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BQ25504: DCM/CCM of BQ25504

Jonathan Geronga
Expert 7930 points
Part Number: BQ25504

Hi Team,

Looking for your comment regarding this inquiry of our customer for BQ25504:

The customer is developing a discrete boost converter with a MPPT instead of voltage controlling for energy harvesting applications. Therefore they are implementing under and overvoltage protection because it is aimed to use a LIC in future. For switching the MOSFET in the boost converter they use a frequency of 5kHz and PWM.
During the development process, they analyzed existing systems such as the BQ25504, and some questions raised.
- According to their research for boosters, the discontinuous current mode (DCM) is for low-power applications more efficient than the continuous current mode. Is the BQ25504 realized in DCM? What is its switching frequency? Their discrete booster does work in DCM.
- Their first measurements showed a very low efficiency 5 to 30% with an input voltage of 40mV and an input power of 400uW which is not satisfying. How can they improve the efficiency of my booster? May they change the value of the coil or the switching frequency?

Please let me know if you have questions with the customer.

Thanks,

Jonathan

  • 0
    TI__Guru**** 166176 points

    Hi Jonathan,

    The BQ25504 only operates in DCM, varying the switching frequency (PFM) up to 1MHz.  The boost charger is only recommended for operation down to  ~100mV input at VIN_DC.  If it is operating down to 40mV with any efficiency, I am surprised.

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    TI__Expert 7930 points

    Hi Jeff,

    Happy new year!

    Upon sharing your answer to our customer, here's the follow up question I got:

    He saw in the datasheet it varies up to 1Mhz. Why does it do this? Increasing efficiency? Does the frequency increase with less input power? There is probably an efficiency increase due to your usage of PFM instead of PWM in my discrete booster.
    He still not understand which devices have the most impact on the efficiency. How he can improve the efficiency of his booster if he has a fixed frequency and using PWM?

    Regards,

    Jonathan

  • 0 in reply to Jonathan Geronga
    TI__Guru**** 166176 points

    HI Jonathan,

    Happy new year to you!

    BQ255XX cannot fun as a fixed frequency PWM converter.  It only has comparators, not error amplifiers, and so must run in discontinuous mode PFM.  Also, the MPPT circuit does not effectively regulate the input of a high Z source so it will stop charge occasionally, which looks PFM as well.  The only ways to increase efficiency would be to use an inductor with lower DCR, place a parallel Schottky diode from LBOOST to ground, from LBOOST to VSTOR and from VSTOR to VBAT. 

    Accurately measuring efficiency with this device is difficult.  I suggest your customer read the appnotes below:

    regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 50 points

    Thank you Jeff and Jonathan for your fast response and help!

    You can close this issue!

    Best regards

    Robert