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bq25504 UV

Sunil Kumar
Prodigy 90 points

Good morning !

I am using bq25504 for ultra low power highly efficint energy harvesting system. I am using 2.3V,10F supercapacitor for storing the harvested energy from the solar panel. I just want to know that as we know the super capacitor with the higher voltages are not available due to technology limitation.I want to use the capacity of the supercapacitor until .7V. But the storage device under voltage protection range is from 2.2V to VBAT_OV. This means I can use only .1V capacity of the supercapcaitor??? please provide me the solution. I know that I can use two supercapacitor in series but then the individual supercapacitor will need to have  voltage balancing to prevent any cell from going into over-voltage and it will also increse the system complexity as well as the extra circuit means the power wastage and it is serious in the ultra low powered applications.

Can we configure the Under voltage UV .7 or .9V?

  • 0
    TI__Mastermind 31120 points

    This part is not really designed for a single cell super capacitor since the lowest UV is 2.2V and one would not get much energy out of a single super cap.

    The main issue is that below ~1.8V the part is in cold start which is very low efficiency.  This should be an abnormal case.  At VSTOR< 2.2V the BAT pin is disconnected from the VSTOR CAP and it allows the VSTOR cap to be charge fairly quickly from the cold start circuit getting back to the efficient boost converter.  The BAT FET has a body diode from VSTOR to BAT that will still charge the storage element if the storage element gets more than 350mV below VSTOR.

    There is one work around that may work if one has around 2.0V solar input (4 solar cells in series--> MMP = 2.0V and Open Ckt = 2.4V).  connect the super capacitor to VSTOR.  Connect the solar panel to an P-ch FET going to VSTOR for direct charge for voltages under 1.8V and connect the solar panel to Vin via a N-CH FET.  Drive the FETs with a signal that goes from low to high when VSTOR rises from below 1.8 to above 1.8V.  This basically bypasses cold start with a more efficient direct charge.

  • 0 in reply to Charles Mauney
    Expert 1790 points

    Why not use a 5V super cap such as Cooper Bussman PB-5R0H104-R ?

    It is not fair to criticize a lowly super cap for lack of energy:  some applications don't need much energy.  After all, TI is call it "ultra low power" device.

    Even a 2.5V super cap, can't you apply the max voltage of 2.5V and get 0.3 V of discharge?  Isn't that about 10% of the power stored?  Is it really a problem to use only 10% of the capacitor, as a storage element?  The tradeoff is: it should have a much longer life than a battery.