Battery Powered PIC Design Help

Discussion in 'The Projects Forum' started by Robert1, May 3, 2009.

  1. Robert1

    Thread Starter New Member

    Apr 22, 2009
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    0
    Wow, what a great board. Thanks to all that are responsible and to all the contributors. I have used it to find information to help me design a battery powered PIC design but had a few questions as to how it all fits together.
    For some reason, I thought this design would be much simpler than it turned out to be. I “just” wanted to power a simple PIC microcontroller circuit with either an AC wall mount adapter or a rechargeable battery. Whenever the adapter is plugged in, it will slow charge the battery (C/10) for 14 hours and then go to a trickle charge (C/10 for 1 second, then turn off for 9 seconds, repeat) to maintain the battery.
    A simple pushbutton will power on and off the device. When the device is powered off (and battery driven) the microcontroller would remove the power to the rest of the circuit and then put itself into sleep mode. I also use a 3.3 volt supervisor that would detect when the batteries are sufficiently drained and turn off the circuit to avoid over-discharging and damaging the batteries.
    The only real guidelines I used were to keep the circuitry as simple (and inexpensive) as possible with parts that are abundant and readily available (Mouser or Digikey).
    This is the circuit I came up with. Is there a simpler way of doing this or am I way off base? If this looks close to being correct, I had a few specific questions on the design.
    1) Do I really need the 548 transistor to turn off the LM317? If disabling the 317 is as simple as pulling the ADJ pin to ground (and there is very little current flow), it seems as though I could just tie the ADJ pin to the PIC output and control it directly. I would tri-state the pin to enable the regulator.
    2) I am relying on the “inherent diodes” of the 2N3906 to form the other side of the “diode-OR” so current does not leak back to the battery. Is this OK or is something going to smoke?
    3) Since the circuit is battery powered, I am trying to keep the current draw to a minimum. Do I really need to pull-up or -down the base of the transistors since this will draw battery current? If so, can I use a really large value like 1M since I really don’t care too much about the switching response time? Also, I am assuming it may be better to use external pull-up resistors on the PIC since the weak pull-up tolerances are so wide they much waste more power than external ones?
    Thanks so much for all the help and any input that can be provided.
     
  2. Robert1

    Thread Starter New Member

    Apr 22, 2009
    8
    0
    Here is the circuit design in JPEG format if that helps
     
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