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power monitor/detector hysteresis for solar panels

Digi-Man
Prodigy 40 points
Other Parts Discussed in Thread: TPS3808G01DBVEVM, TPS3808

My question has to do with monitoring and control of a voltage rail from a solar panel.  I have a circuit that monitors the incoming voltage from the panel and when it reaches 3.0V it turns on a voltage regulator(TPS78033) for the rest of the circuit.  My problem occurs when in low light the voltage rail will rise and turn on the regulator only to quickly shut off when the high current draw forces the rail to droop below the threshold of the monitor and shut off the regulator.  This cycle is not enough time for the CC430 to start up and move to low power mode. So the circuit basically turns on for a short period then turns off while it charges back up to begin the cycle again.  A second thing to note here is that at low light the voltage rise may be very slow or have no rise for indoor panels.
So is there a monitor that will allow me to turn on a regulator at some preset voltage but also set a lower threshold that turns the circuit off. Basically adding some hysteresis to the monitor.  I would consider this a common issue with solar panels under low light situations, you see a voltage on the panel but there is no current to support the voltage.
I could use the regulated voltage plane to hold up the reset line for a bit longer with a feedback circuit but this adds components and does not give me the hysteresis that I am looking for with a small solar panel.  Idealy I would have the monitor on-threshold at 3.3V and off-threshold at 1.8-2.0V this should give the CC430 plenty of time to start up.  Any parts coming up on the roadmap that have this capability?

I have looked at slva360.pdf but I'm not sure how well this would work for solar panels(indoor/outdoor)

Thanks.

  • 0
    TI__Expert 8890 points

     

    I think your idea to use a separate supervisor -like the TPS3808 found in that application note, SLVA360 - is a good one.  I have used similar divices in this type of schmidt trigger arrangement to accomplish similar results.  It might be easier for you to order an Evaluation Module (TPS3808G01DBVEVM) off the web and jumper it into the circuit to see if you can make it work for your application (you will still need to add your own feedback resistor per slva360). 

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 40 points

    I am working on getting one of the evaluation boards to test out the circuit and see how effective it is with solar panels.  Typically with the solar panels there is only 100uA to work with and sometimes even less, so normal methods have not been good and with super low power I hate to shunt any down feedback resistors.  If it works out to be low enough powered then that seems to be the best approach so far.  Although I was hoping that TI would come up with an all in one solution that that would use a couple of setting resistors and a preset turn on threshold.  Do you know of anything coming that would work like that?

  • 0 in reply to Digi-Man
    TI__Expert 8890 points

     

    Unfortunately our roadmap does not have such a part but I have forwarded your idea to the product team.  Regarding the TPS3808 supervisor, note that the current to drive the sense pin is only 25nA max.  You could get away with an external resistor divider to that pin that draws 250nA or less depending on the accuracy you desire.

    Bill