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TPD2S300: CC Line Capacitor selection

Shmuel Davis
Expert 2920 points
Part Number: TPD2S300
Other Parts Discussed in Thread: TUSB320LAI, , TPD6S300

I need to calculate the CC Line Capacitor selection.

I posted the following :-

https://e2e.ti.com/support/interface/circuit-protection/f/389/t/651056

and thus I posted 

https://e2e.ti.com/support/interface/usb/f/1008/t/651328 

From these responses it is still going to be difficult for me to make the choice of the required capacitor (if any is required ?)

Does anyone have any suggestions ?

  • 0
    TI__Genius 10370 points

    Hi Shmuel,

    The first post hyperlink above talks about the CC line capacitance according to USB-PD specs.

    A min of 200pF and a max of 600pF is the spec on the CC lines. According to the TPD2s300 datasheet, there's a min capacitance of 30pF on our device. I would recommend adding a 180pF cap on each CC line. This will add up to 210pF (right above min for PD spec) and I'm assuming that the other device (TUSB320LAI ) won't have an internal capacitance above 390nF.

    If for some reason the cap value on the CC lines is above 600pF with both devices and the 180pF, you can depopulate the cap and add a lower value to the board, but I think that the 180pF cap should be fine :-)

    If this answers your question, PLEASE select  This resolved my issue. 

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points
    Dear Aramis P. Alvarez,
    Thank you for this response.

    In the Proposed capacitor calculations of TPD6S300 data sheet it states that : (For min and max, assume ±50% capacitance change with temperature and voltage derating to be overly conservative)

    Should we consider this also here ? If yes then the target capacitance give your suggested value will (if assume that the other device (TUSB320LAI ) won't have an internal capacitance) gives a TOTAL min value of 120 pF which is below the required 200pF capacitance ?

    Best regards

    Shmuel
  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points

    Hi Shmuel,

    You could try different values of capacitance. I would start with 180pF and see if that solves the problem. If not, you can increase the cap by 50-100pF each time (until you hit the max of 600pF according to USB PD3.0 spec) by depopulating and repopulating a new value onto the board.

    If this answers your question, PLEASE select  This resolved my issue.  

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points

    Dear Aramis P. Alvarez,

    Thank you again for your reply. 

    If the capacitance is not correct - what "problem" are you expecting that the changing of capacitance "on the fly" will fix it ?

    BTW: From my calculations if we do assume ±50% capacitance change with temperature and voltage derating of the "proposed capacitance" :- 

    MIN : 340 pF ==> 340 - (340/2) = 170

    +TPD2S300 MIN capacitance ==> 200pF

    MAX: 320pF ==> 320 + (320/2) = 480
    +TPD2S300 MAX capacitance ==>600 pF

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points

    Hi Shmuel,

    If you would like to know the exact value, this might be difficult because of many variables (temperature, voltage, unknown capacitance of the TUSB320LAI, etc.). The advice that I can give is that the TPD2s300 capacitance on its pin ranges from 30pF to 120pF. According to USB PD 3.0 specs, one will need a min. cap. of 200pF and a max cap. of 600pF on the CC channels. Since the abs max voltage of the CC channel is around 6V, I would recommend a voltage rating of around 50V on the cap (this will significantly lower your derating value). Feel free to try different values of cap on the CC lines since the cap value of the TUSB320LAI is unknown. 

    If this answers your question, PLEASE select  This resolved my issue.  

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points

    Dear Aramis P. Alvarez,

    Thank you again for your reply. 

    If the capacitance is not correct - what "problem" are you expecting that the changing of capacitance "on the fly" will fix it ?

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points

    Hi Shmuel,

    If the total capacitance on the CC channels is outside the range of 200pF -600pF, then the PD communication between both PD controllers might not occur (too much cap might cause too long of a delay).

    If this answers your question, PLEASE select  This resolved my issue.

