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Noise on Vcc line

Discussion in 'Electronic Basics' started by Abe, Oct 16, 2003.

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  1. Abe

    Abe Guest

    Hi,
    I am trying to construct a robot with LMD18245 from National
    Semiconductor as motor drivers and a DS89C420 as the controller. Since
    7 of the LMDs are being used to run 7 different motors, I need to know
    the following:
    1. Do I need to add capacitors across each LMD18245's Vcc and GND (as
    suggested in the datasheet) or do I just add one big cap between the
    Vcc and GND of the PCB which houses all the LMD18245s? The
    microcontroller PCB is separate but gets
    power from the same 12V SLA battery source.
    2. I presume that there will be considerable noise on the 12V Vcc line
    anyways. Now will this affect the 7805 voltage regulator o/p which
    will power the microcontroller?
    3. If answer to Q2 is yes, then will a 15V zener (Cathode connected to
    Vcc) and a 100uF cap across the Vcc and gnd i/ps to the 7805 voltage
    regulator, do the required job of reducing the noise? I am assuming
    that the zener eliminates all spikes above 15V and the cap filters the
    voltage dips below 12V due to the LMD 18245s.

    Thanks in advance
    Abhijit Karnik
    INDIA
     
  2. Steve

    Steve Guest

    This would be best practice. "One big cap" won't do as good a job
    smoothing fast transients (high frequency)... also, the closer you can
    put the caps to the individual ICs the better. If you really wanted
    to drop one of the caps, drop the 100uF and keep the 1uF.
    Not if it's appropriately filtered with a 0.1uF bypass cap on the
    input and output pins. (also see below)
    Well, for my money, forget the zener and just go with a big cap.
    Remember the 0.1uF cap as well and you should be fine.

    Sounds like you might be drawing a lot of current from the 7805, have
    you checked the power calculations?

    nifty
     
  3. Abe

    Abe Guest

    Sounds like you might be drawing a lot of current from the 7805, have

    Not exactly. THe 7805 sources only the uC and some latches.
     
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