Datasheet

LOAD_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 SLUS907H –JANUARY 2009–REVISED JANUARY 2014
Figure 21. Fault Timer Operation During Startup
C
CT
can be chosen for fault-free start up including expected C
LOAD
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 occurs only once for theTPS2421-1 device. For the
TPS2421-2 device, 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 approximately 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 device
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 device enters into current limit during the restart attempt and either latches off (TPS2421-1) or attempts
retry (TPS2421-2).
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Product Folder Links: TPS2421-1 TPS2421-2