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BQ25570: Charging a super cap and li battery using solar cell in indoor application where it outputs 2.3V @ 0.002mA.

Sumeet Patil39
Prodigy 30 points
Part Number: BQ25570
Other Parts Discussed in Thread: BQ25505, TPL5100

Hello Ti,

We have a use case where our chosen solar panel outputs 2.25V @ 0.003mA in an environment where the indoor lighting is used (Normal Tubelights). This is a very critical application, and we need to crack this.

Can TI provide us reference design or some help with BQ25570 for this application? We are just on the threshold of minimum cold start energy required, so that makes me nervous. Please revert back at earliest.

Regards,

Sumeet Patil

  • 0
    TI__Guru**** 155346 points
    Sumeet,

    Unfortunately, we are in the process of issuing a PCN to increase the minimum cold start voltage to slightly more than 600mV. Will that work for your application? Is the indoor lighting insufficient to provide a higher voltage from the solar panel?
  • 0 in reply to Jeff F
    Prodigy 30 points
    Hi, We are getting 2.28-2.3 Volts at 2.3uA in real life environment. The solar cell VMP is 4.2V, but the lighting condition is slightly dim. I measured the light intensity at 172 Lux. So my question is can this voltage and current will allow BQ25570 exit cold start. Please revert back at earliest, as we have to finalize the design in a day or two.
  • 0 in reply to Sumeet Patil39
    TI__Guru**** 155346 points

    Sumeet,

    Cold start operation is only when VSTOR < VSTOR_CHGEN (1.8V) and VIN_DC voltage is below the VIN(CS) threshold which is increasing to 600mV.  If your lowest voltage from the solar cell never drops to 600mV, I do not see an issue. 

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi Jeff,

    Thank you very for the reply. Are there any readily available reference designs. Can we get our design reviewed on a very short notice?

    I am confused between BQ25505 and BQ25570. Which IC should I choose for my application?

    Ideally, our application would wake up in every minute and would transmit a very short message and go to sleep so for that I am using TPL5100 timer with programmable delay.

    However, I am having a small solar cell in the design which will slowly charge a battery of 500 mAH capacity. I am planning to have a secondary bigger capacity battery as a buffer.

    Now I want the solar cell to charge and keep smaller battery always charged and my system should always use power from this battery and use buffer one only when there is no ample charge on the battery connected to energy harvester.

    Can you please give us reference design or any input on parts that I can use.

    The solar cell is one used above, it will not change. 

    2) 2.3V @ 2.9 uA is close to 6.67 uW, so can that low of power be extracted from the solar cell. Since it is a boost converter, I am assuming it will boost 2.3V to say 4.1V so the current will be even less in that scenario. Please advice.

    I look forward to hearing back from you soon.

    Warm Regards,

    Sumeet Patil

     

  • 0 in reply to Sumeet Patil39
    TI__Guru**** 155346 points

    Sumeet,

    If the secondary battery is non-rechargeable, then I recommend the bq25505. It will auto swith external FETs between the primary and secondary batteries. The EVM is at http://www.ti.com/tool/BQ25505EVM-218.  Reference designs using the bq25505 are at http://www.ti.com/reference-designs/index.html#search?keyword=bq25505.