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

Hi, Penny

We found there is a symptom that TPS2384 can't detect PD IC Si3402-B when setting this parallel capacitor as 0.22uF.

The capture is as below:

When changing this capacitor as 0.1uF, Si3402-B could be detected and capture as below

These captures are both in ALT-A mode.

Referring to other PSE IC EVBs, this capacitance is suggested to be 0.1uF.

Could you help check is there any compatibility issue to use 0.22uF?

Thanks. 

Another one question is IEEE802.3af defines the minimum backoff time to be 2 seconds. Does that mean the TPS2384 should be set to ALT-B mode?

Hi Ruei-Ming,

What's detection results when the port cap is 0.22uF and what's the port cap on the PD side? If the port cap is too high it may fool TPS2384 to get the wrong detection result as TPS2384 relies on the port impedance to reduce the detection voltage from 8.8V to 4.4V. Large port capacitance across the port may cause the detection algorithm to be fooled. 

As for the backoff time, IEEE standard specs different timing requirements for Alt A and Alt B. In standard it says

"If a PSE performing detection using Alternative A detects an invalid signature, it should complete a second detection in less than Tdbo min after the beginning of the first detection attempt."

"A PSE performing detection using Alternative B may fail to detect a valid PD detection signature. When this occurs, the PSE backs off for at least Tdbo as specified in Table 33–11 before attempting another detection."

So you can configure either Alt A or Alt B.

Additionally, I want to check why do you pick TPS2384 for your application?  We have TPS23861 which is newer device with better support. You may consider moving forward with TPS23861. Thanks.

Best regards,

Penny

Hi, Penny

1. With the PSE port cap=0.22uF, the PSE continuously detects PD as above figure. With PSE port cap=0.1uF or 0uF, it could detect PD. The output cap in PD side is 0.1uF.

2. If we designed the POE type be Alternative A, could we set the TPS2384 back-off time in Alternative-B mode?

3. This is an older product.

4. We also check PSE reference circuits in other vendors (Maxim, Micro-semi, Silicon-Lab) and TI TPS23861, the output cap is 0.1uF, could you help check the reason of TPS2384 suggests to use 0.22uF output cap?

Hi Ruei-Ming,

Setting Alt A or Alt B basically is setting the back off time. If you design the hard in Alt A mode, you shouldn't use Alt B back off time which violates the IEEE standard. for the port cap, I have to check with our design team. BTW, have you tried to connect TPS2384EVM to your PD board? When we connect TPS2384EVM with TI PD board(input cap is 0.1uF), we didn't see any issues. Thanks.

Best regards,

Penny

Hi, Penny

We have tried connecting TPS2384EVM to this PD device, but the symptom is same.

Is there any update from TI design team?

Hi, Penny
I saw another question in forum: e2e.ti.com/.../682982
You mentioned there are detection mechanism difference between TPS2384 and TPS23861.
Is there any more description or document about the forced voltage and current mode?
Thanks.