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points
    Dear Aramis P. Alvarez,

    Thanks for that. Thus if I understood you correctly then for each capacitor value used will need to be checked when our device is plugged into a specific Host USB port and see if the Host provides power to my device ? I assume there is no easy way to measure the total capacitance on the CC channels ?

    Best regards

    Shmuel
  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points

    Hi Shmuel,

    Measuring the capacitance on the TUSB320LAI  might be a bit tricky, so you're correct. You can also re-post this question about measuring the capacitance on a particular pin of a device to the TUSB320LAI forum.

    If this answers your question, PLEASE select  This resolved my issue. 

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points

    Dear Aramis P. Alvarez,

    Do we have to be concerned about "±50% capacitance change with temperature and voltage derating of the "proposed capacitance" as outlined  Proposed capacitor calculations of TPD6S300 data sheet also for TPD2S300 ?

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points

    Hi Shmuel,

    capacitance margin depends on what type of rated capacitor you're using (temperature, voltage, variation, etc.). It's up to the designer how much margin they would like. Here's a short but great article on capacitor de-rating: https://www.maximintegrated.com/en/app-notes/index.mvp/id/5527. And here's a slide deck that goes over more in detail about capacitor derating: http://www.kemet.com/Lists/filestore/Derating%20Guidelings%20for%20Tantalum%202011%20(3).pdf. 

    In short, I recommend to pick a DC voltage rating around 3 times the nominal voltage (For CC lines, 15-20V will do) and for temperature variation, I would recommend a X7R (15% over the entire temperture range of -55C to 125C). 

    If this answers your question, PLEASE select  This resolved my issue.  

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points
    Dear Aramis P. Alvarez,

    Thank for those articles. I read the first one, your right it is a great article. However there it appears from that article that the variation in capacitance that appears to be of most concern is the drop in capacitance rather than the rise. Also it appears that in certain circumstances there can be a much larger drop in capacitance than -15% - from their measurements there could be a drop in capacitance of up to 67% ? Thus when you suggested 180pF as a "starting value" giving a total of 210pF (when taking the worse case scenario TPD2S300 capacitance of 30pF and assuming that the TUSB320LAI has minimal capacitance) a drop of > 5% of the chosen capacitor would put me out of the range of the spec on the CC lines ? Of course unless I assume that the TUSB320LAI could potentially have up to 100s of pF of capacitance but I have not been given a clear answer yet on the capacitance of the TUSB320LAI ?
  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points
    Hi Shmuel,

    The higher the voltage rating, the closer to the actual capactinace you will be. Also, if you would like to be safe, choose X7R rated caps. As stated in a previous post, please post your question to the TUSB320LAI forum for questions about the TUSB320LAI capacitance. Thank you.
  • 0 in reply to Aramis P. Alvarez
    TI__Genius 10370 points

    Hi Shmuel,

    Since we closed this thread, could you PLEASE select  This resolved my issue  

    Thank you so much for your question and choosing TI parts for your design :-)

  • 0 in reply to Aramis P. Alvarez
    TI__Genius 10370 points

    Hi Shmuel,

    Since we closed this thread, could you PLEASE select  This resolved my issue  

    This will help us understand how to better support our e2e customers! Thank you so much for your question and choosing TI parts for your design :-)

  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points
    Dear Aramis P. Alvarez,

    Thank you for all your help on this but I didn't yet set this thread as "this resolved my issue" since I need a little more info to my response on Jan 22nd, 2018 ?

    Best regards

    Shmuel
  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points
    No problem at all Shmuel. Let me know if you need anymore help on the CC capacitance.
  • 0 in reply to Aramis P. Alvarez
    Expert 2920 points
    Yes please could you look back at my response on Jan 22nd, 2018. Could you please address the issues I raised there ? Thank you so much
  • 0 in reply to Shmuel Davis
    TI__Genius 10370 points
    Hi Shmuel,

    please look at my response on Jan. 22, 2018. Let me know if you have further questions. Thanks again for choosing our parts in your design :-)