According to this application note from Fairchild (btw, I find Fairchild's application notes to be of immense value - thanks to all who have taken the time to write one), the load resistor value for a phototransistor in a common emitter configuration, to obtain switch-like performance, can be calculated with:
VCE = VCC – ICE • RL RL = (VCC - VCE)/ICE
Their example conditions results with a 68kΩ resistor being selected. My question is: is this a minimum value? What affect would increasing this value have on operation? If my values resulted in a 15kΩ RL value, and I used 100kΩ, what behavior might be different?
Background: I'd like to use a GPIO IC that has internal 100kΩ pull-up resistors and am wondering if the pull-ups will work as load resistors. At VCC=3.3V, VCE=0.4V, and ICE probably somewhere around 500µA (LTR-301), my load resistor (according to Fairchild's formula) should be ~15kΩ.