What I have done on some systems is to use two PNP transistors that are controlled by two port pins that
manage the power to the LCD module. One transistor is used to switch the power to the LED backlight on and off. The backlight is turned on whenever the user has pressed a keypad button and will stay on for up to 30 seconds after the most recent keypad press that is detected. The other transistor is used to control power to the LCD controller chip itself. The controller power it turned off whenever the processor is in some type of IDLE mode where the user is not expected to be using the display. (Note that whenever the LCD module power has been off it is necessary to re-initialize the device controller). It is also a good idea to watch how the A & K connections of the LCD module are routed. The high current on these lines can cause noise problems while the transistor turns on and off.

The transistor may also be driven with a PWM type waveform from a PCA channel to obtain an intensity control for the backlight. Select the backlight current limiting resistor to give the maximum brightness you need when the PWM nears 100% active duty cycle.



Good Luck
Michael Karas