Unbalanced Sinking/Sourcing of PICs 2

[mrkenneth]

I know this may be a very rudimentary question, but please bear with me.

Can the amount of current going into a microcontroller (more specifically, a Microchip PIC16) be different than the amount of current leaving it?

I would like to connect the common anode of a LED display to the positive rail, and connect the cathodes to different pins of the PIC (through resistors). The LEDs are directly connected to around 2V, while the PIC is powered by 5V (stepped up by a DC/DC converter). So, the PIC is sinking a maximum of 200 mA (within the limits) when the LEDs are powered on (pins low).

Is this possible? The reason is that so that I can avoid requiring a high-current step-up DC/DC converter to power the LEDs.

Thank you in advance for any insight!

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[itsmoked]

Yes as long as you don't violate the PIC's current restraints which are a total current limit and port limits.

Make sure the 2 volts are enough to run the LEDs.

You may also have a problem because as you force the PIC to sink more and more current its sinking output pin voltage will rise as specified in the specs. Another-words the PIC pin may rise to a volt while the PIC is sinking 200mA so your 2V head may no longer light the LEDs. Most of us would use external transistors if we were going to be sinking 200mA.

 

[MrBananas]

The reason for the voltage rise on the output pin is due to the drain-source on-resistance of the PIC output transistors.

You will have the same problem with external transistors as well unless you either get logic level FETs with low on-resistance or perhaps put some BJTs in saturation. 2V doesn't give much margin on the LEDs. Don't forget to do current limiting if you use external transistors.

 

[itsmoked]

MrBananas; it is also the wire resistance of the two(?) wire bond wires that hook the processor to ground. If you use extrenal transistors you are only talking VceSat = 0.2V .

 

[mrkenneth]

Thanks for all of the information.

The current will probably be much less than 200 mA, but I will have to check the brightness of the LEDs first. Each of the pins will sink less than 25 mA, so I don't think I will use transistors. (Don't want to add too much complexity to the project.)

2V is going to be the minimum voltage. I am guessing that the voltage will hover between 2.5V and 3.0V, so hopefully the LEDs will light up most of the time. ;-) There will be a resistor array to keep the current to the LEDs constant. (This is just a personal project, so reliability is not very important. )

Thanks again!

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