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Battery Monitor Design Help

James Coleman
Intellectual 355 points

Other Parts Discussed in Thread: CC2541, TPS62730

Hello,

I am using the CC2541 in a battery powered application.  One of my design requirements is to monitor the battery voltage and indicate to the system when the battery voltage is low.  I need help with the best design method for implementing this feature.  I believe my options are to divide down the battery voltage and input this value into the ADC or input the battery voltage into the op amp input and then connect the op amp output to the ADC.  Has anyone done this and if so do you have an application circuit that you can recommend?  If not  can anyone provide suggestions on the best method to implement this feature.

Thanks,

James

  • 0
    Guru 18625 points

    Hi James,

    there is a battery detector here:

    http://web.engr.oregonstate.edu/~traylor/ece112/lectures/comparator_ckts.pdf

    Opamp and ADC samples are available in keyfobdemo project, as far as I remember.

  • 0 in reply to kazola
    Genius 4125 points

    The ADC can be configured to use an internal 1.15V reference and internally measure 1/3 VDD. The battery voltage can therefore be monitored with no external components assuming your VBATT is VDD. If not you will need an external divider connected to an ADC pin with either an external or aforementioned internal reference. Note that your divider should use high resistor values otherwise it will drain your battery. If every uA counts then connect the ground leg of your divider to a high Z I/O and only take it low when taking a measurement. 

  • 0 in reply to Eng351
    Intellectual 355 points

    I am using the TI TPS62730 to supply power to the CC2541 VDD pins.  The battery is used to supply power to the TPS62730.  So I believe what you are telling me is that since I am not using the battery to supply power directly to the C2541 VDD pins, then I will have to route a connection from the battery to a voltage divider network and then supply the divider network voltage as an input to the CC2541's ADC.  Is this the correct assumption?

  • 0 in reply to James Coleman
    TI__Intellectual 1600 points

    That's right, if the voltage you want to monitor does not connect to the supply then you will need to route it externally and divided down as you say.  Keep in mind the ADC does have a finite impedance so very large resistors will likely impact your accuracy.

    BK

  • 0 in reply to BugKing
    TI__Genius 12545 points

    BK has the right idea.

  • 0 in reply to Chatto
    Intellectual 355 points

    I have an additional question in regards to supplying power to the CC2541.  Right now my design is using the TPS72630 voltage regulator to supply power to the CC2541's power rails instead of using the battery to supply power to the CC2541.  It is my understanding that using the regulator to supply power consumes less power because I am powering the device at 2.1 volts instead of 3.0 volts.  For my application, I have a requirement that my device has to have a battery lifetime of 6 months to 1 year.  Is my assumption correct that using the regulator to supply the power versus the battery correct??

  • 0 in reply to James Coleman
    TI__Intellectual 1600 points

    Hi,

    You are correct that using the TPS62730 is the best way to save current/power.  The amount of current you save is somewhat dependent on how often the device is going active and also changes as the battery supply voltage changes.  There is a great app note about using the TPS62730 by Abhishek Chattopadhyay, I'd recommend checking it out.

    http://www.ti.com/lit/an/swra365b/swra365b.pdf

     

    BK

  • 0 in reply to BugKing
    Intellectual 355 points

    BugKing,

    Thanks for your help.  One additional question.  Since I will be monitoring the 3V battery voltage, any recommendations on the resistor values to use as the divider circuit to provide the analog voltage to the ADC input?  Is all that is required is a simple divider network?

  • 0 in reply to James Coleman
    TI__Intellectual 1600 points

    Hi,

    You'll need a capacitor as well and possibly a switch if you want to limit the current draw when the ADC is not being used.  You'll find a good post with attached doc from TER here:  http://e2e.ti.com/support/low_power_rf/f/155/t/129638.aspx#464388

    The 2540 adc is the same as the 2530.

    BK

     

  • 0 in reply to BugKing
    Intellectual 355 points

    Thanks BugKing.  This is exactly what I was looking for.  All of the information you provided has been very helpful.