power regulator choice

Discussion in 'General Electronics Chat' started by dcd528, Sep 3, 2011.

  1. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    31
    1
    I am looking for the smallest voltage regulator I can find that will do the job. I am powering a microchip 16f690 chip. My battery is 9v 350mAh. The chip has a supply voltage requirement of approx 3 to 5.5 v. the battery runs a load and is switched by a power transistor.
    I am looking at a 5v TO-92 package which has a max output of 140 mA. How do I determine the maximum input current? The data sheet lists max Vin (35v) but doesn't mention max input current.
    The voltage regulator is jameco Part no. 51182
    thanks
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    For that regulator 78L05, the smallest part would be the SO-8 pkg in terms of the height profile, but in terms of board space and ease of assembly, the TO-92 pkg. is best. There are smaller parts, but they will be surface mount parts.

    PIC 16F690 DataSheet

    A couple others to consider:
    LM2936
    LP2980
     
    Last edited: Sep 3, 2011
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    While the 78L05 is convenient, there is a fixed overhead "tax" i.e. quiescent current of ~5mA, which is considerable for a battery-powered device - along with the ~2v or more dropout (minimum voltage difference from IN to OUT). Input and output caps are required to prevent oscillation. The LM2936 has a far lower dropout and quiescent current requirement.

    You don't say what your output current requirement is. Linear regulators are very inefficient; basically Vout/Vin. Switching regulators can be very efficient, but are more expensive, complex, and require more space.
     
  4. ke5nnt

    Active Member

    Mar 1, 2009
    384
    15
    I believe max input current would be determined by your output current. If I remember right, Kirchoff's law states that at any node in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node, or:

    \frac{n}{\frac{\Sigma}{k=1}} I_k = 0

    For a regulator, input current should equal output current, but be mindful of the max power rating. In a battery operated circuit, you don't want a lot of heat waste.
     
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