Datasheet

100
FT
SENSE
mV
I
R
=
3
650
LIMIT
SUM x
V
I
R
=
TPS2359
www.ti.com SLUS792H –FEBRUARY 2008–REVISED MAY 2013
3.3V Current Limiting
The 3.3-V management power channels include internal pass FETs and current sense resistors. The on-
resistance of a management channel - including pass FET, sense resistor, metallization resistance, and bond
wires - typically equals 290 mΩ and never exceeds 500 mΩ. The AdvancedMC™ specification allows a total of 1
Ω between the power source and the load. The TPS2359 never consumes more than half of this budget.
The 3.3-V fast trip function protects the channel against short-circuit events. If the current through the channel
exceeds a nominal value of 300 mA, then the TPS2359 immediately disables the internal pass transistor and
then allows it to slowly turn back on into current limiting.
The 3.3-V current limit function internally limits the current to comply with the AdvancedMC™ and MicroTCA™
specifications. External resistor RSUM3x allows the user to adjust the current limit threshold. The nominal current
limit threshold ILIMIT equals
(1)
A 3320-Ω resistor gives a nominal current limit of I
LIMIT
= 195 mA which complies with AdvancedMC™ and
MicroTCA™ specifications. This resistance corresponds to an EIA 1% value. Alternatively, a 3.3-kΩ resistor will
also suffice. Whenever a 3.3-V channel enters current limit, its fault timer begins to operate (see Fault Timer
Programming section).
The 3.3-V over-temperature shutdown trips if a 3.3-V channel remains in current limit so long that the die
temperature exceeds approximately 140°C. When this occurs, any 3.3-V channel operating in current limit turns
off until the chip cools by approximately 10°C. This feature prevents a prolonged fault on one 3.3-V channel from
disabling the other 3.3-V channel, or disabling either of the 12-V channels.
3.3-V ORing
The 3.3-V channels limit reverse current flow by sensing the input-to-output voltage differential and turning off the
internal pass FET when this differential drops below -3 mV, which corresponds to a nominal reverse current flow
of 10 mA. The pass FET turns back on when the differential exceeds +10 mV. These thresholds provide a
nominal 13 mV of hysteresis to help prevent false triggering. This feature allows the implementation of redundant
power supplies (also known as supply ORing).
If the 3.3-V channels do not use redundant supplies, then one can clear the 3ORON bit to disable the ORing
circuitry. This precaution eliminates the chance that transients might trigger the ORing circuitry and upset system
operation.
12-V Fast Trip and Current Limiting
Figure 23 shows a simplified block diagram of the circuitry associated with the fast trip and current limit circuitry
within a 12-V channel. Each 12-V channel requires an external N-channel pass FET and three external resistors.
These resistors allow the user to independently set the fast trip threshold and the current limit threshold, as
described below.
The 12-V fast trip function protects the channel against short-circuit events. If the voltage across external resistor
R
SENSE
exceeds the 100-mV fast trip threshold, then the TPS2359 immediately disables the pass transistor. The
12xCL bits set the magnitude of the fast trip threshold. When 12xCL = 1111B, the fast trip threshold nominally
equals 100 mV. The fast trip current I
FT
corresponding to this threshold equals
(2)
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