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TPS82085: TPS82085 output cap issue

Sulyn ZHANG
Expert 8700 points
Part Number: TPS82085
Other Parts Discussed in Thread: TPS82130

Hi Team,

My customer is using TPS82085, but due to load restriction, they have to use output cap higher than 150uF. But in our datasheet, we have recommendation for output cap is 22uF to 150uF. So the question from customer is that:

1. Since they already use bigger than 150uF during first around pp test, is these any risk for this when go to MP? For example, will      TPS82085 be damaged during MP stage after running a while at field side;

2. At the second around pp test(Another slot TPS82085), they do found TPS8208 can't output normal with this big cap, this should trigger the hiccup mode during start up as mentioned in our datasheet.

If customer have to use bigger 150uF, do we have solution to avoid problem or potential risk? Thanks.

Best regards,

Sulyn

  • 0
    TI__Guru*** 142860 points
    Hi Sulyn,

    This E2E thread also discusses this topic and points to the app note which answers your second question: e2e.ti.com/.../547865

    There is likely some ESR in some of your large output caps. This greatly helps stability. This app note helps you measure stability: www.ti.com/.../slva381b.pdf
  • 0 in reply to Chris Glaser
    TI__Expert 8700 points

    Hi Chris,

    Sorry that I have more question even knowing this issue from the application note you recommended. As customer plan to MP very soon, so below important questions needed to be confirmed as well, thanks in advance! Customer need to drive about 550uF output cap in their application, this can't be changed.

    1. As for TPS82085, we have below description:

    It mentioned that "output cap may be possible to be bigger than 150uF, but with reduced load during start up", so how to understand the "with reduced load" here?

    Does it mean the normal lIoad after stable should be reduced? How much need to be reduced? Or it mean that the transient current during start up needed to be reduced to avoid trigger hiccup(For example add external soft start function or add load switch) as showed in our application?

    2. For better solution, I would choose TPS82130, and in our TPS82130's datasheet, we have description that:

    TPS82130 can support up to 400uF output cap. So here what's the Css value was used to meet 400uF output cap here? Or else, with no Css, TPS82130 can drive up to 400uF cap, with bigger Css, TPS82130 can drive higher output cap because of longer soft start time? How to better understand this?

    3. For customer's application, 5V input, 3.3V output with 550uF output cap. If they use TPS82130, can you or some expert help give a reference schematic with recommended external parameters(Especially the Css value and Cff value) to mee this requirement? Thanks a lot!

    Best regards,

    Sulyn

  • 0 in reply to Sulyn ZHANG
    TI__Guru*** 142860 points
    Hi Sulyn,

    1. This is exactly explained in the app note. The load current + the current required to charge the output caps + 1/2 of the inductor ripple current must remain below the current limit value.

    2. TPS82130 is different, because it does not have hiccup current limit. However, it is still best to remain out of current limit during startup. Again, the Css is set to give a long enough startup time to not reach current limit as explained in #1 above.

    3. You should not need a Cff, but you could add a footprint for one. With a 10nF Cff, my calculations show that there will be less than 0.5A output current required to charge the output caps.