TPS2378: Current limit issue with TPS2378.

Hi Charlie,

Inrush is related to IEE802.3 spec. Per standard inrush is limited on the PSE side depending on classification, for Class 1-4 inrush is limited to 450mA and for Class 5-8 inrush is limited to 600mA. Therefore to meet this standard our PSE limits to 400mA on the PSE side. On the PD side, the device must not draw more than 400mA or it will fail inrush. Therefore there is an inrush limit on the PD to 140mA (this shuts down certain features until inrush has passed), this protects against worst case scenario.

Current limit returns to normal operating conditions (.850A) once inrush has passed, which is after 75ms. If current exceeds limit then the MOSFET will open (turn off).

Very Respectfully,

Brandon Beader

Hi Charlie,

Current limit on the PD will switch from its inrush current limit (140mA in this case) to its normal operating current limit (.85A in this case) after successful inrush phase where PD must not consume more than 400mA. This typically is complete around 80ms.

Please watch this short video explaining inrush, this might help to understand.

https://training.ti.com/poe-operation-considerations?context=1137930-1139676-1137929

Very Respectfully,

Brandon Beader