Datasheet

R
SENSE
R
SUM
R
SET
+
SET
SENM
SUM12
IN12
675 mV
+
100 mV
Fast Trip
Comparator
30 mA
PASS
A1
A2
SENP
+
60 mA
OUT12
F T
1.35 V
t C
10 A
= ´
m
6
T F
C T 7.4 10
-
= ´ ´
TPS2458
SLUS916B –FEBRUARY 2009–REVISED MAY 2010 www.ti.com
A system capable of powering an 80-W AdvancedMC™ module consumes a maximum of 8.25 A according to
MicroTCA™ specifications. The above equation suggests R
SET
= 417 Ω. The nearest 1% EIA value equals 422
Ω. The selection of R
SET
for MicroTCA™ power modules is described in the Redundant vs. Non-redundant Inrush
Current Limiting section.
Figure 19. 12-V Channel Threshold Circuitry
Fault Timer Programming
The fault timer of the two channels in a TPS2458 use identical internal circuitry. Each channel requires an
external capacitor CT connected between the CTx pin and ground. When a channel goes into current limit, the
TPS2458 injects 10 mA into the external capacitor. If the channel remains in current limit long enough for the
voltage on the CTx pin to reach 1.35 V, then the TPS2458 shuts the channel down and pulls the FLTx pin low to
declare a fault. If the channel does not remain in current limit long enough to trip the timer, then the CTx
capacitor is discharged through an internal 200-Ω pulldown resistor. The nominal fault time t
F
is shown in
Equation 6.
(6)
(7)
The user should select capacitors that provide the shortest fault times sufficient to allow down-stream loads and
bulk capacitors to charge. Shorter fault times reduce the stresses imposed on the pass FETs under fault
conditions. This consideration may allow the use of smaller and less expensive FETs for the 12-V channels.
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