Datasheet
R
SENSE
R
SUM
R
SET
+
SETx
SENMx
SUM12x
IN12x
675
mV
+
100mV
Fast Trip
Comparator
Current
Limit Amp
+
PASSx
A1
A2
SENPx
30 uA60 uA
100
0 0198
LIMIT SET
( . A )
I R= ´
W
0 675
=
LIM SUM SENSE
SET
I R R
R
.
0 675
SET
LIMIT
SUM SENSE
R
I . V
R R
æ ö
= ´
ç ÷
è ø
12
SENSE SUM
SUMx SENSE
SET
R R
V I
R
æ ö
=
ç ÷
è ø
TPS2359
SLUS792H –FEBRUARY 2008–REVISED MAY 2013
www.ti.com
The recommended value of R
SENSE
= 5 mΩ sets the fast trip threshold at 20 A for 12xCL = 1111B. This choice of
sense resistor corresponds to the maximum 19.4-A inrush current allowed by the MicroTCA™ specification.
The 12-V current limit function regulates the PASSx pin voltage to prevent the current through the channel from
exceeding I
LIMIT
. The current limit circuitry includes two amplifiers, A1 and A2, as shown in Figure 23. Amplifier
A1 forces the voltage across external resistor R
SET
to equal the voltage across external resistor R
SENSE
. The
current that flows through R
SET
also flows through external resistor R
SUM
, generating a voltage on the 12SUMx
pin equal to:
(3)
Amplifier A2 senses the voltage on the 12SUMx pin. As long as this voltage is less than the reference voltage on
its positive input (nominally 0.675 V for 12xCL = 1111B), the TPS2359 sources current to PASSx. When the
voltage on the 12SUMx pin exceeds the reference voltage, amplifier A2 begins to sink current from PASSx. The
gate-to-source voltage of the pass FET drops until the the voltages on the two inputs of amplifier A2 balance.
The current flowing through the channel then nominally equals:
(4)
(5)
The recommended value of R
SUM
is 6810 Ω. This resistor should never equal less than 675 Ω to prevent
excessive currents from flowing through the internal circuitry. Using the recommended values of R
SENSE
= 5 mΩ
and R
SUM
= 6810 Ω gives:
(6)
A system capable of powering an 80-Watt AdvancedMC™ module should provide 8.25 A, +/- 10%, 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 23. 12-V Channel Threshold Circuitry
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