TPS2384: TPS2384 ; PD Detection Function Test Result with Device for or not for PoE

Ruei-Ming Guan

Part Number: TPS2384
Other Parts Discussed in Thread: TPS2378, TPS23861

Hi, Penny

We also done a test by changing the design from ALT-A mode to ALT-B mode, and the PD device could be detected.

Could you help clarify the reason to cause the different detection results between ALT-A mode and ALT-B mode with same PD device?

Thanks.

over 7 years ago

0 Penny Xu over 7 years ago

TI__Genius 15685 points

Hi Ruei-Ming,

Did you operate TPS2384 in auto mode? Thanks.

Best regards,

Penny

0 Ruei-Ming Guan over 7 years ago in reply to Penny Xu

Prodigy 200 points

Hi, Penny

Yes, TPS2384 is in auto mode in our design.

0 Penny Xu over 7 years ago in reply to Ruei-Ming Guan

TI__Genius 15685 points

Hi Ruei-Ming,

What PD did you test with? Thanks.

Best regards,

Penny

0 Ruei-Ming Guan over 7 years ago in reply to Penny Xu

Prodigy 200 points

Hi, Penny

PD we used is Si3402-B.

We had posted this issue before: e2e.ti.com/.../681705

0 Penny Xu over 7 years ago in reply to Ruei-Ming Guan

TI__Genius 15685 points

Hi Ruei-Ming,

Have you tested TPS2384EVM with the Si3402-B PD? Technically, the detection mechanism is the same between alt A and alt B. The only difference is the back off time. Thanks.

Best regards,

Penny

0 Ruei-Ming Guan over 7 years ago in reply to Penny Xu

Prodigy 200 points

Hi, Penny

We'll test TPS2384 EVM with Si3402-B PD.

In previous your reply, you mentioned TPS2384 is forced voltage mode detection and it's sensitive to PD input cap.

Could the increased back off time be helpful for this detection?

We also monitored the detection period in ALT-A is about 75ms but 400ms in Micro-semi PSE IC.

In addition, we also monitored there are capacitors parallel with schottky in Si3402-B EVB.

Do you know what usage of these capacitors?

0 Penny Xu over 7 years ago in reply to Ruei-Ming Guan

TI__Genius 15685 points

Hi Ruei-Ming,

In IEEE standard, Alt A PSE devices has to finish second detection within 2s followed by a failed detection. Alt B PSE has to start second detection after 2s followed by a failed detection.

Longer back off time may help the port settle down before starting a new detection cycle.

The PD input capacitor is required by the standard. During detection, the input capacitance should be in this range:0.05uF to 0.12uF. Thanks.

Best regards,

Penny

0 Ruei-Ming Guan over 7 years ago in reply to Penny Xu

Prodigy 200 points

Hi, Penny

The capacitors parallel rectifier diodes are not in TPS2378 EVM.

These might not PD input capacitors.

0 Penny Xu over 7 years ago in reply to Ruei-Ming Guan

TI__Genius 15685 points

Hi Ruei-Ming,

I got you. I thought you were mentioning the PD port input cap. Yes, you are correct. The caps in parallel with the rectifier diodes are not needed and we don't have these on our PD EVMs. These caps increased the port capacitance and could possibly cause detection failure and noncompliance. Thanks.

Best regards,

Penny

0 Ruei-Ming Guan over 7 years ago in reply to Penny Xu

Prodigy 200 points

Hi, Penny

We have tested the PD device with TPS2384 TPS861 EVMs and got fail results in both two boards.

TPS2384 results:

TPS23861 results:

0 Penny Xu over 7 years ago in reply to Ruei-Ming Guan

TI__Genius 15685 points

Hi Ruei-Ming,

What detection results you got from TPS2384 and TPS23861? you can read from the GUI. Given the failures with TPS2384EVM and TPS23861EVM, I recommend you switch your debug focus to the PD side. Probably you can do sifos test with the SI3402 board and check if it is compliant. Thanks.

Best regards,

Penny

Power management

Power management forum

TPS2384: TPS2384 ; PD Detection Function Test Result with Device for or not for PoE