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BQ24780S: Inductor Current below zero during the low charging current

Chandann27
Prodigy 180 points
Part Number: BQ24780S

Hi,

In the Charge converter using BQ24780S charge controller IC, the Inductor current goes below zero (negative) during discontinues mode, when the charging current is low.

As shown in datasheet figure 26.

I have attached the measurement from our circuit. Green - Inductor current, Yellow - SW node voltage.

What is the effect of this(negative inductor current) on the converter stability? 

Is the current going negative is fine ?

Can you please help to understand better ?

Thanks,

Chandan N

  • 0
    TI__Expert 5330 points

    Hi Chandan,

    The BQ24780s implements DCM by turning off the low side switching FET when inductor current (measured by RSN sense resistor that connects to SRP-SRN) falls below 500 mA for a 10 mOhm sense resistor.  This is implemented using the undercurrent comparator which you can read more on by searching UCP in the datasheet.

    When the low-side FET turns off, its body diode will conduct until the inductor current reaches zero, then the FET will block the current.  Since the high-side FET is also off, the switch node is high impedance.  Parasitic capacitance in the FET combined with the inductor forms a resonant LC circuit, which is why you see the sinusoid in both the SW node voltage and the inductor current.  Even thought the sinusoidal oscillation in the inductor current dips below zero, it comes back over by the same amount and the net is zero.  Also this current isn't going back to the adapter, it is ringing between the switching FET and the output caps.

    To answer the second part of your question, DCM versus CCM operation does affect the frequency response of the feedback loop.  There is a note in the datasheet section 7.3.6.2 that during DCM the feedback compensation is changed accordingly.  So BQ24780s adjusts to keep the loop stable during the transition from CCM to DCM and vice-versa.

    Regards,

    Steve 

  • 0 in reply to Steve Preissig
    Prodigy 180 points

    Hi Steve,

    Thanks for the detailed explanation.

    My understanding from the above is that, the inductor current ringing is not a problem for the converter stability or to the system irrespective of the inductor current (can be 1A or 100 mA) since the net current is zero. 

    Please correct me if am wrong.

    Thanks,

    Chandan N

  • 0 in reply to Chandann27
    TI__Expert 5330 points

    Hi Chandan,

    That is correct.

    Regards,

    Steve