TPS2373-4EVM-758: How to determine the minimum capacitance of C28 of the EVM?

Webber Chang

Tool/software:

Hi,

Good day.

I am currently designing a POE product with TPS2373-4+UCC2897ARGPR and encountered a lack of space to place the electrolytic capacitor.

So, could you please guide me in determining the minimum capacitance of C28 of the EVM?

Sincerely

webber

0 Ke Wang 7 months ago

Hi,

Our PoE team is currently out of office and will be back tomorrow. Sorry for the delay.,

BR,

Ke Wang

0 Diang Xing 7 months ago in reply to Ke Wang

Hi Webber

802.3BT 最worst的250us transient 没有input current，加上100m线 + PSE最低输出电压 + PD 满载load。这时为了不让PSE - PD port关断我们推荐如下的cap。但实际情况这个很难发生，很多设计不会用这么大的cap。

Power Bulk Cap

13W 10-22uF

25W 33-47uF

50W 68-100uF

70W 100uF-200uF

802.3BT还有个和Cport相关的最低要求。考虑到4-paris是两个port，可能需要Bulk >= 40uF for Class-7，8。其余的Cap要求就是和DC/DC相关了。

Best regards,

Diang

0 Webber Chang 7 months ago in reply to Diang Xing

HI Diang,

感謝解釋.
請問EVM C28的目的是用於儲存電流,避免當PD抽重載導致PSE關閉電源?

如果是,有方式可以計算我需要多少電容才能達到這目的?

謝謝

webber

0 Diang Xing 7 months ago in reply to Webber Chang

Hi Webber,

正常工作时C28是为了稳定DC/DC input电压，保证DC/DC在优化的输入电压下运行。这时和SW 250kHz频率、start up时给Vout电容充电也有关系。

PSE-PD inrush Cport需要40uF。如果有250us PSE-PD 断电transient，C28是为了独立提供能量保证PD不落到低于undervoltage （36V）的情况。这时需要100uF-200uF的cap。

Best regards,

Diang

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