I'm working on a battery powered commercial project where I need to run an LCD backlight once in a while. The backlight dominates battery charge consumption.
I'm using a 16x2 LCD module, 80x36mm, yellow backlight. The backlight draws 140mA at 5.0V. I've worked on backlights for mobile phones and I would typically crank up the current to drive the LED very bright and PWM it down to a low duty cycle and get an effectively bright looking display without drawing anywhere near the equivalent 100% duty cycle power for that brightness.
With these modules I'm not seeing the same affect. I can pull Vcc from 5V down to 4.5V and I don't see much change in current at all. So if I PWM at 50% duty cycle I can't seem to drive up the LED current during the on-time to compensate.
From what I could tell these modules use LED strings with a built-in current limiting resistor. My bench data doesn't confirm this. It's almost like there's an internal regulation of some sort.
What is your experience? I'd like to get the RMS current down to 50mA or so with reasonable brightness. What approach(es) do you recommend?