TPS2378: TPS2378 PD Controller Design review

Hi Kapil,

I am working on validation and will get back to you by Wednesday. Thanks for patience. 

Best regards,

Diang

Hi Diang,

Noted, Thank you!!

Thanks for your patience!

Hi Kapil,

Could you let me know where is the PoE input side? Is the 48V_REC a 48V dc adapter? Could you share a block diagram with more information?

Below is one common PoE PD structure with DC adapter. PoE is on the VSS side and DC adapter is on the RTN side. When DC adapter is inserted, APD voltage will rise above threshold and turn off the PD's internal hotswap.  

You may check PMP11257 (https://www.ti.com/lit/df/tidrnh8/tidrnh8.pdf) schematic as a reference.

Best regards,

Diang

Hi Diang,

The two voltage is coming in PD controller as follows.

1. One voltage from PoE Transformer after rectification, protection and filtering. .

2. 2nd Voltage is coming from DC source. The adapter pin power is 48V DC (48V_REC), which I ORed with PoE Transformer Voltage.  

See the image below.

Note: I have taken 3 ground plain.

BGND: PoE 48V

0VS: 48V from adapter after filtering

GND: 48 V DC input from external DC source. 

It seems this link is not opening:

You may check PMP11257 (https://www.ti.com/lit/df/tidrnh8/tidrnh8.pdf) schematic as a reference.

Other than that, I have a query related CLS pin Resistor.

I see the 7.3.3 CLS Classification resistor values for different classes. In my current design I have selected the class-4 with the power range from 12.95 W to 25.5W. Means if I require the power with this range then the PD controller works fine. But If power requirement is less than 12.95W  (like 1W to 12W) so in that case what will happen? Dose PD -controller will work or not?

Let me know your valuable inputs for this PD circuit and Soft start and regulator control circuit.

Regards

Kapil

Hi Kapil,

Thanks for your reply. I will get back to you in 3-5 business days.

Best regards,

Diang

Hi Kapil,

Please check the schematic in the portfolio page of PMP11257: https://www.ti.com/tool/PMP11257

For the class concern: Class4 can be applied for PD input power from 1 - 25.5W. There is no actual minimum power in practice. 

But if the power is too low, causing the PD draw current less than 14mA (or 14mA x 57V = ~0.8W) from the PSE, the PSE may turn off this PD port due to lose of maintain power signature.

Best regards,

Diang    

Hi Diang,

I understand this point then why different value of resistor is given for different classes? 

Hi Kapil,

According to 802.3BT, the classification is based on current reading. TPS2378's CLS pin will output 2.5V when doing classification. A proper resistance for correct classification current. 

Best regards,

Diang 

Hi Diang,

Thanks for the clarification. So As per the class resistor selection do we need to follow the exact value or we can keep some margin. Like for class 4 the recommended value is 63.4 ohm. what will happen If I keep 60 ohm or 75 ohm?

Hi Kapil,

PoE team is doing internal tests and the reply on E2E may be delayed for 3 - 5 business days. Sorry for the delay and thanks for your patience.

Best regards,

Diang

Hi Diang,

It's fine. 

Thanks for patience!

Hi Kapil,

Yes you can have a margin on the classification resistance.

Using (36mA < 2.4V/Rcls < 44mA) & (36mA <2.6V/Rlcs < 46mA), Rcls can be around 57 - 66 Ohm, not including the resistor's +-% error. 

Best regards,

Diang