Industrial LCD monitor power supply voltage question - Page 1
Forum section: electronics / general

author: Milacron
author_rank: Contributor
timestamp: September 01, 2011, 08:39:12 pm
content: Can anyone here explain why, in the Hitachi SX 14Q001* data sheets it has VDD (power supply voltage) at min 3.15, typical 3.3 and max 3.45 volts.....but then in "electrical absolute maximum ratings" it has VDD at 6.0 volts ?This implies to me that the unit could handle 4.97 volts but might not last as long at that figure ? Is that implication correct or completely off base ?(I ask because I need to use that monitor in a control that used an older style 5 voltLCD and outputs 4.97 volts for the VDD pin)*color 5.7" LCD

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 01, 2011, 09:01:53 pm
content: The diagram is clear enough.The VCON is for contrast adjust and this voltage are lower than the base voltage 3.3VAnd so if you need the contrast adjust to operate, you have to follow the rules of the game as is.The VDD it is 3.3V = that's it no exclusions."electrical absolute maximum ratings" = it can take an instant hit with out permanent damage.

author: Unknown
author_rank: Unknown
timestamp: September 01, 2011, 11:48:09 pm
content: Quote from: Milacron on September 01, 2011, 08:39:12 pmThink of the 6.0V as a destruction rating anything above that is likely to cause immediate and likely permanent malfunction.3.3V is the desired supply voltage with the upper and lower figures showing acceptable tolerance.QuoteWhile the unit may be able to handle a 4.95V it will almost not certainly not be able to do this throughout the range of the other specification. Maximum operating temperature for example would almost certainly need to be reduced at the too high supply voltage.QuoteAdding a simple 3.3V LDO regulator to the supply would be a simple enough task. You would also need to look at logic levels to ensure those match.*** a cursory glance at the data sheethttp://www.hitachi-displays-eu.com/doc/sx14q001.pdfsuggests that the 6.0 maximum rating applies to logic level pins. This device is most certainly intended to operate from a 3.3V supply only.

author: Milacron
author_rank: Contributor
timestamp: September 02, 2011, 02:42:24 am
content: Quote from: Kiriakos-GR on September 01, 2011, 09:01:53 pmFor VCON I was planning to use a 10K ohm 15 turn pot to vary the voltage...with the voltage source being the same 4.97 v terminal that powers the VDD. I tried this on an earlier monitor switcheroo and it seemed to work well for contrast adjust. Alas, that monitor still displayed no data, but after further study of the datasheets afterwards, pretty sure that was due to a difference in timing characteristics. (It was an Optrex)I spoke with a Hitachi engineer this afternoon and he thought 4.97 volts would be safe enough but might result in unexpected performance issues.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 02, 2011, 03:07:24 am
content: Quote from: Milacron on September 02, 2011, 02:42:24 amIts your battle choose your weapons correctly.Quote from: Milacron on September 02, 2011, 02:42:24 amYes he had just throw away the responsibility about the advice that he gave to you.There is numerous ways to drop that 5V in to 3V, the simplest one is to add four or five BY127 diodes in series after the 5V stabilizer.

author: Milacron
author_rank: Contributor
timestamp: September 02, 2011, 11:49:47 am
content: Quote from: Kiriakos-GR on September 02, 2011, 03:07:24 amVery interesting, thanks. Can you explain the "stabilizer" more ?

author: ejeffrey
author_rank: Super Contributor
timestamp: September 02, 2011, 12:20:20 pm
content: He means the 5 volt regulator/supply.I wouldn't muck about using diode drops. It will work, but the voltage will vary with current, temperature, and the supply voltage. It will probably still stay within spec, but a 5V to 3.3 V LDO regulator is such a standard item you should just use the right thing. Using diode drops as voltage shifters has its uses, but to 'regulate' a logic supply should only be considered a hack and last resort.Absolute maximums are the safety limits. Exceeding them voids all specifications for the device permanently. Operating limits are the limits under which the device performance is tested or guaranteed. Exceeding them voids all specifications for the device temporarily. The purpose of absolute ratings is to tell you things like how much overshoot your power supply can have at switch-on.

