NHD-TFT40 PWM pin

dttworld

• August 28, 2017 08:07

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

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  Saurabh_B

  • August 28, 2017 09:07

  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

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  dttworld

  • August 28, 2017 10:07

  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. 

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  Saurabh_B

  • August 28, 2017 11:07

  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?

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  dttworld

  • August 28, 2017 13:07

  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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  Paul_B

  • August 29, 2017 13:07

  Morning,
  
  The NHD-TFT40 is designed to work with our 24-bit Parallel RGB Interface TFT displays that require the RGB pixel data to be clocked in continuously.
  
  The backlight driver defaults to 40mA and can boost up to ~24VDC for up to 7-LED strings. Check your display datasheet for the string configuration.
  
  Per your request I pulled the NHD-TFT40 from stock and connected it to the NHD-4.3-480272EF-ATXL#.
  
  Please see the attached images, note that when the PWM signal is tied to GND the backlight turns off as expected.

  

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  dttworld

  • August 29, 2017 15:07

  Thanks for running the test on your end.
  
  I am using the eval board for just the backlight function set to 90mA.
  
   I should be receiving my parts from Mouser on Wednesday and will provide an update after I do some testing.

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  dttworld

  • September 01, 2017 15:07

  So I swapped out the backlight IC with one bought from Mouser. Unfortunately, the PWM pin tied to gnd still doesn't turn off the backlight current. For this test I am actually lighting up 5 LEDs in parallel with the demo board. This really doesn't make sense to me.
  
  Could you run a test where you light up 5 parallel LEDs and see if tying PWM to gnd turns the LEDs off?
  
  Thank you

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  dttworld

  • September 05, 2017 15:07

  I believe I understand my problem now. The backlight converter on the eval board is a boost converter designed to drive a series LED backlight. I actually need a buck converter designed to drive the parallel LED backlight found on the HD-1.8-128160EF-CSXN-F.

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  Spletonna

  • September 30, 2017 15:07

  Ah, I appreciate you commenting back on fixing your issue. It's helped me learn a few things myself.

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