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BQ25504: Question re BQ25504 intended system use

Tim Netherwood
Intellectual 290 points
Part Number: BQ25504
I have a design using a solar panel into a supercapcitor storage bank 5 Farad, whose leakage current (70uA) is equal to the logic and actuator total current. (using your spread sheet)
So I am considering the BQ25504 to squeeze out the last drops of sun-power, but as I don't have a battery how should I use the Vbat and Vstor?
  • should I put a smaller capacitor/supercapacitor on Vstor and the main supercapacitor on Vbat (or ignore Vbat)?
Also as I am using the output from the solar panel to determine daylight/darkness transitions I really need the first 1V of output from the solar panel to not be pulled-up by the cold-start circuit, so I am considering keeping in the blocking diode from the solar panel, maybe two diodes to ensure 1V forward drop guard band.
  • would this be acceptable use?
I am doing trials with either 3V/315mA or 5V/190mA solar panels (both 72cm2 @ 0.9W). But typically in very low day-light levels, 200Lux?
3V 315ma
I'm grateful for any advice at this stage.
  • 0
    TI__Guru*** 143980 points

    Hi Tim,

    The internal PFET between VSTOR and VBAT is only used to protect against under-voltage from a load on VSTOR.  Most super capacitors do not need protection against undervoltage so you can short VSTOR to VBAT.  The 0.01-0.1uF capacitor on VSTOR is required and needs to placed close to IC between VSTOR and VSS (on NC tied to VSS) pins.  I recommend keeping the 4.7uF capacitor as well.

    I don't understand your diodes to avoid cold start.  With such strong solar panels, I suggest using external PFET1 to connect the panel directly to the super cap and then PFET2 that turns on to connect the solar panel to VINDC.  PFET1 would turn off first before PFET2 turns on.  You could use the VBAT_OK comparator to control the PFETs or add a second low IQ comparator from the TPS39 family. This would avoid cold start.

    Keep in mind this is a boost only charger so the input MPP voltage must be below the VBAT_OK voltage.  If you use a 5V panel with 80%*5V=4.2V MPP then VBAT_OK must be >4.2V.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 290 points

    Thanks Jeff, very clear and I'm making progress with a BQ25504 eval board....... and the resistor calculator spreadsheet SLURAQ1. I think I can cope with the cold-start now, but the ideas about the PFETs very good should it be needed :-).

    On the last point you made about the MPP setting, my panel is 5V o/c but unlikely to ever be illuminated that high (nor even inclined to the sun) in British shady conditions where the o/c measured today is max is 4.4 then: but I guess i should still aim for >4.2V for the MPP as that doesn't alter.

    every blessing
    Tim

  • 0 in reply to Tim Netherwood
    TI__Guru*** 143980 points

    Hi Tim,

    The VBAT_OV value must set higher than the max MPP point.  Otherwise, the charger turns on a pull down FET at VIN_DC in an attempt to prevent from overvoltaging the storage element.  If the source it not HiZ, the FET may not be able to pull it down enough and so the VIN_DC - 2*Vdiode will appear at the storage element.

    Regards,

    Jeff