Constant Current LCD Backlight Driver Question

[yanir]

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.

[helloworld922]

What LCD/driver IC are you using (preferably links to the datasheets)?

[yanir]

New haven display: NHD-3.5-320240MF-ATXL#-T-1
Datasheet:
http://www.newhavendisplay.com/redirect.html?goto=www.newhavendisplay.com%2Fspecs%2FNHD-3.5-320240MF-ATXL-T-1.pdf&action=url


My heart is not set on this driver, I just couldn't find anything better yet.
Backlight Driver STP04CM05MTR
Data sheet
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00177556.pdf

[madires]

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?

Since it's a LED backlight you should limit the current to 20mA.

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?

It 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.

[helloworld922]

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 the TLC5916 LED 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

[yanir]

hellowworld922
Thanks 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?

[helloworld922]

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).