LCD TFT requires -5v and +5v ? - Page 1
Forum section: electronics / projects

author: carl0s
author_rank: Unknown
timestamp: May 14, 2018, 12:41:22 pm
content: Can somebody help / explain this one for me? I did watch a video on generating -5v by Dave a while ago, and I sort of kind of followed it.. it was inverting a PWM or something so that the centre line becomes between the two inverted PWMs.. or something.. turning +5v into + and -5v , e.g. a 10v voltage/difference.Anyway, in practical terms, this LCD panel, (which is last on my list to try to get going as it needs the SSD2828 RGB to MIPI converter figuring out, plus it's about 6x less bright than the one I'm going to concentrate on first), well it says it needs -5v VDD and +5v VDD, but it does say that the -5v can be between -5v and 0.3v, so can I just ignore the -5v VDD and put it to ground (0v? )? Or is this a fundamental thing I need to get a grasp on?Here's the datasheet:http://www.internetsomething.com/lcd/DT034XVZ01.pdf

author: glarsson
author_rank: Frequent Contributor
timestamp: May 14, 2018, 12:47:03 pm
content: Is 0V between -0.3V and -5V?

author: daveshah
author_rank: Unknown
timestamp: May 14, 2018, 12:51:45 pm
content: The absolute maximum ratings, which are the bounds for which no damage are guaranteed, are -6.5V and 0.3V for the -5V rail. There is no guarantee of function in this range however.The operating conditions, for which function is guaranteed are between -6V and -4.5V for the -5V rail. Hence -5V must be in this range for the display to function within specification.

author: carl0s
author_rank: Unknown
timestamp: May 14, 2018, 12:53:28 pm
content: Quote from: glarsson on May 14, 2018, 12:47:03 pmSorry my mistake I meant -5v and 0.3v

author: carl0s
author_rank: Unknown
timestamp: May 14, 2018, 12:55:09 pm
content: Quote from: daveshah on May 14, 2018, 12:51:45 pmThanks. I've looked at a few LCD TFT datasheets lately and this is the first one I've seen to show that. Does it seem unusual to you?Am I going to need to re-watch Dave's (I think it was an EEVblog video anyway) video on the subject? Or should I just find some sort of IC that does it all for me?

author: daveshah
author_rank: Unknown
timestamp: May 14, 2018, 01:04:00 pm
content: No, absolute maximum ratings are very common in datasheets, in general if they are missing it's a sign of a poor datasheet.Dave's video was IIRC about driving segment displays which are very low current and don't need a well regulated voltage. I suspect the PWM technique would not be well regulated enough for this display, and a proper charge pump IC (or maybe even inverting boost converter) would be a better fit. There may well be a chip that produces all the needed voltages, as it's a common requirement for modern TFTs in smartphones etc.

author: carl0s
author_rank: Unknown
timestamp: May 14, 2018, 01:05:08 pm
content: Quote from: daveshah on May 14, 2018, 01:04:00 pmYes that rings a bell! I remember now.It looks like there are lots of voltage inverter ICs available.. so I suppose I just need one of these or similar:http://www.ti.com/lit/ds/symlink/tps60400.pdf

author: carl0s
author_rank: Unknown
timestamp: May 14, 2018, 01:27:04 pm
content: Or a MAX749:https://datasheets.maximintegrated.com/en/ds/MAX749.pdfI must admit, I'm still a bit confused.. it doesn't state any supply current limits in that datasheet, nor does my LCD state what the LCD draws.Although it does state a maximum power dissipation in the region of half a watt.. is that what I need to know?Also, the MAX749 sheet says "The MAX749 drives either an external P-channel MOSFET or a PNP transistor."Well, why would I use it with a transistor? Can't I just run my LCD module straight from it? If I use it to drive the base of a transistor, surely that transistor will be connected to +5v and thus negate what I'm trying to achieve? Or is it just saying this because a negative voltage is the only way to switch a PNP via its Base? As you can tell I'm new to all this.

author: Bassman59
author_rank: Super Contributor
timestamp: May 14, 2018, 08:42:00 pm
content: Quote from: carl0s on May 14, 2018, 12:41:22 pmWhy not use a simple charge pump to generate the -5 V rail? LM2660 and its variants?

author: daveshah
author_rank: Unknown
timestamp: May 15, 2018, 07:15:40 am
content: Yep, LM2660 seems like a good bet if you have +5V available already. If not (e.g. running directly off a Lithium battery), otherwise the TPS65133 might be worth a look.

author: carl0s
author_rank: Unknown
timestamp: May 15, 2018, 08:12:55 pm
content: Good shout, thanks guys.