Backlight for NHD-0420AZ-FL-YBW-33V3

Nick Wallette

I needed a 20x4 character LCD module for my project, and this one is a good match.  However, when reading the data sheet, I can't figure out what NewHaven is doing with the backlight.

Typically, the LED backlight is either a direct connection to a string of LEDs, or has onboard current limiting (via a resistor or a current regulator.)  The diagram on page 4 of the PDF shows that the LED-A should be connected directly to the supply, and LED-K should be connected to 0V.  This implies that the backlight is current-limited.

However, the supply voltage doesn't make much sense to me.  The module wants to be powered at 3.3V, +/- 200mV.  OK, that's a pretty tight tolerance, but sure, no problem.  OTOH, the LED wants 3.0V, +/- 200mV.  Uh, wait... 3 point ZERO volts?  I don't get it.

If there's an onboard regulator, is it REALLY so marginal that it can't hang with a common 3.3V supply?  If it's a current-limiting resistor, was its value selected to be at the very threshold of reliability at 3.2V, such that it can't handle another 100mA?  More to the point, why doesn't the data sheet give some clue as to what the backlight circuit IS?  If I knew it was a couple strings of 2 series 1.4V fwd-drop LEDs with 0.01R balancing resistors, I could make some educated engineering decisions...

Also, 280mA supply current for the backlight?  Holy geez, how bright is that thing??

None of this adds up to what I'm used to with typical LCD modules, so I'm really not sure what the correct solution would be.  Do I actually need a 1W-capable buck regulator to drop 300mV on my supply rail?  Or is that an order-of-magnitude typo and I need to add a limiting resistor selected for 28mA with a fwd drop of 3.0V?  If so, why does the data sheet not show this in the diagram?  (It even went to the trouble of showing the contrast potentiometer -- with recommended values!)

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  • Engineering Support
    Community moderator

    Hi Nick,

    We have an onboard current limiting resistor (R7) which is tied in the pin 15(LED+).

    Typically, in our spec we mention the recommended supply voltage and current values not the absolute min and max values. You can provide an another 100mA to the backlight, but it depends on the user to provide a lower or higher voltage than the recommended spec values.

    We provided the 3.3V for the backlight at 280mA, and noticed a drop in voltage to 3.18V.

    This display has a full array backlight, which leads to the high current draw as compared to other LCDs.

    Thank you.

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  • Nick Wallette

    Thanks!  This one was baffling me.

    The datasheet linked from Digikey's product page (NHD-0420AZ-FL-YBW-33V3.pdf) has a table of Electrical Characteristics on pg. 5, with Min, Typ, Max columns.  Backlight Supply Voltage (Vled) is min 2.8, typ 3.0, max 3.2 V.  Backlight Supply Current (Iled) is min 200, typ 280, max 350, condition Vled=3.0V.

    I couldn't figure out how the current would vary, at a fixed voltage, by about +/- 40% unless that was supposed to be read as "you should supply 280mA, +/- 40%, with an expected forward voltage drop of 3.0V, +/- 200mV" -- but then, I would've expected to see Vfwd, not Vled...  since Vled implies a source voltage -- but then why is it such an oddball voltage with such small margins?  So it really didn't make sense to me.  lol

    If it's a fixed current-limiting resistor, then I can calculate an appropriate R on my side of the board to work at 3.3V and stay within the current rating.  Though 200mA really seems like a lot -- that's getting into flashlight territory.  This thing's going to light up the room.  haha  OK, I'll play with those values and see what happens.

    Thanks for your help.

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  • Engineering Support
    Community moderator

    Hi Nick,

    Thanks for your feedback!

    Please let us know if you need any further assistance with your display.

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