On Wed, 13 Oct 1999 09:42:41 -0300 Andres Tarzia <ATarzia@SMART.COM.AR>
writes:
>Hi all,
>
>Most PICs are rated to being able to source up to 20mA and to sink up
>to
>25mA at any pin. I understand that this is an Absolute Maximum Rating
>that
>one cannot stress.
>
>Now, keeping into the safe side, say that I want to source 15mA from
>ALL
>output pins of a 16F84. Having up to 13 outputs, this would mean a
>total of
>nearly 200mA out of the chip. The situation worsens if I choose to use
>a
>16C73, for example, with 22 output pins ALL sourcing 15mA for a total
>of
>330mA. For a 16C74, with 33 output pins, you go into the 1/2 Amp
>range...
>
>Ok, so far so good, one cannot do that. So the question is: How do I
>compute
>the maximum current a given chip can source at a time? I need to drive
>several charges at (possibly) the same time and I am afraid of
>overloading
>or overheating the PIC. I guess that if I find that it is too much for
>the
>PIC I'd have to use transistors for driving my output charges.
>

        I ran into this when I wanted to drive four 7-segment LED
displays with a 16c74 with perhaps 20mA per segment.  Each output could
do it, but we went over the total current capability of the chip.  I'm
looking at DS30390D page 195.  It says the maximum current into the Vdd
pin is 250mA and the maximum current out of the Vss pin is 300mA.  In
addition, the maximum current sourced or sunk by a port is listed as
200mA.
        Not enought to drive all the segments simultaneously, so I went
to a smaller PIC (the 16c622a) and an Allegro UCN5832A driver (serial in
shift register driving a 32 bit latch driving 32 open collector outputs,
each of which can sink 100 to 200mA).

Harold


Harold Hallikainen
harold@hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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