Datasheet

TPS2458
www.ti.com SLUS916B –FEBRUARY 2009–REVISED MAY 2010
Enable Functions
The TPS2458 provides three external enable pins for the AdvancedMC™ slot. Pulling the EN3 low turns on the
3-V channel. Pulling the EN12 pin low turns on the 12-V channel. If the EN12 pin goes high, the TPS2458 pulls
the PASS and BLK pins to ground. Pulling the OREN pin low turns on the reverse blocking logic in the 12-V
channel. If the OREN pin goes high, then the BLK pin remains low. Each of the three enable pins has an internal
200-kΩ pull-up resistor to VINT.
Power Good (PG) Outputs
The TPS2458 provides two active-low open-drain outputs that reflect the status of the two output voltage rails.
The power good output for each channel pulls low whenever the voltage on its OUT pin exceeds the PG
threshold. The 3.3-V channel has a nominal threshold of 2.85 V and the 12-V channel has a nominal threshold of
10.5 V.
Fault (FLT) Outputs
The TPS2458 provides two active-low open-drain fault outputs, one for each channel. A fault output pulls low
when the channel has remained in current limit long enough to run out the fault timer. A channel experiencing a
fault condition automatically shuts down. To clear the fault and re-enable the channel, turn the channel off and
back on using the appropriate ENx pin.
Current Limit and Fast Trip Thresholds
Both channels monitor current by sensing the voltage across a resistor. The 3.3-V channel uses an internal
sense resistor with a nominal value of 290 mΩ. The 12-V channel uses an external sense resistor that typically
lies in the range of 4 mΩ to 10 mΩ. Each channel features two distinct thresholds: a current limit threshold and a
fast trip threshold.
The current limit threshold sets the regulation point of a feedback loop. If the current flowing through the channel
exceeds the current limit threshold, then this feedback loop reduces the gate-to-source voltage imposed on the
pass FET. This causes the current flowing through the channel to settle to the value determined by the current
limit threshold. For example, when a module first powers up, it draws an inrush current to charge its load
capacitance. The current limit feedback loop ensures that this inrush current does not exceed the current limit
threshold.
The current limit feedback loop has a finite response time. Serious faults such as shorted loads require a faster
response in order to prevent damage to the pass FETs or voltage sags on the supply rails. A comparator
monitors the current flowing through the sense resistor, and if it ever exceeds the fast trip threshold it
immediately shuts off the channel. Then it will immediately attempt a normal turn on which allows the current limit
feedback loop time to respond. The fast trip threshold is normally set 2 to 5 times higher than the current limit.
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