LFS: 5-7" low power monochrome LCD Display - Page 1
Forum section: electronics / general

author: madshaman
author_rank: Frequent Contributor
timestamp: February 09, 2014, 05:42:42 pm
content: Hi, I don't know the display market too well, but I do know what I'm looking for:- monochrome transmissive LCD (with at least 16 grayscale levels) that I can attach my own backlighting to (want to use an EL panel ala Japanese Gameboy "Light"; yes I own one ;p, and they are not to sacrifice)- low power consumption but still allow 30-60 effective fps- minimum resolution: 320x240 but would prefer at least 400x400It's not vital whether the display comes with a controller/driver or not, but would prefer.I assume grayscale ability is a combination of the display being designed for such and also the controller/driver supporting it.It's for a one-off personal project so I'm not opposed to salvaging the display from something else, but it would be nice to be able to build another easily.I've hunted around digikey/mouser and also all the duino-type web-stores and can't find what I want at anything like a reasonable price; either that, or there's something that does what I want but didn't realise it.

author: tom66
author_rank: Super Contributor
timestamp: February 09, 2014, 09:20:04 pm
content: It's not monochrome, but you're probably limited to what you can get nowadays. Colour displays are just so cheap because they are produced more than greyscale.http://www.topwaydisplay.com/Pub/Manual/LMT057DNAFWU-Manual-Rev0.1.pdfHard to understand typo in datasheet, but it looks like it draws 0.02A (20mA; not 0.02V) from the +3.3V. It also requires a +5V, +15V and -10V supply. The latter two are typically <10mA when using the display. The +5V current will typically vary with displayed complexity: the more pixel-to-pixel transitions across each row, the higher current. I'd target 100mA at minimum, 500mA at maximum. 0mA when display is OFF, and significantly lower current if frame rate is lower or if you can tolerate some blurring on frame changes.LCDs are charge driven: pixels can retain perfect image for up to 100ms typically, and acceptable image up to 1 second with minimal flicker (especially in an image with high contrast.) So, if you can drop VSYNC rate, +5V current will be much lower.VCOM is a couple mA supply which can be set by potentiometer from VGH to VGL. VCOM will need to be adjusted to minimise display flicker and improve image lifetime during long frame pauses.You can use a boost converter to generate 5V from 3.3V, then a diode-split multi-stage charge pump for each 15V and -10V. The 15V and -10V do not need to be well regulated. A voltage range of 2V either way will typically not harm the image quality or display performance. Lower voltage on the VGL voltage will lead to vertical line bleed, typically. Lower voltage on the VGH rail will lead to poorer contrast ratio, red/brown/orange whites and slower display updating (blur on change, etc.) Higher voltage on either will cause minimal artefacts, although current draw will be higher and the maximum limit of +32V/-22V and +45V between VGH and VGL must not be exceeded.http://www.ti.com/lit/an/slva398a/slva398a.pdfMaybe a bit complicated for a hobby project if you're interested in a challenge then it could work out well.

author: madshaman
author_rank: Frequent Contributor
timestamp: February 10, 2014, 04:14:36 am
content: tom66, thanks for the great reply. To make sure I understand, what you're saying is that if I develop my own driver circuitry there are tricks I can use to minimise power consumption (such as you describe).To be honest, the displays in my shopping carts are all colour tft displays as that's the best price-point per capability I could find. They all come with on-board driver chips which usually refer you to the driver IC's data sheet for timing details etc. I chose displays which met my specs, it least from the driver IC's point of view. I didn't really dig deeper into the on-board driver ICs to see of they supported any kind of power saving ability; I should look there as well.Feeding clocked bits to something is definitely much easier but this is a personal project for fun, so if there's a real possibility of optimising power usage by developing my own driver, I'm game for it.Thanks again.

author: tom66
author_rank: Super Contributor
timestamp: February 10, 2014, 10:27:12 am
content: Yep - just be aware for any reasonable size display you are going to want to use a fairly powerful microcontroller or dedicated display controller to do the heavy lifting. The display has no memory. It has the DAC, gamma correction, TFT source driver and TFT gate driver, but nothing else. If anything, the power supply design is an interesting challenge. Here's one idea, producing +14.5V, +5V and -9.5V from a single +3.3V supply. All supplies must be active while updating the panel.