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BQ24640: Efficiency at low charge currents

ArTzy
Genius 14839 points
Part Number: BQ24640

Hi Experts,

Please advise to the query from Cx:

I am currently planning to use the BQ24640, which I know is designed for high currents, but is able to charge at lower currents by changing the sense resistor. I am trying to determine what components play a role in reducing the efficiency at low currents, in order to see if optimization of those components would create a more efficient solution. More specifically, I'm trying to quantify how changing the sense resistor and subsequent sense current will affect the overall circuit efficiency. Our current mode of thinking is that the FETs has some function of control over this efficiency but want to get a deeper understanding of how this effects our circuit. Our two main questions are:

[1] What components have control over the system efficiency? Or what is the dominant component that controls efficiency?
[2] Is the supply charging currents a function of the FET characteristics? What role, if any, do the FET characteristics play in the switching currents present in the system?

Thank you.

Regards,
Archie A.

  • 0
    TI__Mastermind 33285 points

    Hello Archie,

    1. The components that control the efficiency from the input to the output are the DC losses in the high side and low side FETs, the DCR of the inductor, the sense resistor, and any losses to and from the battery and input.

    2. The charging current does not depend on the FET characteristics. The charge current regulation is determined by Section 7.3.2. However, if the device gets too hot as in Section 7.3.11 Thermal Protection will be active. The inductor current ripple is determined by the input and output voltage.

    As a side note, please pay attention to the charge current regulation accuracy. For 1.5 mV or less across the sense resistor the charge accuracy is greater than +/-50%.

    Regards,

    Mike Emanuel