Producing -5V to a TFT display - Page 1
Forum section: electronics / beginners

author: braden_sun
author_rank: Newbie
timestamp: March 04, 2020, 03:26:10 pm
content: Hi everyone,I am trying to drive a TFT display ,datasheet linked below, with the 32F769IDISCOVERY, datasheet linked below, MIPI DSI interface (page 27). The display takes a -5V, 5V and 1.8V input. My issue is the Microcontroller has a 5V and 1.8V pin but no -5V. Now I know to produce -5V you would normally just feed the 5V signal to gnd and the gnd signal to the voltage input but in this case I am confused since there are multiple gnds on the Microcontroller and display connectors and it seems like a bad idea to put 5V on any of the gnds?Thanks for the help!https://docdro.id/lDkouQRhttps://www.st.com/content/ccc/resource/technical/document/user_manual/group0/7c/c9/c0/5f/4e/9b/44/eb/DM00276557/files/DM00276557.pdf/jcr:content/translations/en.DM00276557.pdf

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 04, 2020, 03:30:45 pm
content: By "grid" do you actually mean ground?You will need an additional -5V supply source, you cannot reverse and use at the same time the existing +5V feed, that would short something out.

author: fcb
author_rank: Super Contributor
timestamp: March 04, 2020, 03:36:52 pm
content: How much current do you need on the -5V rail, and how accurate does it need to be.In the past if built a charge pump out of a couple of diodes and capacitors and hung it on a clock line, great for creating a sniff of -ve.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2020, 07:27:22 pm
content: I've flicked through the data sheet and couldn't find anything about it requiring a -5V rail. If all that's required is a few mA, then a charge pump IC, such as the ICL7660, is the usual way to do it.https://www.renesas.com/eu/en/www/doc/datasheet/icl7660.pdf

author: braden_sun
author_rank: Newbie
timestamp: March 05, 2020, 01:08:38 am
content: Thanks for the replies. The datasheet for the display doesn't list any current ratings so I think maybe I should go with an inverting buck converter until I can assess how much current it draws?

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2020, 08:43:50 am
content: Quote from: braden_sun on March 05, 2020, 01:08:38 amSorry, I couldn't find anything on the datasheet which states a requirement for a -5V supply and the first link you posted was blocked by the firewall. I did look at page 27, as you mentioned and also did a search for -5 but didn't find anything. Perhaps I've missed it? I don't have the time to manually read through the entire data sheet.Could you please take a screenshot of the part of the documentation where it says it needs a -5V supply and upload it as an attachment here (click "Attachments and other options" underneath the part where you type to upload the file)? I think you might be mistaken.

author: Yansi
author_rank: Super Contributor
timestamp: March 05, 2020, 09:18:14 am
content: What kind of damn display does need a negative voltage supply in a year 2020?Why there are no current consumption specs in the fakin datenblatt? I wouldn't dare use such TFT. Too much trouble.And don't forget you also need a constant current boost LED driver for the backlight strip (12.8V 40mA).

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2020, 10:26:16 am
content: I'm guessing the data sheet was the first link, which I can't access? Please upload things here, where possible, rather than using external hosting services.Yes, it is crazy the current consumption isn't mentioned. I would connect it to a bench PSU first to determine the current consumption. Unfortunately, the tolerance of +/-0.1V is too tight for a charge pump to work, unless the current draw is tiny. An inverting buck-boost regulator can be used to deliver -5V, from a +5V supply.

author: Yansi
author_rank: Super Contributor
timestamp: March 05, 2020, 11:24:33 am
content: This is a MIPI serial interfaced display - trying to characterize what should already be in the datasheet is ...You would need to at least make a breakout board, somehow interface to the MIPI serial lines, then programm it, apply test patterns to the screen, observe current change vs. temperature.TOO MUCH HASSLE with that junk.Either contact the manufacturer, or pick a different enemy.

author: braden_sun
author_rank: Newbie
timestamp: March 07, 2020, 07:46:26 pm
content: Thanks for the advice. I am trying to make a digital motorcycle speedometer and want it to be a circular display. The trouble I am having is all the large circular displays (~3.4 inch) are from China with very bad documentation. I know I could use a square display with a circular active region but that ends up making my final package larger than I would like.I guess I was hoping that the MIPI DSI spec would have the information I need as far as how to drive the screen and since the micro controller I plan to use (https://www.st.com/en/evaluation-tools/32f769idiscovery.html) supports MIPI that it wouldn't be too difficult...

