current drive of IO pins

[stw]

Hi All,
I need to drive a 75ma load on existing hardware. Has anyone ever used IO pins ganged together to increase the current source/sink capabilities. for example

output_b(0x0f); //set low 4 bits of PORTB

and tie them together to essentially create a 5V source with higher current capability.

Its a reach I know. Just wondering if its possible.
Thanks

[Ttelmah]

Lots of caveats.
You certainly can do this to some extent. However, remember that when delivering it's maximum output current, a pin is only warranted to give the minimum Voh voltage. So probably something like 4.2v. Then there is a separate specification for the maximum output on _all_ the ports, and on some chips, maxima for the individual ports. There is also the maximum power dissipation figure for the chip, and if each pin is dropping 0.7 to 0.8v, with a total of 80mA drawn, something over 1/4W will be being dissipated by the drivers here, and once you add the chip's output etc., you may start to run into problems here....

Best Wishes

[SherpaDoug]

I have the datasheet for the 16C5X PICs on hand. It lists the sourcing limits as 20ma per pin, 40ma per 8 bit port, and 50ma into the VDD pin. So paralleling more than two pins is fruitless. Sinking currents are 25/50/150ma. Your chip may be a little different, but probably not much.
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[stw]

Thanks for the reply. As i said I thought it might be a reach to do something like this. Of course I dont have hardware on the board for this function and was trying to find an easy way out.

[rnielsen]

You should use an external driver for higher current requirements. Something like a ULN2003 can deliver 500mA per channel.

Ronald