+5V / -5V supply circuit for use with LED driver (driving big LED displays)

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thoraxe

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Background:
I've started down a project where I want to drive three honkin' large 7-segment LED displays. These digits use a ~8V supply to drive them (I'm not going to use the decimal point). I can definitely verify that the digits barely turn on until at least 6V. I am currently using an 8V voltage regulator to step down from 13.8V (this is going in a vehicle):
https://www.kingbrightusa.com/product.asp?catalog_name=LED&product_id=SC23-11SRWA

I was going to use the MAX7221 LED display driver to run them with the LEDcontrol library:
https://www.maximintegrated.com/en/products/power/display-power-control/MAX7221.html

According to the 7221 datasheet, to run a 2.3" display requires the use of a MAX394 switching circuit, some individual MOSFETs, and a +5/-5 voltage supply for the MAX394. This forum post mentions using a MAX681 to get +10/-10 out of +5V, but I need half that. The product information indicates the MAX681 is a "doubler", so would supplying +2.5V to it produce a +5/-5 output?
https://forum.arduino.cc/t/can-you-...itive-voltage-out-of-the-same-source/101213/7

This app note talks about ways to drive common anode displays (I currently have common cathode ones but I can reorder anode ones if it simplifies things):
https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1196.html

This is also a lot of extra circuitry just to drive some digits and a significant extra cost. The 1.5" 7-segment LED display looks like it can operate at 3.8-4.6V:
https://www.kingbrightusa.com/category.asp?catalog_name=LED&category_name=KC1-Digit 1.5in&page=1

There's also a 1.8" one, but it requires 5.4-6.9V:
https://www.kingbrightusa.com/category.asp?catalog_name=LED&category_name=KC1-Digit 1.8in&page=1

That being said, this comment in this thread indicates that the 7221 is a current device, and not a voltage device, but, still:
https://forum.arduino.cc/t/max7219-voltage-to-low-0-2v/880284/10

Questions:

1. Can the MAX7221 drive the 1.5" LED display directly? The 1.8"?

2. If I want to use the 2.3" digits and need to run the MAX394 + MOSFETs, what is the right/best way to get +5/-5? Somehow I don't think a pile of voltage regulators is the right choice.

3. Is there a simpler way to run the 2.3" LEDs?
 
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Those appear to be common cathode. Those need high side drivers which is more complex when voltages are
different.

With common anode you could have driven them with 16 channel constant current drivers like the TLC5928,
which operate as shift registers you can daisy-chain (like 74HC595s).
There are various 8 and 16 channel LED low-side drivers like this with open-collector and/or constant current
sinking outputs, another being the STPIC6D595 (which is pretty much just a '595 with different output drivers).
 
Other than the common cathode/anode issue, "can drive" is mostly knowing the current capability vs desired LED current. Do use current control (vs driving with a fixed voltage).
 
jonr said:
Other than the common cathode/anode issue, "can drive" is mostly knowing the current capability vs desired LED current. Do use current control (vs driving with a fixed voltage).
Click to expand...

If you look at the datasheet for the MAX7221, it makes the following claims:

Code: 
Current
DIG 0–DIG 7 Sink Current............................................500mA
SEG A–G, DP Source Current .....................................100mA

The 2.3" display claims 30mA forward and 155mA peak. I'm still not entirely sure if that means it can drive it or not. The datasheet for the 7221 as well as the app note are pretty clear about using some other circuitry to drive these digits. The MAX7221 does not even offer "driving with a fixed voltage." However, from my hazy memory of college circuit physics, if you don't provide sufficient voltage across the LED junction you don't get any current... so... there's that.
 
MarkT said:
Those appear to be common cathode. Those need high side drivers which is more complex when voltages are
different.

With common anode you could have driven them with 16 channel constant current drivers like the TLC5928,
which operate as shift registers you can daisy-chain (like 74HC595s).
There are various 8 and 16 channel LED low-side drivers like this with open-collector and/or constant current
sinking outputs, another being the STPIC6D595 (which is pretty much just a '595 with different output drivers).
Click to expand...

The nice thing about the MAX7221 is that you only need one of them to drive up to 8 characters. With the shift registers, you end up needing at least two chips to drive three digits. Space isn't exactly at a premium given the footprint of these LEDs (almost 6" x 3" for the three digits).

I'm confused about the TLC5928 versus the STPIC6D595. They both claim to be current-sink. I think I'm misinterpreting your grammar. It sounds like you are saying that common anode, generally, is easier with shift registers, and all of the ones you list could be used to drive common anode displays. Is that correct?

It seems like it might be a toss-up with the PCB wiring for 7221 versus shift register. With the 7221 I'll likely end up with more individual chips because of the requirement for the extra transistors to drive the LEDs. With the shift registers, I'll need a LOT more current-limiting resistors and several of the registers along with the wiring to all of their serial inputs. Granted, they can be daisy-chained. But it's still a lot.

Hmm... this is difficult.
 
The MAX7221 can supply up to 40ma/segment or anything less than that. The LED can handle 30ma/segment or anything less than that.
 
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