AN4116
APPLICATION NOTE
4
©2002 Fairchild Semiconductor Corporation
by 4.5V (from 3V to 7.5V) using 2
µA. When Cfb is 47nF,
delay time is approximately 100ms. Fairchild Power Switch
will not shut down within this time. Increasing Cfb to get a
longer delay time can become a problem, because Cfb is an
important parameter in determining the SMPS dynamic
response time.
One method to delay the shutdown time is to add a resistor
between the F/B pin and GND and to subtract the amount of
the delay current. When the 4. 7M
Ω resistor was used
experimentally with Cfb of 47nF, shutdown time was almost
doubled to 180~200ms. When Vfb voltage is 7.5V, the
current flowing to the 4.7M
Ω resistor is approximately
1.6
µA.
To obtain the same results, a zener diode (approx. 3.9V) can
be series connected to a capacitor (47nF) which can then be
parallel connected to Cfb as shown in Figure 2.4.
Figure 2-4. Fairchild Power Switch (FPS) Long Delayed Shutdown
+
-
3
2
1
D2
Cfb
Cd
KA431
8
0.9mA
0
0
0
Idelay
7.5V
Vz=3.9V
Vo
D1
Fairchild Power Switch(SPS)
1
2
3
4
OLP
Latch
0
2uA
Vfb
4
Vfb*
(LM431)
2.3.2 Over Voltage Protection
Fairchild Power Switch has self protection features that
function even when abnormal states occur such as an open or
short circuits in the feedback loop. When the feedback
terminal shorts as viewed from the primary side, the
feedback terminal voltage becomes zero and prevents
switching from starting. If it opens, the protection circuit acts
as an over voltage protection circuit. When there is an
abnormal state or a possibility of opening due to improper
soldering etc. in the secondary side feedback circuit, the
primary side continues to switch using the maximum set
current until the protection circuit starts to operate. In such
instances, it is common for the secondary side voltage to
become greater than the rated voltage, which can lead to a
fuse blowing or, more seriously, a fire if a protection circuit
is not in place. Even if this was not the case, ICs immediately
connected to the secondary output without a regulator can be
destroyed. Therefore, the Fairchild Power Switch employs
the over voltage protection circuit to protect against
feedback anomalies. The Fairchild Power Switch VCC is
proportional to the output voltage. When the Fairchild Power
Switch VCC exceeds 33V, the over voltage protection feature
is triggered. Therefore, VCC must be maintained at less than
30V during normal operation.
2.3.3 Over Current Protection (OCP).
The existing concept of Ipeak control does not go beyond
limiting the amount of current during normal operation. The
OCP block prevents damage to Fairchild Power Switch from
abnormal states, such as a diode or a load short. A diode or a
load short causes a large current to flow through the
SenseFet for a short time. This can be tens of amperes. The
leading edge blanking circuit sets the minimum turn on time
at 600nS.
Tens of amperes for 600nS could destroy the
Fairchild Power Switch and so the OCL block senses this
instantaneous current and latches like the existing protection
circuit.