FSL137MRIN Просмотр технического описания (PDF) - Fairchild Semiconductor
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FSL137MRIN Datasheet PDF : 15 Pages
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4. Protection Circuits: The FSL137MRIN has several
self-protective functions; such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
Output-Short Protection (OSP), Over-Voltage Protection
(OVP), and Thermal Shutdown (TSD). All the
protections are implemented as auto-restart. Once a
fault condition is detected, switching is terminated and
the SenseFET remains off. This causes VCC to fall.
When
VCC B
B
falls
to
the
Under-Voltage
Lockout
(UVLO)
stop voltage of 7.5 V, the protection is reset and the
startup circuit charges the VCC capacitor. When VCC
reaches the start voltage of 12.0 V, normal operation
resumes. If the fault condition is not removed, the
SenseFET remains off and VCC drops to stop voltage
again. In this manner, the auto-restart can alternately
enable and disable the switching of the power
SenseFET until the fault condition is eliminated.
Because these protection circuits are fully integrated
into the IC without external components, reliability is
improved without increasing cost.
Power
VDS
on
Fault
occurs
Fault
removed
increasing until it reaches 7.0 V, when the switching
operation is terminated, as shown in Figure 22. The
delay for shutdown is the time required to charge CFB
from 2.5 V to 7.0 V with 2.0 µA. A 25 ~ 50 ms delay is
typical for most applications. This protection is
implemented as auto-restart.
VFB
Overload Protection
7.0V
2.5V
t12= CFB×(7.0-2.5)/Idelay
t1
t2
t
Figure 22. Overload Protection
VCC
12.0V
7.5V
t
Figure 21.
Normal
operation
Fault
situation
Normal
operation
Auto-Restart Protection Waveforms
4.1 Overload Protection (OLP): Overload is defined
as the load current exceeding its normal level due to
an unexpected abnormal event. In this situation, the
protection circuit should trigger to protect the SMPS.
However, even when the SMPS is in normal
operation, the overload protection circuit can be
triggered during the load transition. To avoid this
undesired operation, the overload protection circuit is
designed to trigger only after a specified time to
determine whether it is a transient situation or a true
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 more than this
maximum power, the output voltage (VOUT) 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 2.5 V, D1 is
blocked and the 2.0 µA current source starts to
charge CFB slowly up. In this condition, VFB continues
© 2012 Fairchild Semiconductor Corporation
FSL137MRIN • Rev. 1.0.1
11
4.2 Abnormal Over-Current Protection (AOCP):
When the secondary rectifier diodes or the
transformer pins are shorted, a steep current with
extremely high di/dt can flow through the SenseFET
during the minimum turn-on time. Even though the
FSL137MRIN has overload protection, it is not
enough to protect the FSL137MRIN in that abnormal
case; due to the severe current stress imposed on the
SenseFET until OLP is triggered. The internal AOCP
circuit is shown in Figure 23. When the gate turn-on
signal is applied to the power SenseFET, the AOCP
block is enabled and monitors the current through the
sensing resistor. The voltage across the resistor is
compared with a preset AOCP level. If the sensing-
resistor voltage is greater than the AOCP level, the
set signal is applied to the S-R latch, resulting in the
shutdown of the SMPS.
Drain
6,7,8
OSC
3R
VFB* R
PWM
Gate
Driver
LEB (300ns)
QS
QR
VCC Good
RSENSE
VAOCP
GND
1
Figure 23. Abnormal Over-Current Protection
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