Edge-lit backlight (i.e. white-on-blue LCD) and Core backlight control

[Wilba]

I just got two LCDs with edge-lit backlight that according to the datasheet use 60mA instead of the typical 280mA of LCDs with a LED array backlight.

As I don't really want to burn out the white LEDs with too much current, I'm trying to work out if I could accidentally do this with the Core backlight control circuit, which I believe does supply up to 280mA. I know it will probably work OK, as many people have used white-on-blue LCDs before and probably didn't do anything special, but if there's the chance I could be oversupplying the white LEDs and potentially burning them out instantly or reducing their lifespan, I really want to know NOW!

So any explaination of the backlight control circuit, or sharing experiences with edge-lit backlights would be most helpful.

[pay_c]

I put a 47 Ohm resistor in front of the LCD backlight pin in nearly all my apps just to go sure (brings down max current to below 100 mA). I also did that on Midiboxes and all LCD are working pretty fine indeed. So, I do not know anything *if* this could be harmful, but just put a resistor in front of it and you *certainly* will be fine.

Greetz!

[Wilba]

On closer inspection, these edge-lit displays just have three white SMD LEDs in parallel connected directly to the A and K pins on the right side, and these pins are connected directly to pins 15 and 16. There is no current-controlling circuitry as you would find in displays with LED arrays (components are missing from the PCB!). I'm surprised that they don't mention this in the datasheet.

So the 60mA in the spec is just 3 x 20mA for each LED. With a 3.5v forward voltage, the voltage drop of a current limiting resistor would be 5v - 3.5v / 0.060 amps = 25 ohms. I didn't have one handy, but 5v through a 100ohm resistor lit up the backlight to what I assume was less than half brightness. By the way, these aren't white-on-blue, they're actually black-on-grey... a bit like this:

I might not drive it so bright though... so it's still a grey background for that retro old-school LCD look  ;D

[Sasha]

I never had any of edge-lit displays, but according to pictures they all looks like light is not spread well all over the background. Is it so obvious or it is how camera see it? It looks especially ugly on negative LCDs where background should be black. Is there something that could be done about it?

[Wilba]

There appears to be a rectangular piece of glass with metal film around the edge, behind the LCD layer, and a 1mm gap between the two. It seems quite possible to add another few white LEDs to the other side, or insert some kind of diffusing semi-transparent sheet in this gap. I've only briefly tested this one at low brightness and it's not as bad as it looks in photos, maybe at maximum brightness the bright spot on the right edge will be more noticeable.

Maybe I'll get another one to experiment with, and stick a few RGB LEDs on each end for a colour-blended look  ;D