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[FAQ] TPS23861: How does it protect itself (current protection, thermal protection)? What’s the difference between ICUT and ILIM?

Brett Colteaux
Expert 3700 points
Part Number: TPS23861

How does TPS23861 protect itself (current protection, thermal protection)? What’s the difference between ICUT and ILIM?

  • 0
    TI__Expert 3700 points

    (Refer to TPS23861 datasheet section 8.4)

    When sensed temperature exceeds the thermal shutdown threshold, all ports are powered off and put in Off mode.

    The TPS23861 provides three levels of overcurrent protection. During tSTART period immediately following power up, inrush current protection is provided as described in the Inrush Protection portion of the datasheet. This protection allows the input capacitance of the PD to charge to the full input voltage on the power interface while ensuring the pass FET remains within its safe operating area.

    Following the end of the tSTART period a two-tiered current-limit protection scheme is applied to the ports. The first level (i.e., lower current) is the ICUT current limit. The ICUT current-limit threshold is set using the ICUTnm CONFIG registers and includes a timeout, tOVLD, set using TICUT field in the TIMING CONFIGURATION register. When the TICUT timer times out because the ICUT current threshold is exceeded, the port is powered off, and the ICUTn bit in the FAULT EVENT register is set with the option of asserting an interrupt. This delay in powering down the port provides protection against spurious power downs during moderate load transients.

    The second level of powered-on current-limiting protection is the ILIM current limit. The ILIM current limit is a hard limit. That is, hardware protection including voltage foldback is imposed when the ILIM current threshold is reached. This second level of protection is invoked in the event of extreme overload or short circuit. The ILIM current-limit value is set using the POEPn bits in the PoE Plus register. Also, when the ILIM value is reached, the ILIM timer is started. When the ILIM timer times out, the port is powered off and the ILIMn bit in the Start/ILIM Event Register is set with the option of asserting an interrupt.

    PoEPn bit should be set according to ICUT value for host to ensure the ICUT and ILIM relationship (Check Table 15 in datasheet). Additionally, If ILIM and ICUT are set to same value, it is a race condition between ILIM and ICUT in TPS23861 and it depends on the firmware as to the winner. 

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