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Switching power source between coin cell and debug header automatically?

rswl88
Intellectual 385 points
Other Parts Discussed in Thread: CC2540

I am running my CC2540 directly off a coin cell battery.

I've noticed that the CC debugger has a 3.3V pin that can be used to power my system.

Is there a simple circuit that I can use to enable the system to switch to the cc debugger power source automatically when the debug header is connected?

  • 0
    TI__Guru 57865 points

    Hi,

    If you are referring to the CC2540 Keyfob you can short circuit R1 placeholder with a 0ohm resistor (or just some lead or scrap metal). By that you'll be able to power the keyfob from the debugger.

    Best Regards.

  • 0 in reply to J Lindh
    Intellectual 385 points

    What I am after is a way to do it automatically.  Is there any circuits that could be built to get around this?

    Currently, I am thinking of placing a diode at the 3V output from the coin cell, but I am afraid the voltage drop across the diode will be too high.

  • 0 in reply to rswl88
    Guru 31280 points

    Hi,

    You know, it's quite a challenge.

    May be this kind of question should be asked on the analogue forums, not the RF one.

    Don't forget that CC253x/4x are sleeping devices which means that they are switching 

    power, drawn current can be from 25mAmps to several uAmps, so you should be careful

    when placing an active part. You shouldn't be afraid of voltage drop, since CC254x can

    operate from 2.1Volts. So as long as the supply voltage is around 2.7 volts and voltage

    drop on diode doesn't exceed 0.5 volts. I recommend you to use schottky bariers, typically

    they have very low drop voltage ~0.2V

  • 0 in reply to Igor Sherer
    Genius 4125 points

    Schottkys are easiest but if you need full 3.0V from battery then you will need a NCH fet, NPN and schottky to switch the battery. Connect like this:

    1. Battery +ve to load +ve

    2. 3.3V debug to load +ve via Schottky 0.3V diode.

    2. NCH Drain to battery -ve

    2. NCH Source to GND

    3. NCH Gate connected with pull up resistor (4K7 etc) to battery +ve

    4. NPN transistor with collector connected to NCH gate, emitter to GND.

    5. Voltage divider from 3.3 debug to NPN base to give 0.6V when debug = 3.3V (this is important as you dont want the battery disabled before this voltage has reached 3.3V

    6. Large cap between circuit +ve and GND 10uF+