Datasheet

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SLUS562A − JUNE 2003 − REVISED SEPTEMBER 2003
14
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APPLICATION INFORMATION
Figure 20
INPUT TRANSIENT RESPONSE
C
LOAD
= 100 µF
R
LOAD
= 50 Ω
R
SNS
= 20 mΩ
C
IRAMP
= 3900 pF
I
INB
= (2 A/div)
−48V_RTN (5 V/div)
(Offset 44 V)
t − TIme − 100 µs/div
GATE (5 V/div)
V
DRAIN
(5 V/div)
PG
(50 V/div)
In order for downstream loads (bricks, etc.) to operate through the distribution bus transient, it is important to
properly size the filtering capacitance to supply the needed energy during the OFF-time of the pass FET. In this
example, once the RTN node stabilizes at about 3.5 V higher than the original potential, about 4.5 V develops
across the FET, indicating approximately a 1-V droop across the brick input. Therefore, due to the fast response
of the TPS2398/99 devices, the 100-µF capacitor achieves excellent hold-up of the brick input voltage. Actual
requirements depend heavily on the individual application. Whether the device turns back on in either
current-ramp or current-limit mode depends in part on the size of the ramp capacitor (C
IRAMP
) and the input
capacitance of the pass FET. But in any case, the circuit turns back on in a controlled-current manner after
rapidly clamping the potentially damaging spike.