FSFM300 Fairchild Semiconductor, FSFM300 Datasheet - Page 11

• Manufacturer Part Number: FSFM300
• Description: The FSFM260N/261N/300N is an integrated Pulse Width Modulator (PWM) and SenseFET specifically designed for high-performance offline Switch Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

FSFM260N / FSFM261N / FSFM300N • Rev. 1.0.1
© 2009 Fairchild Semiconductor Corporation
where, I
specification; N
and primary side, respectively; I
source at feedback pin with a typical value of 0.9mA; and
ΔI
eliminated. In case of capacitor in the circuit 1µF, 100V is
good choice for all applications.
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
FSFM260 Rev: 00
lim_comp
Vds
Vcc
12V
8V
FSFM260 Rev. 00
R
LIM
Figure 18. Constant Power Limit Circuit
lim_spec
Power
Figure 19. Auto Restart Operation
on
is the current difference that must be
a
Vfb
I_lim
operation
and N
Normal
is the limit current stated on the
Drain
GND
Vcc
occurs
p
Fault
are the number of turns for V
V
situation
R
DC
Fault
Y
fb
C
removed
is the internal current
Np
Y
Fault
L
Na
+
-
V
compensation
network
y
=
V
operation
DC
Normal
×
N
N
a
p
t
CC
11
components,
increasing cost. Once the fault condition occurs,
switching is terminated and the SenseFET remains off.
This causes V
stop voltage, 8V, the protection is reset and the internal
high-voltage current source charges the V
via the Vstr pin. When V
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 (V
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 (V
the 5µA current source starts to charge C
V
reaches 6V, when the switching operation is terminated,
as shown in Figure 20. The delay time for shutdown is
the time required to charge C
5µA. In general, a 10 ~ 50ms delay time is typical for
most applications.
CC
6.0V
2.5V
V
. In this condition, V
FB
FB
Figure 20. Overload Protection
). If V
CC
T
the
1
to fall. When V
FB
reliability
exceeds 2.5V, D1 is blocked and
T
Overload protection
12
FB
= Cfb*(6.0-2.5)/I
CC
continues increasing until it
B
reaches the UVLO start
is
from 2.5V to 6.0V with
CC
improved
reaches the UVLO
delay
FSFM260 Rev: 00
B
O
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CC
) decreases
slowly up to
capacitor
T
2
without
t