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Driving Backlight LED

Hi, I have some questions about the proper voltage and current to provide for the LED backlight.

1. What is the correct forward voltage and forward current? The DT024DTFT datasheet page 7 lists forward current and has symbol (VF) typical 15mA. 15mA seems reasonable, but shouldn't the symbol be (IF)? The datasheet list forward voltage symbol (IF) typical 12V. 12V seems pretty high is this correct? I was under the impression I would be able to drive it with 3.3V. Also shouldn't the symbol be (VF)?

2. Does the DT024DTFT LED backlight require a current limiting resister or does it come with one? If one is required what is the suggested resister value?

3. Does the LED backlight need to be ON for me to see anything on the screen?

Replies

engineering_seacomp's picture
engineering_seacomp
March 9, 2020

Hi JSteward,

1. The correct backlight forward voltage and current are 12V and 15mA. The datasheet does incorrectly list Forward Current as "Vf" and Forward Voltage as "If". They should be reversed as you have pointed out.

The 3.3V refers to the display's Power Supply Voltage (powers the display drivers), which is separate from the Backlight Voltage. The backlight consists of 4 LEDs in series with forward voltages of 3V each, hence the 12V backlight voltage (see page 4 of the datasheet for backlight circuit diagram). 

2. A resistor should be chosen to provide the backlight LEDs with 15mA. If you are using a boost driver such as the CAT4139 to drive the backlight (recommended), you will need a current set resistor such as the one in the reference design shown here:

https://www.screencast.com/t/MW5S5Vmy5

The 20 ohm resistor (R24) sets the current to 15mA, and the boost driver automatically generates the voltage needed.

3. The backlight does need to be on in order to observe anything on the display.

Thanks!

JSteward's picture
JSteward
March 9, 2020

Thank you for your speedy reply and thank you for taking the time to thoroughly answer my questions! I was able to provide the 12V with an external power supply and use a current limiting resister to provided the 15mA. I can now see my demo program running on the display :) Thanks for the tip with the boost driver I plan to include it in the next revision of my PCB design.