Constant Current LCD Backlight Driver Question - Page 1
Forum section: electronics / projects

author: yanir
author_rank: Regular Contributor
timestamp: January 02, 2013, 09:20:30 pm
content: If an LCD Backlight requires 20mA @ 19.2 V and I use a driver IC that supplies 80mA, does the backlight just take the 20mA it needs or do I fry backlight?Or to put it another way does a constant current source always put out the specified current regardless of load (seems to be by definition) and how does this impact parts that require less current?Thanks in advance.

author: helloworld922
author_rank: Regular Contributor
timestamp: January 02, 2013, 09:35:25 pm
content: What LCD/driver IC are you using (preferably links to the datasheets)?

author: yanir
author_rank: Regular Contributor
timestamp: January 02, 2013, 09:39:53 pm
content: New haven display: NHD-3.5-320240MF-ATXL#-T-1Datasheet:http://www.newhavendisplay.com/redirect.html?goto=www.newhavendisplay.com%2Fspecs%2FNHD-3.5-320240MF-ATXL-T-1.pdf&action=urlMy heart is not set on this driver, I just couldn't find anything better yet.Backlight Driver STP04CM05MTRData sheethttp://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00177556.pdf

author: madires
author_rank: Super Contributor
timestamp: January 02, 2013, 10:17:54 pm
content: Quote from: yanir on January 02, 2013, 09:20:30 pmSince it's a LED backlight you should limit the current to 20mA.QuoteIt puts out up to the specified current. The magic lays in R = U / I :-) If your part sinks more current than the max. allowed one it simply overheats or breaks.

author: helloworld922
author_rank: Regular Contributor
timestamp: January 02, 2013, 10:41:23 pm
content: The driver will try to sink the set current amount, and I can't see anything in the LCD datasheet which would suggest if it has any circuitry in there to limit the current, so you could be driving 80mA through the LED and fry it.In theory if you use a large enough external set resistor you might be able to convince the driver that the target current is 20mA, but there's no guarantees with that driver because the datasheet doesn't actually specify what resistance is needed for smaller target currents.I've recently used theTLC5916LED driver. On Digikey it's a cheaper chip (~$1.65 vs. $2.16 USD, single unit), gives you 8 output channels instead of 4, it's specified for regulating 3mA-120mA/channel with a maximum LED drive voltage of 20V, and works beautifully with 3.3V or 5V logic.p.s. I sound like I'm a salesman for TI or something, I'm not

author: yanir
author_rank: Regular Contributor
timestamp: January 02, 2013, 10:49:55 pm
content: hellowworld922Thanks I'll check out that part.I was looking this is the wrong way. I thought the driver was supplying the current to the LEDs but instead the driver is sinking current downstream of the LEDs causing the specified current to flow through them. Do I make sense now?

author: helloworld922
author_rank: Regular Contributor
timestamp: January 02, 2013, 11:03:34 pm
content: Yep, those are both sink drivers so they need to be downstream of the LED.From what I understand most IC's are better at sinking current then sourcing it (has to deal with hole mobility vs. electron mobility). It's possible to make LED source drivers, though personally I've never seen/used one (keep in mind I'm fairly new to electronics myself).