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Common Anode v.s. Common Cathode

E

EEng

Jan 1, 1970
0
It has always been my contention and design technique to sink current
where possible, rather than source it, thus I prefer common anode
wherever possible for displays and other driven devices and I write
all firmware routines to provide lows for execution rather than
highs.. The reasons are obvious on most data sheets that most devices
can sink more than they can source.

Recently while designing a 12 character 16seg LED display
(hostile/rugged environment) I was surprised to find that the majority
of display drivers by almost 10 to 1 prefer common cathode so that the
driver is sourcing. Obviously this doesn't fit with my experience.
For the display and driver being used, an All Lamps On condition would
require in excess of 2A. I doubt most drivers can source that.

I spoke with the company and they informed me that they too had
problems with this, and so now all their display drivers have built in
10% duty cycle multiplexing, also built in. The problem with this is
it requires a charge pump to maintain the command word and I can't
have a charge pump anywhere inside the product (highly sensitive RF
device). The company did manage to find one of their static devices
that fulfill all my needs except for retention of the command word,
but that's fine as I was planning on using a Look Up Table and EEPROM
to store critical command data. This static device also has the
ability to store display command/data words as long as power is
applied and that works too, since it will be stored in EEPROM as well.

Seemingly the best of both worlds, but I'd like a consensus opinion
regarding preferences, not needs for Common Cathode v.s. Common Anode.
My personal choice as stated is common anode. Other opinions and
explanations are greatly appreciated.

EEng
 
S

Spehro Pefhany

Jan 1, 1970
0
Seemingly the best of both worlds, but I'd like a consensus opinion
regarding preferences, not needs for Common Cathode v.s. Common Anode.
My personal choice as stated is common anode. Other opinions and
explanations are greatly appreciated.

With a multiplexed display you have to sink *and* source ALL the
current. So it boils down to the current level, the number of each
driver involved (if any, as some can perhaps be directly driven from a
microcontroller or PLD), and the cost of each.

All told, ,my preference is also for CA.

Best regards,
Spehro Pefhany
 
E

EEng

Jan 1, 1970
0
With a multiplexed display you have to sink *and* source ALL the
current. So it boils down to the current level, the number of each
driver involved (if any, as some can perhaps be directly driven from a
microcontroller or PLD), and the cost of each.

All told, ,my preference is also for CA.

Best regards,
Spehro Pefhany


Interestingly enough, we have in fact gone to a multiplexing driver
for our 12 character 16seg display. Check out the Maxim 6954 and
6955. For our purposes, we can't use anything with a charge pump so
these work fine. We'll have to cascade two of them, but that's fine,
and yes, they're being driven directly from a PIC using SPI protocols.
 
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