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bq25504: Adding PFET to compensate for low current output from the dc source

MOHAMMAD ALHASSOUN
Prodigy 50 points
Part Number: BQ25504
Other Parts Discussed in Thread: BQ25570, SN74AUP1G14, SN74AUP1G06, SN74AUP1G04

Hi,

I am reading the user's manual for bq25504 in order to use it as a booster converter for the output of an RF energy harvester. Since the output power is less than PIN(CS), the manual, in page 14, suggested the use of PFET between the system load and VSTOR and the gate of the PFET is driven from the inverted VBAT_OK.  I have two questions about this implementation

1. Are there recommended  types of  PFET for the connection? 

2. Is there a specific configuration to get an inverted VBAT_OK? 

Thank you

  • 0
    TI__Intellectual 2050 points

    Mohammad,

    Answers below.

    MOHAMMAD ALHASSOUN said:

    1. Are there recommended  types of  PFET for the connection? 

    Any standard PFET should work fine.

    MOHAMMAD ALHASSOUN said:

    2. Is there a specific configuration to get an inverted VBAT_OK?

    Simply use and inverter logic gate.

    Once VSTOR charges VBAT above the VBAT_OK threshold, the VBAT_OK output will go HIGH. Otherwise the output is LOW. By sending this output signal through an inveter logic gate, you can properly control the external PFET.

    The PFET will remain open until the gate is pulled LOW. The inverter will take the HIGH signal from VBAT_OK when VBAT is charged above the VBAT_OK threshold and drive the PFET gate LOW connecting the system load to VSTOR. This method allows for VSTOR to enter and maintain a voltage high enough for proper main boost mode charging prior to loading VSTOR with the system.

    Was there any other questions I can help you with?

    Regards,

    Joe

    If your questions were successfully answered, please hit the green "Verify Answer" button.

  • 0 in reply to Joe Stephan
    Prodigy 50 points

    Thank you Joe,

    This in fact brought another question. Is there a passive way to derive the PFET, since my system is driven from an energy harvesting circuit? would the use of NFET work?

    Thanks

  • 0 in reply to MOHAMMAD ALHASSOUN
    TI__Intellectual 2050 points
    Since VBAT_OK changes with the VSTOR voltage, it would be constantly changing the voltage applied to the gate of the NFET.
    With a PFET and an inverter the voltage on the gate of the PFET would be constant (except when transitioning)

    What is your concern with using an inverter?

    Are you wanting to drive the external FET without using the BQ25504?

    -Joe
  • 0 in reply to Joe Stephan
    Prodigy 50 points
    Great! Thank you,

    My concern is I don't have a power source to power up the inverter, so I'm looking for passive technique to invert VBAT_OK.
  • 0 in reply to MOHAMMAD ALHASSOUN
    TI__Intellectual 2050 points
    Ok I understand now. You could power the inverter off of VSTOR or VBAT.

    Are you planning on charging up a battery or supercap? Or are you using this device just for boosting the output of your energy harverster?

    Also, take a look at the BQ25570. This device is the BQ25504 but also has a regulated buck output rail! you could use the integrated buck to supply a regulated voltage for your inverter.

    -Joe
  • 0 in reply to Joe Stephan
    Prodigy 50 points
    That's great, I'll look it up.

    To answer your questions, No, the output of the booster will be fed to the load. Also, do you recommend any low-power inverter?

    Thank you very much, Joe
  • 0 in reply to MOHAMMAD ALHASSOUN
    TI__Intellectual 2050 points
    TI has many inverter devices. I would take a look at SN74AUP1G04, SN74AUP1G06, and SN74AUP1G14 for starters. Each are low power, single gate inverters. If you have further questions about inverters I recommend taking that discussion to the Logic forum.

    Let me know if you have further questions about the energy harvester devices.

    Regards,
    Joe
  • 0 in reply to Joe Stephan
    Prodigy 50 points

    No that's it. Thank you very much Joe

  • 0 in reply to MOHAMMAD ALHASSOUN
    TI__Intellectual 2050 points
    Okay great I will close this post then. Just because it is closed does not mean you can no longer post here. So, if you have questions in regards to this in the future feel free to ask here! Otherwise start a new post.

    Glad I was able to help.

    -Joe