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Voltage Control of TPS61202EVM, SuperCap voltage input

Aaron Visser
Prodigy 140 points
Other Parts Discussed in Thread: BQ25505, TPS61202

Basically I'm designing an thermal energy harvesting setup with supercaps. I've done a bunch of different configs, but lets say for this I'm using two 1F 2.7V in series, so that I get 0.5F  5.4V. 

I currently am using two TI evms, and they work great.   I have the bq25505eevm that collects the energy from peltiers and the TPS61202EVM that boosts/regulates the collected power from the supercaps to a constant 5V.

However. I don't want the 61202 to turn on and start draining the supercaps until the the caps are at a certain voltage.  say 4.7 or 5V or somevalue. and then I want the 61202 to turn off when the caps reach an output voltage of 1.3V or something. Honestly, the turn off point doesn't matter because eventually the 61202 doesn't output enough voltage anyways.


The problem is the 61202 turns on when the supercap output voltage is at 0.5V.  But then what happens is that I can never get enough voltage into the caps to do anything useful....

Any suggestions would be amazing.

Thanks!

Aaron Visser 

  • 0
    TI__Guru*** 143950 points

    The bq25505 boosting charger can be configured with VBAT_OV = 5.0V.  It will continue to charge the super caps until they reach 5.0V.  Then you could use the VBAT_OK signal to turn on/off your system.

    If you still want to use the TPS61202 following the bq25505, you could set the VBAT_OV and VBAT_OK voltages >2.1V and then use VBAT_OK to turn on the EN of the TPS61202.  Keep in mind the TPS61202 turns on its powerFET and pulls current from the super caps up to its current limit.  It doesn not have a MPPT (input voltage regulation) feature to prevent collapsing its input source. 

     

     

  • 0 in reply to Jeff F
    Prodigy 140 points

    Once the supercaps are up to 5V, I'm very happy with the tsp61202 to take as much current as it wants.  My tests have been great so far.  All that I need is to only turn on when the caps are at a level I'm happy with.

    How would you suggest I wire the VBAT_OK signal to manage the EN of the TSP61202... 

    (Thanks for your quick reply!!!)

  • 0 in reply to Aaron Visser
    TI__Guru*** 143950 points

    You have several options:

    1.  Set bq25505's VBAT_OK to go high when supercap (tied to VBAT of bq25505 I assume) voltage goes above a certain voltage > VBAT_UV = 2.1V and connect to TPS61202 EN pin.  When VBAT_OK is low, TPS61202 is completely disabled.

    2.  Use TPS61202's UVLO pin, with resistor divider from VIN, to partially disable TPS61202 when VIN drops below a certain voltage (>2.1V).  TPS61202 will turn on faster but will consume a bit more current in the partially disabled state.

     

  • 0 in reply to Jeff F
    Prodigy 140 points

    Beautiful. Time to wire this bad boy up. :)

  • 0 in reply to Aaron Visser
    TI__Guru*** 143950 points

    The VBAT_UV feature is really for batteries to prevent overdischarge.  Most supercaps don't have an undervoltage requirement.  So you can likely tie VSTOR=VBAT and lower that 2.1V threshold down to 1.9V.  Since the bq25505 goes into cold start when VSTOR < 1.9V, you don't want to collapse the battery below that voltage unless you have to.

  • 0 in reply to Jeff F
    Prodigy 140 points

    Can I set the OK_Hyst and OK_Prog and VBAT_OV? There is little to no documentation about setting OK_Prog.   

    I see that OK_Prog has to be > VBAT_UV which is 2.1.    

    So in my ideal case I'd put VBAT_OV to 5.4V, OK_Hyst to 5V (or even 5.4V) and OK_Prog to 2.11

  • 0 in reply to Aaron Visser
    Prodigy 140 points

    Is OK_Prog the same as VBAT_OK ?

  • 0 in reply to Aaron Visser
    TI__Guru*** 143950 points

    The link below is to a spreadsheet that will help you size the resistors for the levels below:  http://www.ti.com/lit/zip/sluc484

    I am still confused about why you need the TPS61202 if you are planning to set VBAT_OV > 5.0V.  If you use the TPS61202, you could turn it on using VBAT_OK as soon as the bq25505 has charged the supercaps >1.9V, of source there is the risk that it will pull a slug a current that will drop VBAT=VSTOR below 1.9V, forcing it into cold start so you probably want the OK_PROG_HYST threshold be higher. 

    OK_PROG_HYST is the threshold that takes VBAT_OK high as VSTOR is rising.  If you connect VBAT=VSTOR, thereby disabling the VBAT_UV function, you can set OK_PROG down to 1.9V.

  • 0 in reply to Jeff F
    Prodigy 140 points

    I need the TPS61202 because I'm powering an Arduino and a whole bunch of other stuff off the supercaps., and I want to have a constant 5V signal. Based on some tests, it seems to be the best I've found so far in terms of performance. But of course there's so many chips out there. Lol.

    If I don't have the TPS61202, then the linear discharge of the supercaps makes the voltage quickly unusable for the arduino. (or whatever other MicroProcessor I want to use.

    If I turned the 61202 on at >1.9V, then the arduino almost instantly drains the supercaps.  My tests have shown that if I can fully charge the supercap (~1 minute of heat applied to the peltiers), then I can get enough juice out of the caps to adequately power my system for the right amount of time.  

    The problem right now with the peltiers, is that I only have a limited amount of time to get the most power out, since they are passively cooled (don't have enough energy to run a fan or anything). 

    totally willing to expand on my usecase.... :)

  • 0 in reply to Jeff F
    Prodigy 140 points

    Thanks for the spreadsheet link (I had found it.) Was just a tad confused regarding how VBAT_OK and OK_PROG are basically the same thing. :)

    The info graphic is REALLY helpful.

  • 0 in reply to Aaron Visser
    TI__Guru*** 143950 points

    VBAT_OK is the name of the feature and the output signal.  It is controlled by the OK_PROG and OK_PROG_HYST thresholds. 

    I understand your need now.  So, basically, if you knew the amount of energy required for your system to do its thing, as well as the input source power profile, for a given time period, you could set OK_PROG_HYST lower than the max and the get enough energy from the caps before they deplete.

    Which peltiers are you using?

  • 0 in reply to Jeff F
    Prodigy 140 points

    Exactly.  Right now I'm just getting a prototype going to pass on to my production team (I'm the skunkworks).  I have a whole bunch of peltiers in series. Some Lairds, one from the enocean dev kit i ripped apart, would getting you model numbers help?  Ideally I'd have a custom peltier made out of a mould, or lots of 1x1mm peltiers all linked in series... to wrap around my heat source.

  • 0 in reply to Jeff F
    Prodigy 140 points

    Jeff,

    Just wanted to follow up on this.  I got the replacement surface mounted resistors and used the spreadsheet to make the voltage adjustments. And it works perfectly.  Absolutely fabulously.

    Thanks for your quick responses and suggestions. :)

    Aaron