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BQ25703A: Inductor selection setting (IADPT pin) for 1.5µH inductor

Benny Goemans59
Prodigy 25 points
Part Number: BQ25703A
Other Parts Discussed in Thread: BQ25703, BQ25713

Hi,

the datasheet for the BQ25703A mentions that it's possible to set inductor values via a resistor and lists the possible resistor values for 1µH, 2.2µH and 3.3µH.
It doesn't mention the 1.5µH inductor, which as far as I can tell from other places in the datasheet is also supported. Should this one use the 1µH value instead?

"Recommend 800 kHz with 2.2 μH or 3.3 μH, and 1.2 MHz with 1 μH or 1.5 μH.
Host has to set the right PWM frequency after device POR." (p37)

Is there any formula to calculate the resistor value? As far as I can see this value is used to calculate the power consumption and the better it is matched to the inductor, the better the consumption could be calculated.

Thanks in advance,
BG

  • 0
    TI__Expert 5330 points

    Hello Benny,

    The main purpose of specifying the inductor value is that it is used to reconstruct inductor current during the phase of the converter duty cycle where the hi-side switch is off and the low-side switch is on.  This is necessary because the current sense resistor can only measure the inductor current during the reverse portion of the duty cycle.

    BQ25703 then uses this reconstructed inductor current for the average current mode control of the device.  Setting the wrong inductor value, even if it is close, will adversely affect the feedback loop for the switching regulator, which will impact transient response, accuracy, etc. 

    The resistor values are sampled into "bins" at startup, so it is not a continuous range where the resistor value can be calculated.  I am sorry about the misleading reference to 1.5 uH on p.37, but for the reasons listed above, you should change your inductor value to match the supported resistor bins on table 3 -- 1.0, 2.2 or 3.3 uH.

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    Prodigy 25 points

    I removed all references to the 1.5µH inductor from internal documentation and added that it isn't supported.
    A 1.0, 2.2 or 3.3 µH inductor will be selected instead.

    One small extra question though. Is there a recommendation for inductor accuracy/tolerance? Most of them are specified to be up to 20% or even 30% off.
    Could a large inaccuracy affect the operation of the BQ25703/BQ25713 device?

    Thank you for clarifying this.

  • 0 in reply to Benny Goemans59
    TI__Expert 5330 points
    Thanks for your understanding Benny. Glad I could help.

    Steve