RA8875 humming when backlight-PWM-value < 255

[local_dani_21]

Hi adafruit

I use a RA8875 to drive your 5" TFT and have set it up using SPI. Drawing bitmaps works well. I'll build a living-room installation with several displays and I want to be able to turn the screens on (PWM 255) and off (PWM 0). However, as soon as I set the backlight to a value < 255 using

Code: Select all

tft5.PWM1out(0);

, I get a humming/buzzing until I set the value back to 255. The humming is too loud as I would want this in the livingroom.

BTW: I'm using the +25mA backlight-jumper.

Do you have a suggestion what I could do to remove the humming and have an all quiet installation at all brightness levels?

Thank you very much, Dani

[adafruit2]

sounds like the backlight driver is oscillating. its an artifact of the PWM rate on the RA8875. not sure if its possible to change the PWM frequency. if you can, try driving the LITE pin from an external microcontroller with varying frequencies until you find one that isn't noisy.

[local_dani_21]

Hi adafruit

That I find a very unexpected answer. Do I summarize this correctly: You sell a RA8875-breakout board along with a 5" TFT 800x480 pixel display and sketch the ability to dim the backlight using the onboard PWM backlight driver over SPI and then tell me that this driver chip is oscillating and I should use an extra microcontroller to take over the job of this driver?

Please tell me there is another approach to that problem. Why else would your technicians have put this driver on the board in the first place if it doesn't work as it should?

Thank's for going over it once more.
Regards, Dani