Hey Gang,
Does this seem like a reasonable design for driving big 7-segment common cathode displays?
The parts I have been asked to look at use 2 strings in parallel of 4 LEDs in series per segment, with 40mA/Segment current flow.
I'm thinking TPIC6B595 to pull p-channel MOSFET gates low to turn them on, and resistors to pull the gates high to turn them off.
This is the cheapest p-channel somewhat low Rds mosfet I came across in "low" quantities, I'm looking at 210 to 240 of them (not sure if just 7-segment yet).
I'm not aware of any p-channel 8-mosfet array's.
8 channel PNP arrays such as Allegro A2982 have 1.5V drops across their output, while this method only loses (0.04A * 0.7ohm) = 28mV so not losing brightness due to driver loss.
TPIC6B595's are ~70 cents at avnet.com, so driver cost/digit would be about
68 cents for shift register (qty 30)
21 cents for Resistors (estimate a pennty each at qty)
70 cents for transistors (250 qty)
so $1.60 per digit, seems pretty reasonable.
