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BQ25700A: Current ripple in CV mode

Beginner
Intellectual 2381 points
Part Number: BQ25700A

Hi Team,

I create this thread to follow up  http://e2e.ti.com/support/power-management/f/196/p/831718/3103453#3103453?jktype=e2e

This is the same project w/ the same settings. In 1st and 2nd build, customers creates both 20pcs to evaluate. They can 1 pcs w/ current ripple in each build. 

Already adding more 60uF cap on Vsys or VBAT to avoid PFM mode. However this current ripple is not the similar w/ what we saw.

Kindly help to explain this behavior and provide solutions to resolve

Test Procedure

•Before testing, charging battery until it enters CV mode.
•Let gauge to monitor system information – included voltage and current.
•Plot voltage/current by time as below.

Test Result 

  • 0
    TI__Expert 3320 points

    Hi There:
      

         This may be caused by current regulation issue at PFM mode. Could try to add 68nF cap between SRN and SRP pins on the EVM to see if there is any improvement?

          

    Regards

    Shishuo

  • 0 in reply to Shishuo Zhao
    TI__Intellectual 2381 points

    Hi Shishuo,

    We will try this on next Monday. In parallel, could you explain why PFM mode will cause this symptom and then this changed will help for this?

  • 0 in reply to Beginner
    TI__Expert 3320 points

    Hi There,

         From the waveform we find that the oscillation mostly happens at small charge current before termination. During that time converter is in PFM mode and we have some experience that during small current PFM mode the current regulation is good which could result in charging current regulation.

    Regards

  • 0 in reply to Shishuo Zhao
    TI__Intellectual 2381 points

    Hi Shishuo,

    My understanding is the current ripple is caused by PFM. Could you kindly explain more detail here? 

    This is the main concern for customers. They think this is abnormal thus they would like to change solutions.

  • 0 in reply to Beginner
    TI__Expert 3320 points

    Hi There,  

         Under PFM mode the current signal is tiny and discontinuous, therefore it is more sensitive to board noise. That is why we suggest to add 68nF cap between SRN and SRP to filter differential noise. If above issue is solved after adding 68nF cap, my understanding could be verified. 

    Regards

  • 0 in reply to Shishuo Zhao
    TI__Intellectual 2381 points

    Hi,

    We tried to add 69nF cap between SRN and SRP but if fails to resolve this symptom. Schematic is sent to you offline. Please check. 

  • 0 in reply to Beginner
    TI__Expert 3320 points

    Hi There,

         I have checked the schematic. Your schematic currently only have 76uF ceramic capacitor however we suggest to have 47uF POSCAP + 70uF ceramic capacitance to reduce PFM mode ripple. Could you try to add 47uF POSCAP to see if there is any improvement. Based on previous support experience, increase system capacitor should help on this issue. 

    Regards

  • 0 in reply to Shishuo Zhao
    TI__Intellectual 2381 points

    Hi Shishuo,

    I will ask customers to do this test right away.

    Meanwhile, here are more questions w/ your comments. 

    1. Your comment is to add TA POSCAP. Could we replace by TA SMD type?

    2. If this works, does it means POSCAP can optimize frequency or your comments is just to add more capacitor?

    3. Once this solution works, could we have 100% confident to ensure this won't be happened in other PCBA? They are worried about this is a single solution for one of them PCBAs.

  • 0 in reply to Beginner
    TI__Expert 3320 points

    Hi Ake,

    1. Your comment is to add TA POSCAP. Could we replace by TA SMD type?

        Yes, I believe as long as effective capacitor is higher than 47uF, it should work.

    2. If this works, does it means POSCAP can optimize frequency or your comments is just to add more capacitor?

        If it works, it means that enough output capacitance is necessary for both CCM regulation  and PFM regulation for BQ25700A.

    3. Once this solution works, could we have 100% confident to ensure this won't be happened in other PCBA? They are worried about this is a single solution for one of them PCBAs.

       We are confident, because we do see similar issue is successfully fixed by adding system cap.