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BQ51003: Slow Step response -- what can we do?

Scott Brenneman
Genius 3295 points
Part Number: BQ51003
Other Parts Discussed in Thread: BQ51013B

We're struggling to deal with dips in the OUT voltage of the bq51003 when there's a load current step from 125 mA to 250 mA.  Our system MCU is being reset as a result.  We've found that we can add super-capacitors (0.15 Farad) to deal with this, but we have very limited space and the super-caps are too large.  We also find that the quoted 90 ms step response is optimistic compared to what we're seeing which is 150 to 200 ms or longer.  We're looking for other ideas to try instead of the brute force approach of added bulk capacitance.  To that end, here are some thoughts and questions:

- It appears that the bq51003 monitors the voltage on RECT to determine how much power to request from the wireless power transmitter.  Is that correct?

- Could the large bulk capacitance on OUT be slowing the step response by masking the load step?

- Is there a way to "pre-load" the system so that the bq51003 requests more power in anticipation of the load step?

- Any other ideas?  We can't be the first people to have had this sort of design challenge!

Thanks,
Scott

  • 0
    TI__Guru* 75656 points

    Hello Scott

    A couple of things to try:

    1.) Set the current limit higher.  This should increase the voltage at RECT to better support a load step.  The BQ51013B is P to P compatible and has higher RECT voltage.

    2.) Increase capacitors at the RECT pin, this voltage is input to output LDO.

  • 0 in reply to Bill Johns
    Genius 3295 points

    Hi Bill,

    1.  You're saying that the 51013B is required to set the current limit higher?  Or is it just a higher-current part?

    2.  Will increasing the capacitance on the RECT pin slow down the STEP response?  Am I correct in believing that the voltage on RECT is how the part determines that it needs more power from the transmitter?

    Thanks,

    Scott

  • 0 in reply to Scott Brenneman
    TI__Guru* 75656 points

    Hi Scott

    The BQ51013B will set the voltage at RECT higher at lower current vs the BQ51003.  Setting the RECT voltage higher is key to surviving a load step.  The BQ51013B can go higher and needs to support a larger load step vs BQ51003.

    Increasing C on RECT will not slow down step response.

    The voltage at RECT is determined by output current, at low current the voltage is increase to support a load step.  The RX will send control error packets to TX indicating error from target voltage at the RECT pin.  The TX will then adjust operating point to hit the target.  At low current voltage at RECT is running about 7V and as load is increased the voltage is reduced to  about 200mV above output voltage.  The setting of ILIM will determine the slope of  RECT voltage vs output current.

    Updated Nov 6

  • 0 in reply to Bill Johns
    Genius 3295 points

    Hi Bill,

    Thank you for the quick response.  We'll look into this more closely.  One detail question: in your second-to-last sentence, I think some text might be missing; it says "...as load is increased the voltage is reduced to about 200mV.", but I don't think RECT is reduced to 200mV, but rather is reduced by 200mV, or reduced to 200mV above something.  Assuming I'm correct, what's the missing phrase?

    Thanks again,

    Scott

  • 0 in reply to Scott Brenneman
    TI__Guru* 75656 points

    I have updated the post, it should be 200mV above output voltage at max load.  Also see data sheet figure 5, graph of V-RECT vs output current with different output current limit settings.