LCD Backlight Sags during screen writes with Teensy 4.1

Thundercat

Well-known member
Hi, I have a 2.4" Waveshare screen I'm driving with a Teensy 4.1 using 3.3v output for the screen. It's a MIDI project with 4 faders also connected.

When I drag faders, I write to the screen to show graphics. However, the backlight gets brighter when I drag the faders - even though there's no code to brighten the LCD backlight when the faders are dragged.

It makes for a kind of flickering effect that is unpleasant, especially since areas of the screen I'm not writing to become brighter, and it makes it unpleasant to use the unit, especially when light is lower and the effect is pronounced.

Note I have this exact same project also running on a 2.0" screen and this doesn't happen.

I've been advised by a tech guy that I need to supply the screen with an independent 3.3V output using a linear regulator. He is of the opinion that the brightness fluctuations are caused by the Teensy processing data when I'm dragging the faders. He also advised trying a 470uF capacitor for the screen power to decouple the Teensy from it.

Does anyone concur with this assessment, or have a differing suggestion?

If I do add a 5V-3.3V linear regulator, is it OK to use the VIN 5V pin to derive the power for the linear regulator?

Would anyone also advise adding a capacitor to the Backlight Pin of the LCD screen as well?

Thanks for any insights. This one is frustrating as the project works well otherwise.

Best,

Mike
 
The Teensy 4.1 delivers a maximum of 250mA at the 3.3V output (see picture). In the specifications of Waveshare display there is no data on the power consumption of the backlight. You have to measure it..
I think the display uses too much power with the backlight. I would connect the display via a small LM 1117 3.3 volt regulator.

Greetings from germany. Rolf

Teensy-4-1.png
 
This pin is necessary for the power supply of the Teensy. You can also connect a 3.3 volt regulator for the display in parallel.

20230521_133946.jpg
 
Fantastic Rolf thank you!

That’s actually exactly what I’ve done.

The voltage regulator I’m using is a Toshiba and the data sheet says it needs a 1uF cap on both input and output, so I’m using that but I also had added a 100nF just as you showed.

Thank you again.
 
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