How can I best drive and switch high side power (for multiplexing) between the 7 displays with as few components as possible, for under $2. I am trying to simplify a design.
I have a custom board design with seven common anode 7-segment displays. I plan to build a large amount of them where I want to minimize components as well as keep component costs down.
Currently, on the high side, I use a SN74HC595 to switch power between 7 IMD10AT108 (combined NPN/PNP on 1 chip) drivers which power the displays.
One IMD10AT108 (NPN/PNP driver) powers each display. That makes 8 components to drive the high side of 7 displays. I'm not satisfied with this, although it works great. For comparison, I'm able to handle to low side of all 7 displays with only 2 components…chip + resistor.
It seems "odd" that I can't find a single chip shift register style solution to this at low cost, like is quite easy to do on the low side via a number of different chips and methods. (Yes, I see $5 chips (qty 100) are available do to this.)
I'd be happy to switch over to common cathode if that made a solution available. I am also looking to stay with something that can be driven via shift register logic, because these board chain together to add more displays in a chain.
This seems to be a question that has been asked in a number of forums, and is a common design problem for people trying to drive a number of multiplexed 7-segment displays.
A PNP array chip seems like the easy answer, until you start looking and find out they don't exist. To me, this seems like a common problem without a simple (cheap) answer.
Since the design is about to be finalized, I'm making one last ditch effort to find an alternative. Thanks for any input.
Best Answer
To multiplex the high side, all you need is a shift register circulating a single zero and a resistor and PNP for each digit, like this: