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Science Forum Index » Electronics - Misc Forum » IC Power source / sink limits
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| Pete Verdon |
 Posted: Thu Feb 01, 2007 7:12 am |
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Guest
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Hi,
I'm a software developer dabbling in hardware for the first time,
building a device controlled by a PIC microcontroller, specifically a
16F914:
http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en020956.pdf
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.
2. 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 |
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| GM |
| Posted: Thu Feb 01, 2007 10:35 am |
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Quote: Any advice would be welcome.
Pete
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 |
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| Pete Verdon |
| Posted: Thu Feb 01, 2007 12:15 pm |
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GM wrote:
Quote: Why not using an LED driver IC? In that way, the driver IC is responsible
for providing the necessary current and not the uController
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 |
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| CWatters |
| Posted: Fri Feb 02, 2007 1:07 pm |
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"Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> wrote in
message news:52e091F1njmhjU1@mid.individual.net...
Quote: Hi,
I'm a software developer dabbling in hardware for the first time,
building a device controlled by a PIC microcontroller, specifically a
16F914:
http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en020956.pdf
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.
Do they have to be 20mA LEDs? Lower power LEDs are available. |
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| Pete Verdon |
| Posted: Mon Feb 05, 2007 4:42 am |
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CWatters wrote:
Quote: "Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> wrote
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
Do they have to be 20mA LEDs? Lower power LEDs are available.
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 |
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| CWatters |
| Posted: Mon Feb 05, 2007 4:33 pm |
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"Pete Verdon" <usenet@verdonet.organisation.unitedkingdom.invalid> wrote in
message news:52o90nF1p5tdsU1@mid.individual.net...
Quote: CWatters wrote:
"Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> wrote
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
Do they have to be 20mA LEDs? Lower power LEDs are available.
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
Might still be cheaper to replace them than add a driver IC or transistors.
Depends how you are doing the "wiring". |
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