TPS23754: Could TPS23754 detect the passive PSE injector? if not, any circuits recommended to achieve this?

Hi Jeff,

Thanks for reaching out to us!

For TPS23754, T2P pin can be used as an indicator for Type-1 (af) or Type-2 (at). But the T2P will also be low when PPD is higher than 1.55 V or APD is higher than 1.5 V. 

For passive PoE injector, if PPD (triggered by the voltage between Vdd - Vss) is used, it will also make T2P low. May I know what is the voltage of the passive PoE injector? The PD's input voltage is 36 - 57 V for standarded PSE. If the passive PoE injector has a lower voltage (like 24 V), then using the circuit below could help the MCU recognize the passive injector.

Best regards,

Diang

Hi, Diang,

Very appreciate for your information, the voltage range of the passive PoE injector is 50V +/- 3V. Please kindly advise the solutions.  

Thanks a lot,

Jeff

Hi Jeff,

Sorry that I did not see a very reliable method to tell the passive PoE injector or af, since both should make T2P High.

The difference of a passive PoE injector and an active one is the passive PoE injector does not do negotiation with the PD. Which means passive PoE injector shows no detection and classification voltages.  

One possible way I could think is using a circuit below. When with the af PSE, it will go through classification (when Vdd-Vss = 9 to 22 V) pulse which will make CLS = 2.5 V for 10-75 ms. Then the voltage of C1 (Vc1) can be charged to about 2.5 V - 0.7 V = 1.8 V when with suitable R1 and R2 values. When with the passive PoE injector, CLS will also be 2.5 V for a much shorter time when Vdd-Vss = 9 to 22 V, then Vc1 could be much less than 1.8 V.

R1 and R2 values should be much larger than the classification resistance Rcls since this detection cannot influence the classification current as the table below. A bleeder resistor (Rbleed) needs to be large enough too since it need to hold the Vc1 voltage higher than a certain value unit the MCU is powered up.

When MCU is powered up, the Vc1 voltage and T2P could be monitored, if they are both High, it will be af. T2P High and Vc1 Low it will be passive PoE injector. A High threshold voltage of Vc1 also need to be carefully designed.   

Best regards,

Diang

Hi, Diang,

Is there any way we could differentiate the passive POE injector or the standard unit by talking to TPS23754 after the CPU is powered up?

or any other TI parts could be used to do this, instead of TPS23754 (for AT mode)?

Thanks a lot,

Jeff

Hi, Diang,

Sorry for late reply.

Thank you for your help.

Jeff