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IC Power source / sink limits

Hi,

I'm a software developer dabbling in hardware for the first time, building a device controlled by a PIC microcontroller, specifically a

16F914:

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This IC is described as "high power, suitable for direct connection of LEDs", which is mostly what I need. However, my LEDs are 20mA ones, and the spec sheet says that the maximum power source capability of the chip is 200mA. Presumably this means I can only drive ten such LEDs, since there are situations in which they must all be on simultaneously. Unfortunately I need each chip to run up to 16 LEDs, plus a small number of other things (logic connections to other ICs, pushbuttons, etc).

I have thought of two solutions to this - or possibly three if we include the option of just not worrying about it and assuming the specs have a large enough margin (not something I would want to do without knowledgeable advice). The other two are:

  1. Since the chip has both a 200mA source capacity and 200mA sink, connect half the LEDs with their positive sides to the chip and drive the pins high to light them, and the others the other way round and drive the pins low. I have no idea if this is a sensible proposal, but I can't immediately see a problem. Maybe as a software engineer I see things too abstractly.

  1. Pulse the LEDs so that only half of them are on at any given time, so quickly that the flicker will not be noticeable. I don't entirely like this idea, partly because it will mean the LEDs are dimmer and partly because I'm wary of introducing high-frequency noise into the circuit.

Any advice would be welcome.

Pete

Reply to
Pete Verdon
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Why not using an LED driver IC? In that way, the driver IC is responsible for providing the necessary current and not the uController

Regards GM

Reply to
GM

Because I didn't know that such things existed :-) ?

At this point, I already have the parts for the project and can't reasonably get my hands on others. We were also operating on a very strict budget; fortunately we were able to obtain the PICs for free.

Cheers,

Pete

Reply to
Pete Verdon

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Do they have to be 20mA LEDs? Lower power LEDs are available.

Reply to
CWatters

Unfortunately yes. Although we realised that we had power issues, and asked the person obtaining the parts to get the lowest-power ones he could, 20mA is what we've ended up with. To be fair, we had also specified LEDs with built-in resistors to ease assembly (we've got about

200 of the things to wire up in total) which may have limited the choice somewhat.

Pete

Reply to
Pete Verdon

Might still be cheaper to replace them than add a driver IC or transistors. Depends how you are doing the "wiring".

Reply to
CWatters

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