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'Maximum current draw from a given PIC'
1999\10\13@083941 by

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.

Regards,
Andres Tarzia
Technology Consultant, SMART S.A.
e-mail: atarziasmart.com.ar

You wrote:
>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.

You have to check the maximum Power dissipation the device can handle.
Pdis=VDDx{IDD-SUM IOH} + SUM{(VDD-VOH)xIOH} + SUM(VOLxIOL)

Holp this help.

Marcelo Y.
m_yamamotouol.com.br

In addition to a current limit on individual I/O pins there are current
limits for banks of pins.  For the 16F84 they are given in section 10.0 of
the Datasheet.
Max current sunk by PORTA          80mA
Max current sourced by PORTA       50mA
Max current sunk by PORTB          150mA
Max current sourced by PORTB       100mA

These are "Absolute Maximum Ratings".

I don't have the 16C7x datasheet handy, but IIRC, they are specified in two
sets of banks.

There is also a spec for max power dissipation for the chip given which
covers overall current handling capability - 800 mW for the 16F84.

Hope this helps,
Jonathan

> {Original Message removed}
>Most PICs are rated to being able to source up to 20mA
>and to sink up to 25mA at any pin.

My understanding of the data sheets is that this would apply to any pin that is
not a supply pin, i.e. this is the maximum current you could put through the
protection diode.

>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 limitation here is what voltage drop you can stand across the output
transistor in the PIC. If you are driving an LED for example, you can compensate
for the transistor drop by reducing the value of the series resistor. The only
problem you then have is the thermal dissipation in the transistor inside the
PIC. With just one or two turned on, there is probably no problem. If the
possibility exists to have many turned on, then you will need to look at any
curves that may exist for the chosen chip, and calculate the total power
dissipation you could have, and relate that to the maximum for the chip. You
then get into a loop of checking if this could be a steady state situation, and
what the operational ambient temperature may be, or is it going to be a dynamic
situation e.g. operating a security door lock for say 5 seconds, in which case
the temperature rise may not be a problem (but you would be wise to check). For
the number of "high" current drives you want, it would probably be wise to have
an external driver chip such as a ULN2000 series chip (the one I know best is
ULN2007), but they do come in other flavours). each chip has several drivers
capable of high currents, with protection diodes for inductive spike
suppression.

Andres,

> 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...
...snip...
>So the question is: How do I compute
> the maximum current a given chip can source at a time?

There are also absolute max specs for each Port (or group of them),
and for Max current drawn from Vdd or sunk to Vss.

There is also an absolute max spec for power dissipation on the
chip, the data sheet shows how this is specified.

Make sure you have the complete data sheet (http://www.microchip.com).

Notice that the Output High Voltage and Low Voltages are specified
at a much lower than that maximum current (+8.5 mA, -3.0mA for
my PIC)  No guarantee, but I'm guessing that this specification point
is the designers' expected maximum for normal operation when
many Port pins are used.

>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.

Yes, and that's sometimes desirable, to protect the PIC pins from

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

On Wed, 13 Oct 1999 09:42:41 -0300 Andres Tarzia <ATarziaSMART.COM.AR>
writes:
{Quote hidden}

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
haroldhallikainen.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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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. ... 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?

You don't have to compute it - it's already in the spec sheet.  In the same
"absolute max ratings" section, there are:

Max output current sourced by a single I/O PORT 40mA
"    "          "  sinked       "       "       50mA
Max current out of Gnd pin                      150mA
Max current into V+ pin                         50mA

(remembering someone odd ideas about current flow direction, all the
current "sunk" by IO pins should be coming out of the Gnd pin...)

BillW

>>Most PICs are rated to being able to source up to 20mA
>>and to sink up to 25mA at any pin.
>
> My understanding of the data sheets is that this would apply to any pin that i
s
> not a supply pin, i.e. this is the maximum current you could put through the
> protection diode.

No, the protection diode doesn't enter into that.  The protection diode is
to protect against externally applied voltages above Vdd or below Vss.

The output sink and source limits are the maximum currents through the
output driver FETs.  If you exceed those, the voltage outputs will not
be met, and the part may malfunction.

Note that in addition to the per pin source and sink limits, there are also
per-port limits.  You can't source or sink 20 mA out of ALL of the pins.

Thanks to all who took the time to answer.

I'll go for output transistors or maybe some IC buffer for protecting the
PIC.

Regards,
Andres Tarzia
Technology Consultant, SMART S.A.
e-mail: atarziasmart.com.ar

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