# Driving LCD segments with an LC generated sine wave to save power?

All of the LCD drivers I have seen use square waves to generate the RMS voltage across the segments. Since a segment acts as a capacitor, this seems to needlessly use coulombs to constantly charge and reverse the polarity by shunting to the supply rails.

Could it be more power efficient to generate the same RMS voltage using an LC oscillator where the glass is the C (or at least part of it)? There would still be R losses, but these could be significantly less than the losses of the digital drive.

Is there a reason why this is not done? Or is it done and I just haven't seen it?

NOTE: This would only apply to static segments and not multiplexed displays. I also know that the power requirements for the existing digital drives are already very small (nA's), but static LCDs are often used in extremely power and space constrained applications so any reduction of power could be worth it.

• I've wondered this myself, though your circuit would not be suitable as it would create a DC bias on the LCD. LCDs behave primarily capacitively, so it would seem like this approach should improve efficiency. I suspect, though, that LCDs probably have a sufficient series resistance that the actual improvement in efficiency would be modest. Feb 8, 2017 at 16:39
• @supercat Ah, yes- good point on the DC bias. I think this could be overcome using a bi-polar drive? Alternate which direction you push on alternate pulses so the bias on the damping sine waves cancel out? You would still get power savings by not having to dump all that charge on every cycle, yes? Feb 8, 2017 at 17:00
• No. You are still transferring the same amount of charge to the glass, and using saturated switches (square waves) is the most efficient way to do that. This is especially important when you're talking about hundreds or thousands of individual row and column drivers. Besides, no matter how you implement them, inductors take up a lot of room and add cost. It doesn't matter whether you're talking about a simple wristwatch or a 4K monitor, space and budget are overriding concerns. Feb 8, 2017 at 17:09
• perhaps the best approach reduces the square-wave toggle rate on LCD until the subjective eye response sees flicker: in the 1 - 10 toggles-per-second ballpark? Feb 8, 2017 at 17:32
• @DaveTweed It seams like if I drive the LC with pulses of alternating polarity and always wait for the previous ring to die off before sending the next pulse, then though symmetry the DC bias is zero. Am I missing something? Feb 8, 2017 at 21:57

Simply switching the segments means a loss proportional to $fCV^2$ so reducing f, decreasing C (smaller segments) and reducing the voltage will minimize power consumption.