Low Current LCD Backlight

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crazyray
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Posted by crazyray: Fri. Sep 18, 2009 - 04:56 AM
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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?

Thanks,
Ray

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Learning and Information, General Electronics
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Jepael
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Posted by Jepael: Fri. Sep 18, 2009 - 12:34 PM
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Look how the backlight gets current. I've seen 0R resistors on the LCD PCB so they can change the module to one with resistance while keeping the actual LED module identical.

You could also change the backlight to be driven with a boost converter.

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DocJC
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Posted by DocJC: Fri. Sep 18, 2009 - 02:06 PM
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If it is a commercial product, as mentioned, then you are most likely using a known vendor for your LCDs. This means you should have a real data sheet, and also access to their own Tech. Support staff.

My calculations, using a Crystal Fontz 16x2 LCD with LED string BL, Vf of 4.2 V, Ibl ~ 140 mA, (Max 260 mA), would use an R of 5.7 ohms at 5V supply. If the V+ is then reduced to 4.5V, the current should drop significantly, to 52 mA. So much for the simple model...

If you do end up using an LCD with a simple series LED string(s) BL, then note that you have the option of using a true, constant current source, LED driver instead of a simple series resistor.

The driver could be an LED driver chip, made by many companies, or your own design with an op-amp, and a small Isense resistor. Many of the drivers incorporate a PWM control signal.

The trade off is cost, parts count, complexity, and board space vs improved battery life. A true constant current driver is much more efficient than a simple series resistor, which wastes a lot of energy in heat... Although they still use a series resistor, it is a very small resitor for sensing the current, and hence wastes much less energy.

JC

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crazyray
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Posted by crazyray: Sat. Sep 19, 2009 - 12:17 AM
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Thanks for the comments. That's a good point on the voltage vs. current mode sourcing. I thought all LCD modules had an on-board resistor to set current for a given supply voltage.

For example a module I've been using specifies the backlight as an LED array for 5V, 160mA. I assume I'd have to have physical access to the backlight resistor and replace it with a 0 Ohm jumper in production, then use a constant current source. I hadn't considered that.

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Bronze Marktheusch
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Posted by theusch: Sat. Sep 19, 2009 - 04:07 PM
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I'd also poke at different LCD models/types. We have a current (pun intended) app with a 4x20. There are certain types and certain color combinations that have MUCH better backlight current draw characteristics.

[I find it a bit strange that a 50% PWM on an otherwise OK LED backlight would even be barely noticeable to the eye.]

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Bronze Marktheusch
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Posted by theusch: Sat. Sep 19, 2009 - 04:19 PM
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OK, here is what you gotta do:

-- I like the "white on blue" better myself, and they seem like they have advantages.

-- I can't tell you the difference between transmissive, reflective, and transreflective off hand but it does make a difference as we'll see ...

-- Let's pick the 80mmx36mm 5V 2x16 models from this page:
http://www.newhavendisplay.com/i...

This one has a 20mA backlight:
http://www.newhavendisplay.com/s...
This one has a 120mA backlight:
http://www.newhavendisplay.com/s...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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jayjay1974
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Posted by jayjay1974: Sat. Sep 19, 2009 - 05:52 PM
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Efficiency will be the same whether you use a linear current source or a resistor. The excess power has to go somewhere, it's dissipated in either the resistor or the transistor.

If you use a switching current source, things are different of course.

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crazyray
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Posted by crazyray: Fri. Sep 25, 2009 - 10:42 AM
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Thanks for the comments guys. I also just noticed that the backlight current was a lot different based on LED color.

I was also surprised to see how much difference there was visually between 100% and 50% duty cycle. I've worked with customers that power LED backlights and this is not the behavior they saw. I was on the development teams that designed the LED driver circuit ICs. We tested them with discrete LEDs rather than LCD modules. The modules I have don't behave like I expected. Maybe I just need to try some other modules.

It definitely looks like I can get in the overall power dissipation range I need just by doing that.

Thanks again.

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