3.3v dc/dc, dual supply from Li-Ion batt. & USB

Discussion in 'General Electronics Chat' started by takao21203, Mar 19, 2013.

  1. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Requesting your engineering assistance for this problem.

    I want to use USB or Li-Ion battery together with a dc/dc converter IC.
    Output is 3.3V

    Made a prototype today, works fine. 1uH inductor + 4.7uF ceramic caps.

    I also want to use USB 5 volts. How can I do this?
    One way of course is a diode from the battery, but 0.2 to 0.3V loss even with a good schottky.

    Any idea how I can do this with a MOSFET or BJT (for instance BC327 PNP only drops 90mV)? And I only have N-channel digital MOSFETs here.

    -When the battery is switched ON, and USB is connected, the battery should disconnect.

    The dc/dc chip is started with a pushbutton, later this is turned off briefly with a microcontroller (2.2K pullup).
     
  2. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Also I am going to add a Li-Ion charger IC: The MCP73832.

    I have never used these components before.
     
  3. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    This is what I built in the meanwhile.

    It is a solution. I can start the circuit from either USB or battery, disconnect one of them, and it stays on. The battery voltage remains the same, means no charging current flows.

    It is hard to believe the voltage accross the transistor is 0.00 volts.

    Sacrifying 0.6V from the USB does not matter too much here.

    What is important is to have a constant voltage from the Li-Ion battery.
     
  4. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Changed the BJT to a p-CH MOSFET, since it was draining a few mA base current even with the DC/DC off.

    I was not able to figure out if this is due to the nature of the BJT circuit, or the fact the first chip I used was partially fried. It was drawing a few mA even when switched off.

    I figured out about this after I had the MOSFET installed already with the DC/DC chip exchanged (was quite a fiddling to resolder a new one).

    Having a diode from the USB is still a minor ridicule since power is wastet, but for the time being, does not matter.

    What if it is a USB host, and 1/2 hour more of battery operation is crucial? Such mobile devices tend to fail from empty battery exactly when you need them the most, and can not recharge as well.

    The DC/DC chip used costs about $1.50, + coil 50 cent + caps 10 cent. Running at 3 MHz, it even needs a 1uF cap at the enable input.

    I am not sure if to add a small 16F USB PIC (which I maybe want to use for a 16f54 programmer), or a PIC32 + serial TFT display. Maybe both things.

    Do you have any suggestions how I could get rid of the blocking diode from USB as well? If I use a MOSFET here too they should be locked against each othet. There are even special ICs for that.

    As of now, a partial solution was archieved, which is sufficient for the time being.

    The datasheet for the MOSFET is wrong actually, source and drain swapped.
     
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