The MPS is an electrical signature presented by the PD to assure the PSE that it is still present after operating voltage is applied. For a Type 1 or Type 2 PD, a valid MPS consists of a minimum dc current of 10 mA, or a 10-mA pulsed current for at least 75 ms every 325 ms, and an AC impedance lower than 26.3 kΩ in parallel with 0.05 μF. Only Type 1 and Type 2 PSEs monitor the AC MPS. A Type 1 or Type 2 PSE that monitors only the AC MPS may remove power from the PD.

To enable applications with stringent standby requirements, IEEE802.3bt introduced a significant change regarding the minimum pulsed current duration to assure the PSE will maintain power. This applies to all Type 3 and Type 4 PSEs, and the pulse duration is ~10% of what is required for Type 1 and 2 PSEs. The MPS current amplitude requirement for Class 5-8 PDs have also increased to 16 mA at the PSE end of the ethernet cable.

If the current through the RTN-to-VSS path is very low, the TPS23731 automatically generates the MPS pulsed current through the VSS pin, with an amplitude adjusted such that its net current reaches a level high enough to maintain PSE power. The TPS23731 is also able to determine if the PSE is of Type 1-2 or Type 3-4, automatically adjusting the pulsed current amplitude, duration and duty-cycle, while minimizing power consumption. Note that the IEEE802.3bt requirement for the PD is applicable at the PSE end of the cable. That means that depending the cable length and other parameters including the bulk capacitance, a longer pulse duration may be required to ensure a valid MPS. For that purpose, the TPS23731 provides auto-stretch capability which is used to cancel the impact of such system conditions on the effective pulsed current duration. See Figure 8-4.

When APD is pulled high or when DEN is pulled to VSS (forcing the hotswap switch off), the DC MPS will not be met. A PSE that monitors the DC MPS will remove power from the PD when this occurs.