8.4.4 Foldback Protection (ILIM)

The TPS23861 features two types of foldback protection mechanisms for complete MOSFET protection. During inrush at port power on, the foldback is based on the port voltage as shown in Figure .

After the port has been powered on and Power Good is valid, a dual-slope foldback current limit is applied, providing protection against partial and total short-circuit at port output, while still being able to maintain the port powered during normal transients in the TPS23861 input voltage or load current. Refer to Figure 58. Note that setting the POEPn bit selects the 2x curve while clearing it selects the 1x curve.

The TLIM timer starts counting down when the current-limit threshold in Figure 57 and Figure 58 is reached. When it reaches zero the port is powered down. If the ILIM current-limit condition is cleared while the TLIM timer is counting, the TLIM timer counts up, at a slower rate, without exceeding the maximum count corresponding to the setting in the TLIM field which is located in the Timing Configuration Register.

If the port experiences a short circuit, the TPS23861 forces zero volts on the gate of the external FET to protect it from destruction. Within microseconds the foldback circuit engages, and until the ILIM timeout is reached, the port current is controlled according to the foldback schedule.

The ILIM current limit is to be applied such that its setting is greater than the setting of the ICUT current threshold. That is, an ICUT current-limit condition should occur before the TPS23861 asserts foldback control over the port current via an ILIM current limit. To this end, the ILIM current limit must be set greater than the ICUT current threshold. This must be accomplished by the host except when in Auto Mode (or the AUTO bit is set). In that case, the settings for ICUT and ILIM will be properly set based on classification results before the port is powered on.

Figure 58. Foldback Port On (ILIM vs V_DRAIN)