author: Yansi
author_rank: Super Contributor
timestamp: March 08, 2020, 09:59:46 pm
content: MIPI is proprietary shite if I am not mistaken, usually requiring NDAs signed to get access to required documentation. MIPI equipped microcontroller indeed required, but you still need to know exactly how to configure the LCD controller inside, as MIPI displays have autonomous controllers in them, even with frame buffers for displaying static images with less consuption - not trivial to use as the old DPI (data parallel interface) displays were.Same problem is with most Chinese LCDs equipped with CTP (capacitive touch controllers). Very hard to obtain documentation for the mostly very obscure and complex controller chips (don't get it wrong, sometimes the rather little number of pins does not mean they are trivial to configure). And when you obtain the documentation for the CTP controller chipset, you just realize you need also whole config file from the manufacturer of the CTP glass itself, so you can configure the glass drivers and sense amplifiers.I would suggest to not use obscure round displays. Use a readily available rectangular one with a circular bezel. Most if not all modern car dashbords are made using standard rectangular TFTs having just fancy bezels in front of them.

author: shved
author_rank: Regular Contributor
timestamp: March 09, 2020, 07:49:38 pm
content: I can confirm that charge pump is a good option. I have exactly that kind of circuit in 2 commercial projects. In one of them i used MCU's hardware PWM peripheral for LCD update clock, as well as -4v charge pump.

author: tooki
author_rank: Super Contributor
timestamp: March 09, 2020, 10:50:54 pm
content: Quote from: Yansi on March 05, 2020, 09:18:14 amLots of displays need it. Many generate it internally, some don’t. It’s for negative bias for contrast. It’s more common on large-digit character displays, as well as on wide-temperature LCDs. It generally requires minuscule amounts of current, so a charge pump like the 7660 normally does the trick.

author: Till Matte
author_rank: Contributor
timestamp: April 24, 2020, 03:09:47 am
content: i think the negative voltage requirement was from the request of its IC driver ILI9881, so you need to generate the -5V on your driving voltage, in input it through the MIPI interface, from pin 7~8 on this lcd. the -5V should be related to the VLG voltage for thetft lcd, tft lcd driving voltage is from VGH-VLH, when used the VLH voltage in -5V, that means you don't need a higher VGH voltage in your tft driving system, actually, it really save some cost in DC-DC circuit in your tft driving system.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 24, 2020, 01:35:05 pm
content: There is a possibility also to use isolated 5V/5V DC-DC converter (1-2 W). They are quite small and not very expensive today.

author: kripton2035
author_rank: Super Contributor
timestamp: April 24, 2020, 02:44:18 pm
content: Quote from: Vovk_Z on April 24, 2020, 01:35:05 pmhttps://www.mornsun-power.com/html/products/35/0.25-3w-smd.html

author: tooki
author_rank: Super Contributor
timestamp: April 24, 2020, 06:05:34 pm
content: Quote from: Vovk_Z on April 24, 2020, 01:35:05 pmIt's way more expensive than a 7660 charge pump and its prerequisite capacitors. The negative bias current of an LCD is microamps, so absolutely no need for a watt or more.If we look on LCSC for Chinese clones, the cheapest 5V DC-DC converter is $1.40, while a 7660 clone is 20¢.

author: shved
author_rank: Regular Contributor
timestamp: April 24, 2020, 07:30:23 pm
content: Quote from: Vovk_Z on April 24, 2020, 01:35:05 pmPWM peripheral is free in most modern MCU's(1 pin) + 2 lines of code + 2 diods + 2 caps. Enough said.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 25, 2020, 06:04:56 am
content: Yes, pump IC is cheaper then isolated DC-DC, but PWM peripheral method is the cheapest of cause (with it's drawbacks).

author: Mp3
author_rank: Frequent Contributor
timestamp: April 25, 2020, 06:14:18 am
content: I've used many of those cheap Chinese DC buck/boost converters that can provide an isolated bipolar power signal. I have had good experiences with most ( basically anything that looks halfway decently built ) and I have several that have been in heavy use in DIY amplifiers and headphone amplifiers.If you ONLY need -5 and not +5 there may be a cheaper way to invert the signal, but if you need +5 to go somewhere too , it's quite an easy option.Also , last night I bought a bunch of small IC's off Aliexpress that provide an isolated 5V source, but only up to 1W. They are very small and I imagine might work for you.https://www.aliexpress.com/item/32840613546.html