author: Milacron
author_rank: Contributor
timestamp: September 02, 2011, 01:15:07 pm
content: On another forum I moderate, (which is not electronics related but some EE's hang out there) we were discussing this issue and one member suggested the following...---------------------------------The new display might be 5v/3.3v compliant. You would have to call up a engineer at hitachi and find out if it will allow 5v power and logic to be sure. If it is not 5v compliant you will need to use a voltage regulator and a level translator. (links below)http://www.maxim-ic.com/datasheet/index.mvp/id/1966http://para.maxim-ic.com/en/results.mvp?fam=bidirtranslators--------------------------------My question to you guys is- Do you agree that in addition to a voltage regulator, a level translator is needed...and if so, why ?

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2011, 01:36:54 am
content: Quote from: Milacron on September 02, 2011, 01:15:07 pmThe gates in a logic device are designed to operate at a certain voltage. To keep the device small and efficient, there is not much headroom designed into the gates. So if you feed 5 V logic signals into a 3.3 V logic device you might cause it to malfunction or even damage it. A general safe design principle is that if a device is specified to operate at 3.3 V you need to keep your voltage levels at 3.3 V.

author: Milacron
author_rank: Contributor
timestamp: September 03, 2011, 02:10:51 pm
content: Quote from: IanB on September 03, 2011, 01:36:54 amEr....Ah....did that answer the actual question about the level translator ? I mean, whatisa level translator ?

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2011, 03:54:27 pm
content: Quote from: Uncle Vernon on September 01, 2011, 11:48:09 pmYes, the absolute maximum ratings are not conditions under which the device should be subjected to continuously.It may also malfunction or not work at all if the supply voltage exceeds 3.45V, even though it will withstand it and return to functional operation when powered down and back up again.Some ICs even include a negative voltage in the absolute maximum ratings i.e. -0.3V to 7V even though the device is only designed to work from 2V to 5.5V.

author: Unknown
author_rank: Unknown
timestamp: September 03, 2011, 04:00:58 pm
content: Quote from: Milacron on September 03, 2011, 02:10:51 pmAs the name implies, it translates logic signals from one level to another, eg. 5V CMOS to 3.3V CMOS. Searching for level shifter or level translator on Google should provide a decent number of helpful hits. Sparkfun has a tutorial with some quick-and-dirty methods, you don't necessarily have to buy an expensive IC to do it.Absolute maximum rating is about not releasing magic smoke, not about working within specs, or at all.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 03, 2011, 05:32:49 pm
content: Quote from: IanB on September 03, 2011, 01:36:54 amNot even might. Unless the inputs are specifically listed 5 V tolerant, feeding 5 volt logic into a device operating on 3.3 volts (even a device rated to operate on 5V -- all that matters is the actual supply voltage) is almost guaranteed to cause SCR latchup. This will at least stop the device working until you power cycle it, and has a good chance to destroy the chip.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 05, 2011, 10:47:25 pm
content: Quote from: ejeffrey on September 02, 2011, 12:20:20 pmCorrectThanks. LM7805The diode has an very stable internal resistance, the BY127 its huge in size for the specific application,and I will give 95% chances for success with out drawbacks.

author: Milacron
author_rank: Contributor
timestamp: September 06, 2011, 02:52:35 am
content: Quote from: Kiriakos-GR on September 05, 2011, 10:47:25 pmAll of the LM7805 on Newark site have 10 or 20 volt input with 5 volt fixed outup. I need 5 volt input with 3.3 volt output.... does a simple 3 pin semiconductor like the LM7805, but 5 to 3.3 volts exist ?

author: IanB
author_rank: Super Contributor
timestamp: September 07, 2011, 03:51:27 am
content: Quote from: Milacron on September 06, 2011, 02:52:35 amYes, you could look at the MCP1700-330 for instance:http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010642This design from Lady Ada illustrates voltage reguation and level shifting to interface 5 V circuitry to 3.3 V circuitry:http://www.ladyada.net/make/logshield/design.htmlI think other projects from Lady Ada also include 3.3 V level shifting if you browse around a bit.