TPS2421-1_16 Datasheet, PDF (15/31 Pages) Texas Instruments – 5-A, 20-V Integrated FET Hot Swap

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TPS2421-1, TPS2421-2
SLUS907I – JANUARY 2009 – REVISED JANUARY 2015
9.4.2.2 Fault Timer
The fault timer is active when the TPS2421 device is in SOA protect mode or the current is above I_SET. Figure 13
illustrates operation during non-faulted start up (C_LOAD = 470 µF and I_VOUT = 1 A in a 12 V system). C_CT charges
at approximately 35 µA until TPS2421 device exits SOA protect mode, discharges quickly (approximately 40 µA)
to approximately 0.16 V, and then decays slowly (approximately 1.4 µA) towards zero.
2V/div
FLT
0.5V/div
CT
1A/div
ILOAD
5V/div
VOUT
5ms/div
Figure 13. 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.
C_CT
=
(1+ C_{LOAD_TOL} + C_{CT_TOL} ) ´ t_ON
40000
(11)
9.4.2.3 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.
9.4.2.4 Start up into a Short
The controller attempts to power on into a short for the duration of the timer. Figure 11 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 10.
9.4.3 Shutdown Modes
9.4.3.1 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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