Datasheet

TPS2359

SLUS792H –FEBRUARY 2008–REVISED MAY 2013

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Fault, Power Good, Overcurrent, and FET Status Bits

The TPS2359 I

C interface includes six status bits for each channel, for a total of 24 bits. These status bits

occupy registers 7, 8, and 9. The following table summarizes the locations of these bits:

Table 14. TPS2359 Status Bit Locations

(1)

12A 3A 12B 3B

FUNCTION

Power Good (PG) R7[0] R7[2] R7[4] R7[6]

Overcurrent Time out Fault (FLT) R7[1] R7[3] R7[5] R7[7]

Momentary Overcurrent (OC) R8[0] R8[2] R8[4] R8[6]

Overcurrent Fast Trip (FTR) R8[1] R8[3] R8[5] R8[7]

12-V Block FET Status (12xBS) R9[0] R9[3]

12-V Pass FET Status (12xPS) R9[1] R9[4]

3-V Block Status (3xBS) R9[2] R9[5]

3-V Gate Status (3xGS) R9[6] R9[7]

(1) For a description of each bit, refer to Detailed Description of Registers section.

Current Limit and Fast Trip Thresholds

All four channels monitor current by sensing the voltage across a resistor. The 3.3-V channels use internal sense

resistors with a nominal value of 290 mΩ. The 12-V channels use external sense resistors that typically lie in the

range of 4 - 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, then it

immediately shuts off the channel. The channel turns back on slowly, allowing the current limit feedback loop

time to respond. One normally sets the fast trip threshold some 2 – 5 times higher than the current limit.

Product Folder Links: TPS2359