7 segment LED displays: Common cathode vs. common anode

[japlytic]

With 7 segment LED displays, are common cathode units more common than common anode units?
From what I see, common cathode units have simpler drive circuitry.

[Curious.George]

Re: 7 segment LED displays: Common cathode vs. common anode

With 7 segment LED displays, are common cathode units more common than common anode units?
From what I see, common cathode units have simpler drive circuitry.

It depends on how you intend to drive them.

Common cathode is great if you intend to multiplex the displays cuz you can sink 8 x Isegment with a "digit driver" and just have to source 1 x Isegment (eight times) for each segment -- regardless of how many digits you're driving.

But, if you can't easily source Isegment and need to add an external buffer (eight of them), then the calculus can change -- install a large source driver per digit and you can probably cheaply sink Isegment with an open-collector/drain gate/buffer.

For a non-multiplexed display, common anode usually wins as you can source from V+ and sink through gates/buffers with ballast resistors.

[eccerr0r]

Re: 7 segment LED displays: Common cathode vs. common anode

The CA and CC drivers are logically equivalent to each other minus the fact one is the complement of the other. The logic is not any more complicated except the fact that humans rather think of "high" as "on" and CC works this way.

However in the past, the fact that cheap/smaller silicon transistors tend to favor N-channel and NPN transistors due to semiconductor physics. Because of this, circuit design was heavily influenced. Transistors show their highest gain when in common emitter/source configuration and CA is favored when directly driving LEDs. The popularity of the 7447 (BCD-to-7 seg) and ICL7107 (LED digital panel meter) ICs caused CA to proliferate.

As seen by the sheer number of wires that need to be hooked up to multidigit displays, multiplex was designed to reduce board complexity. Because of this, CC became more popular just to minimize the number of high current devices which are cheaper implemented with NPN transistors as digit drivers. Common collector or PNP segment drivers are now needed for this configuration (common drain was not an option in the past...)

This is not to say you have to follow these "rules." I tend to do my circuits with whatever I have on hand, sometimes I have one or the other and will have to keep these things in mind when working with the opposite of preferred, or suffer brightness issues. On a recent project I used CA multiplex because I have a boatload of 2N3906's that conveniently works for CA digit drivers, plus the common source segment drivers of the microcontroller sink plenty of current to make the LEDs shine brightly - which is a major problem in multiplex designs due to the duty cycle.

IMHO CA should be more popular in general due to the tremendous advantages it gives to the circuit to make it cheaper. However CC is still around for those who rather think "high" as "on."

As an aside, I don't know the history of germanium transistors which hole carriers appear to be better, but the digital use of LEDs were pretty much all silicon era. But this is swaying off topic...

[redwire]

Re: 7 segment LED displays: Common cathode vs. common anode

Digit common is the highest current, up to 8 LED's worth if you include the decimal point, muxed. So a 20mA/segment display could use 40-160mA.

IC's and logic gates will source less current than they sink, both bipolar and CMOS, because of P-ch devices performing worse on IC's. A PNP or P-fet on a mixed IC just performs poorly due to semiconductor physics. So CA is not popular for muxing control from an IC.
Modern LED display driver like TM1638 can accommodate either CA or CC displays but note the difference in current capability. source 25mA or sink 140mA (great for CC).

Direct drive (no mux) common anode was popular on old SN7447, or common cathode with a -ve voltage power supply like on old MOS-LSI clock IC's MM5387.