I'm trying to control the back-light of a TC1602A-09T 16X2 Character LCD by using an output pin on a Teensy 3.2 microcontroller. This is the circuit I have at the moment:
The LED- pin would be connected to the backlight ground pin of the LCD and it would be pulled to ground via a signal from the teensy microcontroller. R1 limits the current into the base of the NPN transistor while R2 will pull the base to ground when the backlight should be off. My question is about R3. I put R3 into the circuit so that the LED- pin on the LCD would not be floating when Q1 is turned off. Is this necessary to do? Is there any harm in just leaving the pin floating?
Regardless of whether or not R3 is necessary in this particular application, I was trying to determine what the best value of R3 should be such that the power dissipated through it when Q1 is on would be at a minimum. Using the data I obtained from LTspice I created this graph:
LTspice clearly shows that the higher the resistance value, the lower the power dissipated in the resistor. If this is the case, is there any reason not to choose the highest value resistor that one can get in order to minimize power consumption? Are there any other considerations when choosing a pull-up resistor that help in determining the correct value of the resistor?