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BQ25504 simplified resistor network for non-battery, capacitor storage.

lloyd konneker
Expert 1790 points
Other Parts Discussed in Thread: BQ25504

I am trying to reduce the number of resistors in a design using BQ25504.

The design uses a super cap, I don't need battery management.  There is no undervoltage or overvoltage danger to the super cap.  When the super cap is charged to VBAT_HYST (the increasing threshold), VBAT_OK will assert high and switch my load.  That will drain the cap until it reaches VBAT_OK.  The load is non-electronic, just a motor.

I am thinking I can use just the three resistor network for BAT_OK and BAT_OK_HYST (and omit the resistor networks for VBAT_OV and UV.)   From my reading of the datasheet, I can tie the VBAT_OV pin to the OK_HYST pin (also tied to the same node of the three resistor network) and tie VBAT_UV pin to the OK_PROG pin.

Should that work?

  • 0
    Expert 1790 points

    I experimented with this design.  It seems to work.   A report is here.

  • 0 in reply to lloyd konneker
    TI__Guru**** 166996 points

    Another option would be to tie VBAT_UV pin to VSTOR and the VBAT_OV pin to GND and tie VSTOR and VBAT together also.

    As a safety feature, you might want to use the VBAT_OV resistors so that the super cap doesn't overcharge if the load switch doesn't drain it as expected.

  • 0 in reply to Jeff F
    Expert 1790 points

    Thanks.

    Your option might be easier to route on the PCB.  (Which BTW is single sided except for the trace in question.  I hope to upload a free Fritzing PCB soon.)  Now I need to understand why your option works and whether that is documented in the datasheet and I overlooked it.

    In my application, the supercap is rated 5V with surge to 6V, and my understanding the BQ25504 will put a max of 5.5V on VBAT.   As the datasheet says, the OV is failsafe.  My application is not that critical, a toy, but where weight is critical: it is not so much the weight of the extra resistors as the weight of the PCB underneath them!

    You must have been on vacation, TI is catching flack for lack of responses.  You didn't read the report, power harvesting is a $1B market.  Or you did read the report and you are working on a big design win instead of toys. ;)

  • 0 in reply to lloyd konneker
    TI__Guru**** 166996 points

    The 5.5V max rating is just that, a rating.  The part does not have over voltage protection circuitry other than the VBAT_OV.  A risk for all boost converters is that their output could run away without feedback regulation. 

    The datasheet is a bit lacking for non-battery charging applications.

  • 0 in reply to Jeff F
    Expert 1790 points

    I noticed there is a recent datasheet revision.

    So your suggestion, including tieing VBAT_OV to ground lets the converter run unregulated?  That is non-intuitive, I expected to tie VBAT_OV to some high voltage, not 0 volts.

    Yes, I am confusing the specs.  The max rating on VIN is 5.5V, and recommended operating max for VIN is 3V.  As you say, the converter could boost that to higher voltages.

    My understanding is that the converter is pulsing with high voltage (much above VBAT_OV, say much above 5V?) spikes.  The CSTOR and  CFLTR (documented in the revised datasheet) capacitor damp those spikes.  CSTOR and CHVR should be rated to 10V as the EVM BOM specifies.  But the supercap only sees a lower average voltage, and the spikes are not seen to exceed the max surge voltage for a supercap???

  • 0 in reply to lloyd konneker
    TI__Guru**** 166996 points

    A boost converter dumps pulses of current into the output capacitor.   There may be small spikes on the output capacitor but not much above the regulated voltage.