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BQ24297: Use of PMID and charging a supercap

Jeff Coletti
Expert 8755 points
Part Number: BQ24297

Hi Support team,

I have a customer with a very non standard application with the bq24297. They are using the bq to limit the current into there system and also using the bq to provide additional current from the super cap when needed.

I can send over a block diagram, but essentially they have a 500mA source that goes directly to the bq,(VUSB), they have a 47F super cap on the VBatt. Now my concern stems from the fact that they use the  PMID to source their circuitry. Nothing is connected on Vsys. the circuit is usually idle-low power mode, but when active it draws more current than the 500mA. What they are trying to do is during idle periods, charge up the cap and run the system from PMD, but when they need to turn on the high power cktry they want to operate off VUSB (current limited to 500mA) and draw excess current from the super cap on VBATT.

Is this a valid set up? Is it OK to draw from PMID when VUSB is present? Is it OK to leave VSYS unconnected?

Thanks

Jeff

  • 0
    TI__Mastermind 25800 points

    Hey Jeff,

    Customers may leave VSYS floating and pull current from their VBUS source through the PMID pin. 

    However, I see two issues with their setup:

    1) You mentioned with want to use the BQ24297 to limit current into their system. However, the charger can only limit charging current when using our IINDPM feature (to limit input current). As such, any current through PMID is essentially unregulated. In other words, the effect of going beyond the input current limit of 500mA (either by hitting the IINDPM setting in registers or by hitting the VINDPM setting from an overloaded power supply) is to pull back as much current from the charging portion (into the BAT pin, super cap here) to maintain either VINDPM or IINDPM, whichever takes priority. 

    2) These conditions will also not allow the super cap to discharge through the charger and out of the PMID pin. For one, you have an input source that is higher in voltage than the supercap it is charging. There is an additional body diode on the Q2 FET from SW to PMID, adding a forward diode drop to the super cap voltage to PMID. So you have a 5V (possibly lower if overloaded as described in (1)) trying to source current from a lower potential. Second and most importantly, the charger is trying to operate the buck converter during all of this. As such, it would be trying to drive current into SYS/BAT.

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Expert 8755 points
    Hi Joel

    We want to regulate current on VBus, and then supplement the current on Vbus with current from the super cap.
    As for the operation, I thought we could use the inductor to boost the battery voltage to help supplement the current going out PMID. My understanding was that PMID was to be used for VBUS when in an on the go application.
    Am I correct here, or am I missing something, trying not to set on any ones toes here, I just took over the account last week.

    Thanks Jeff Coletti
  • 0 in reply to Jeff Coletti
    TI__Mastermind 25800 points

    No worries Jeff.

    The thing to note here is that we only regulate current through VBUS by decreasing the current into the SuperCap (BAT pin).

    The other problem is that the charger cannot both charge the SuperCap and operate the OTG boost mode. The internal converter is shared for both Buck-mode charging and Boost-mode OTG power. When a switch is required from Buck to Boost, there is about a 30ms delay to switch the converter operating logic. 

    Again, my suggestion is really to put an inline boost converter (if needed) on the SYS pin. In this case, when VBUS is pushed to the Input Current Limit your customer sets, it will provide less and less charging current into the SuperCap until the point where it will discharge the cap.


    Regards,

    Joel H