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BQ25756: Inductor Selection

Umut Öznar
Prodigy 140 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25758

Tool/software:

Hello,

I am designing a charger circuit with the BQ25756. My charge current and input current limit are 20A, so the inductor’s DCR is a critical parameter for my design. However, when I checked the TI design tool, it states that the inductor should have a minimum DCR of 5mΩ. Could you please explain the reason behind this requirement?

The inductor I am planning to use has a DCR of 1.53mΩ. Would using such an inductor cause any issues in the application?

Best Regards,

Umut Öznar

  • 0
    TI__Expert 5905 points

    Hi Umut,

    From our FAQ page:

    • What happens when the IC starts up and starts switching?
      The IC does the inductor detection first. A series of current pulses are sent through the inductor at startup to determine the time constant of the system. Furthermore, the system will power up assuming the DCR of the inductor is around 11mΩ. When the converter output current exceeds about 1.9A, this assumption will be reevaluated. This reevaluation occurs slowly with small changes taken every 3.5ms. The time-constant calculations are done from the input supply in forward mode and from the output side in reverse mode depending on how start up happens. The time constant calculation happens one-time each power up whereas the DCR adjustments are always running.

    This means that we have a time constant restraint for the inductor. The datasheet for the BQ25758 lists the time constant in the "Table 6-1. MODE Pin Resistance Configuration Options" which isn't in the BQ25756 datasheet since there is no mode pin on the BQ25756.

    The DCR of 1.53 will likely cause issues in the circuit.

    Best regards,
    Michael Bradbourne