Datasheet

OUT_TOL CT_TOL ON
CT
(1 C C ) t
C
40000
+ + ´
=
ILOAD
VOUT
CT
2V/div
0.5V/div
1A/div
5V/div
5ms/div
FLT
TPS2421-1
TPS2421-2
www.ti.com SLUS907G –JANUARY 2009–REVISED MAY 2013
Figure 21. Fault Timer Operation During Start up
C
CT
can be chosen for fault-free start up including expected C
OUT
and C
CT
capacitance tolerance as shown in
Equation 11.
(11)
Normal Operation
When load current exceeds I
SET
during normal operation the fault timer starts. If load current drops below I
SET
before the fault timer expires, normal operation continues. If load current stays above the I
SET
threshold the fault
timer expires and a fault is declared. When a fault is declared a TPS2421-1 device turns off an can be restarted
by cycling power or toggling the EN signal. A TPS2421-2 device attempts to turn on at a 3.7% duty cycle until the
fault is cleared. When I
LIM
is reached during a fault the device goes into current limit and the fault timer keeps
running.
Start up into a Short
The controller attempts to power on into a short for the duration of the timer. Figure 19 shows a small current
resulting from power limiting the internal MOSFET. This happens only once for TPS2421-1. For TPS2421-2, the
cycle repeats at a 3.7% duty cycle as shown in Figure 18.
Shutdown Modes
Hard Overload - Fast Trip
When a hard overload causes the load current to exceed ~1.6 × I
LIM
the TPS2421 immediately shuts off current
to the load without waiting for the fault timer to expire. After such a shutoff the TPS2421 enters startup mode and
attempts to apply power to the load. If the hard overload was caused by a transient, then normal startup can be
expected. If the hard overload is caused by a persistent, continuous failure then the TPS2421 goes into current
limit during the restart attempt and either latches off (TPS2421-1) or attempts retry (TPS2421-2).
Overcurrent Shutdown
Overcurrent shutdown occurs when the output current exceeds I
SET
for the duration of the fault timer. Figure 8
shows a step rise in output current which exceeds the I
SET
threshold but not the I
LIM
threshold. The increased
current is on for the duration of the timer. When the timer expires, the output is turned off.
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