Datasheet

( )
IMAX
IMAX
201k 201,000
R 57.4 k
I 3.5
W
= ´ = W
( )
IFLT
FAULT
200k 200,000
R 80 k
I 2.5
W
= ´ = W
TPS2420
SLUS903E –JANUARY 2009–REVISED MAY 2013 www.ti.com
Start Up Into a Short
The controller attempts to power on into a short for the duration of the timer. Figure 20 shows a small current
resulting from power limiting the internal MOSFET. This happens only once for the latch off mode. For the retry
mode, Figure 24 shows this cycle repeating at an interval based on the C
T
time.
Shutdown Modes
Hard Overload - Fast Trip
When a hard overload causes the load current to exceed 1.6 × I
MAX
the TPS2420 immediately shuts off current
to the load without waiting for the fault timer to expire. After such a shutoff the TPS2420 enters into startup mode
and attempts to apply power to the load.
If the hard overload is caused by a current transient, then a normal startup can be expected with a low probability
of disruption to the load, assuming there is sufficient load capacitance to hold up the load during the fractions of
a millisecond that make up the fast trip/restart cycle.
If the hard overload is caused by a real, continuous failure then the TPS2420 goes into current limit during the
attempt at restart. The timer starts and eventually runs out, shutting off current to the load. See the fast trip
Figure 22 and Figure 23. When the hard overload occurs the current is turned off, the PG pin becomes false, and
the FLT pin stays false. The FLT pin becomes true only when the fault timer times out.
Overcurrent Shutdown
Overcurrent shutdown occurs when the output current exceeds I
MAX
for the duration of the fault timer.
Overcurrent shutdown is the circuit breaker type protection of equipment. Figure 23 shows the step rise in output
current. The increased current is on for the duration of the timer. At conclusion of the timer, the output is turned
off.
Design Example
The TPS2420 Design shown in Figure 25 supports 12 V to operate a hot plugged disk drive.
The 12 V specification for a disk drive is approximately 1-A operating current and 2-A typical spin-up. Selecting a
2.5 A setting for I
FAULT
would allow some margin for the operating current and satisfy the start current
requirements.
Calculate R
RFLT
using equation Equation 10 or select it using Table 2.
(10)
The I
MAX
setting, 3.5 A, is set by R
RMAX
in Equation 11.
(11)
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