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Buck/boost choice for powering the CC2650 with 3 x AAAs

Evan Davey
Intellectual 915 points
Other Parts Discussed in Thread: CC2650, TPS61221, TPS62740, TPS22910A, TPS709

Hello,

We need to power a CC2650 from 3xAAA batteries (due to the enclosure choice over which we have no control), so a power range of 2.7-4.5V.  Additionally we would like to support 3V peripherals (e.g. a blue LED).    Are there any recommendations for an appropriate buck/boost convertor for the low power / RF?  Something like the TPS61221?  Would we buck/boost to 3.3V then use the internal regulation of the CC2650 or give it separate regulation with something like the TPS62740 (meaning potential additional complications like logic translators). 

We have considered dropping 3.3V peripheral support and using 3xAAA 1.2V rechargeables for 2.7-3.6V (but then need over-voltage protection in case alkalines are inserted).  Or using only two of the batteries for 1.8-3V with the 3rd inserted as a dummy (but this seems wasteful).

What is the best reverse polarity scheme?  The TI design's appear to rely on the TPS22910A.

What is the best way to monitor battery voltage when operating on the regulator (assume we lose the internal battery monitor support)?

Any help appreciated

Cheers,

Evan

  • 0
    TI__Guru** 106705 points
    Hi Evan,

    I would buck-boost to 3.3 V and run the CC2650 off that. No need for TPS62740. Or even simpler, if you drop the blue LED, buck down to the minimum level you would require for other LEDs. Going down to 1.8 V would allow you to configure the CC2650 in "external regulator mode" which would simplify the design. In that case I would of course recommend the TPS62740.

    For reverse polarity protection I would just go with an N-channel MOSFET. Drain on the negative battery terminal, source is your circuit ground node and the gate is connected to the positive battery supply.

    For battery monitoring you need a resistive divider connected to the ADC. Even better would be to make this switchable. One of TPS62740 siblings has a switchable supply output exactly for this purpose.

    For more advice on specific regulator I would recommend to post on the Power Management forum.

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    Intellectual 915 points
    Hi Fredrik,
    I was thinking of supporting 3xstandard AAAs using an LDO like TPS709 then feeding that into the CC2650 internal regulator. Any concerns with that approach?
    Cheers,
    Evan
  • 0 in reply to Evan Davey
    TI__Guru** 106705 points
    Hi Evan,

    That should work fine!

    Cheers
  • 0 in reply to Fredrik K
    Intellectual 915 points
    Thanks. We're looking for the path of least resistance for this revision but might look to add buck/boost later if this project phase is successful.