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BQ25798: Stability issues - max values of output capacitance

Tomasz Ciesla
Intellectual 690 points
Part Number: BQ25798


hi, we're using BQ25798 to charge 3S Li-Ion battery pack using 12V HVDCP. IC works with 1uH inductor, 1A charging limit and 1.5 MHz switching frequency. We did notice acoustic noise and wrong ADC readouts when the Power Stage switches from buck to boost operation towards the end of charge (Uusb ~11.6 V, Ubat = 12.45V, Ibatt <0.3A) Disabling PFM in forward mode solves the issue. The issue is not present in BQ25798EVM. Can it be due to too high MLCC capacitors used on PMID and SYS causing unstable loop operation? If yes are there max recommended values?

BQ25798EVM:

  • PMID: 3x10uF (C2012X5R1E106K125AB) = 3x1.67uF@12.5V = 5uF@12.5V + 33uF tantal 
  • SYS: 5x10uF (C2012X5R1E106K125AB) = 5x1.67uF@12.5V = 8.35uF@12.5V
  • BATT: 2x10uF (GRT188R61E106ME13D) = 2x1.5uF@12.5V = 3uF@12.5V

Our board

  • PMID: 3x22uF (C3225X7R1E226M250AB) = 3x14.8uF@12.5V = 44.4uF@12.5V
  • SYS: 3x22uF (C3225X7R1E226M250AB) = 3x14.8uF@12.5V = 44.4uF@12.5V
  • BATT: 2x22uF (C3225X7R1E226M250AB) = 2x14.8uF@12.5V = 29.6uF@12.5V 

additionally our board has similar issues in buck mode when working with 2.2uH inductor and 1.5MHz

  • +1
    TI__Guru**** 169156 points

    Hi Tomasz,

    There is no max capacitor and more capacitance should help the issue.  Did you follow the recommended layout in the datasheet and on the EVM?  Are the 0.1uF decoupling/bypass capacitors on PMID and SYS placed as shown below:

    Switching noise is worse in boost mode so ADC measurements are expected to be worse.  We do not spec the ADC voltage and current accuracy for this reason.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 690 points

    I did patch the PCB to move 100nF C1 & C2 as close to BQ25798 as possible and it solved the problem. Funny thing is that distance form chip of C3, C7 and C6 doesn't affect it much. Probably one of them (C1 or C2)is crucial.