. When VCC reaches the UVLO
stop voltage, 8V, the protection is reset and the internal
high-voltage current source charges the VCC capacitor
via the Vstr pin. When VCC reaches the UVLO start
voltage, 12V, the FSFM260/261/300 resumes normal
operation. In this manner, the auto-restart can alternately
enable and disable the switching of the power SenseFET
until the fault condition is eliminated (see Figure 19).
3.1 Overload Protection (OLP): Overload is defined as
the load current exceeding a pre-set level due to an
unexpected event. In this situation, the protection circuit
should be activated to protect the SMPS. However, even
when the SMPS is in the normal operation, the overload
protection circuit can be activated during the load
transition. To avoid this undesired operation, the
overload protection circuit is designed to be activated
after a specified time to determine whether it is a
transient situation or an overload situation. Because of
the pulse-by-pulse current limit capability, the maximum
peak current through the SenseFET is limited and,
therefore the maximum input power is restricted with a
given input voltage. If the output consumes beyond this
maximum power, the output voltage (VO) decreases
below the set voltage. This reduces the current through
the opto-coupler LED, which also reduces the opto-
coupler transistor current, increasing the feedback
voltage (VFB). If VFB exceeds 2.5V, D1 is blocked and
the 5µA current source starts to charge CB slowly up to
VCC. In this condition, VFB continues increasing until it
reaches 6V, when the switching operation is terminated,
as shown in Figure 20. The delay time for shutdown is
the time required to charge CB from 2.5V to 6.0V with
5µA. In general, a 10 ~ 50ms delay time is typical for
most applications.
VFB
FSFM260 Rev: 00
Overload protection
6.0V
FSFM260 Rev: 00
Normal
operation
Fault
situation
t
Normal
operation
Figure 19. Auto Restart Operation
3. Protection Circuit: The FSFM260/261/300 has
several self protective functions, such as overload
protection (OLP), over-voltage protection (OVP), and
thermal shutdown (TSD). Because these protection
circuits are fully integrated into the IC without external
2.5V
T12= Cfb*(6.0-2.5)/Idelay
T1
T2 t
Figure 20. Overload Protection
© 2009 Fairchild Semiconductor Corporation
FSFM260N / FSFM261N / FSFM300N • Rev. 1.0.1
11
www.fairchildsemi.com
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