hi Mike,
My goal is to minimize total current draw to as low as possible. The leds are rated for a nominal 20mA but I should be able to drop that to 10-15mA... they do not have to be super bright. By cycling through the enable lines I am effectively pwm the entire display so depending on the duty cycle (I assume 25%) then the brightness will be lowered by that as well but now I understand from what you are saying that PWM does not reduce current draw on an individual led.
with the design you drew if I am cycling through 4 fets then at any time I only ever have a maximum of 8 leds on. If each led consumes 20mA current then the total current draw would be 160mA. I understand what you mean about pwm vs. perceived brightness... in my case I don't care if it is dimmer I just need to save as much mA as possible!
My other alternative is to use 4 SRs and but connect the enable pin of each one to uC pin and do the same thing (just load all 4 registers with my led states but then cycle through the enable pins at some rate).
btw, I am totally inexperienced using FETs. Is there a part number you would recommend? Do I need an resistors with it?
Also that is a clever idea to retask the col driver pins!