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NHD-TFT40 PWM pin

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dttworld:
Hi -

I have 2 "NHD-TFT40" boards that do the same strange thing. When the PWM pin is connected to ground the display current does not shut off.  Can someone at Newhaven replicate this test?

Thank you

Saurabh_B:
Hello,

Can you tell me what pins are drawing current?

The display VDD itself will still be powered without the backlight, for this you would need to trigger the display's standby mode. (DISPOFF)
After triggering this, you will still get current draw for the DC/DC converter used to boost the voltage. (5V input)

The PWM signal only controls the brightness of the backlight. The backlight isn't tied to any circuits on the display that would cause it to shut down or pause.
The PWM signal doesn't stop the current drain for the backlight voltage booster either, to stop this you would need to shut down the power being provided to the +5V input of the NHD-TFT40

dttworld:
Hi -

I am trying to control the backlight current using the PWM signal. Right now I am using the NHD-TFT40 board to supply 90mA to the backlight off an NHD-1.8-128160EF-CSXN# display. I don't understand why tying the PWM pin to GND doesn't turn off the backlight completely (PWM duty cycle = 0%). This is also labeled as a /shutdown pin on the FAN5333BSX backlight IC. 



Saurabh_B:
How are you tying this to the 1.8" TFT?

There are multiple cathodes for the backlight on this display. Do you have a schematic for how this is setup?

dttworld:
Hi -

I am connecting to the display through a FPC breakout board. The backlight connections have been double checked and things are wired properly (including multiple cathode connections). The backlight lights up properly but does not shutoff with PWM pin tied low. I am suspecting a problem with the backlight IC and have ordered some "genuine" parts from Mouser Electronics.

Please verify in your lab that the PWM works as intended. I'm not going to draw conclusions until I do an IC swap.

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