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BQ25505: Non-standard configuration allowing temporary removal of load from EH source

Andrew Holmes
Prodigy 30 points
Part Number: BQ25505

I am using a BQ25505 in an energy harvesting application. For reasons specific to the application, I need to periodically remove the load applied to the energy harvester for ~200 ms before re-applying it, and I want to do this without changing the mode of operation of the BQ chip. The solution I have come up with is to put an SPDT analogue switch on the Vref_samp pin, with its COM terminal connected to Vref_samp, its NC terminal connected to a 10 nF capacitor (other end at GND), and its NO terminal connected to Vstor. So, when the switch is not activated I have a standard BQ configuration, but when the switch is activated Vref_samp is raised above Vindc and the DC-DC converter stops loading the source.  

My question is: does this solution seem reasonable to you? I have tried it and it seems to work, but I am wondering if I am going to get caught out (as I have done in the past with this chip when straying from the App Note circuits!). 

Many thanks. 

  • 0
    TI__Guru*** 148665 points

    Hi Andrew,

    I would not have do it that way but I don't see any reason it wouldn't work.  As long as the analogue switch has low leakage when turned off and doesn't cause the VREF_SAMP capacitor to discharge more quickly, I see no issue.  Just curious why you didn't use the same analogue switch tied between /EN and VSTOR, with a pull down resistor on /EN?  Too much leakage through the pull down resistor?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi Jeff, 

    I think the alternative solution you are suggesting (to temporarily disable the BQ via the ~EN input) is one I had tried previously that didn't work as expected.  I found that when the BQ came back up it did not immediately apply the correct MPPT set-point; it seemed to load the harvester too heavily when coming out of the disabled state. I posted an earlier question about this a few weeks ago on this forum. (I think it may have been you that replied.)

    Regards,

    Andrew 

  • +1 in reply to Andrew Holmes
    TI__Guru*** 148665 points

    Andrew,

    Oh, now I remember.  Your way is best.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thanks Jeff. I will go with it and hope I don't get any nasty surprises!