Newhaven Display Forum

Newhaven Products => Character LCDs => Topic started by: mh on April 28, 2014, 02:56:13 PM

Title: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: mh on April 28, 2014, 02:56:13 PM
Hello,
I'd like clarification on what the schematic shows to drive the backlight for this device. Basically, no resistors for the cathodes. Is this correct? I plan to drive the anode with 3.3V, same as the rest of the unit. The specs say 1.9V to drive red, 3.0v for green and blue, at 30mA for each. Is that max current handled internally, or should resistors be added to each cathode?
Title: Re: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: Michael_L on April 29, 2014, 10:19:24 AM
There are no current limiting resistors on the backlight circuit of this display.  You would need to add these to limit the current.
Title: Re: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: mh on April 29, 2014, 10:50:30 AM
Hi, thanks Michael.

I guess the Vf on those diodes needs to be inferred from the typical voltage vs current in the datasheet? Really, that datasheet is pretty unclear, especially when the reference circuit shows the each LED's cathode going to ground.

Please provide some guidance (LED appx Vf at ~3V would be nice); I'd rather not have to add constant current circuitry to drive a couple of LEDs.
Title: Re: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: mh on April 30, 2014, 02:46:39 PM
*crickets*
Title: Re: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: Michael_L on April 30, 2014, 02:56:28 PM
The requested information is below:

Red LED Vf = 1.9V
Blue LED Vf = 3.0V
Green LED Vf = 3.0V

This information can also be seen in the mechanical drawing on page 3 of the display's datasheet.
Title: Re: Driving backlight for NHD-C0220BiZ-FSRGB-FBW-3VM
Post by: mh on April 30, 2014, 04:15:00 PM
Thanks. I hadn't noticed that tiny print. It actually doesn't list Vf, but rather suggested supply, ie 1.8-2.2v. Note that the current listed there contradicts what's listed on page 5. Just stating each LED's Vf would be so much simpler.

Anyway, thanks for the answer. I will work with that.