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NHD-4.3-480800EF-CTXP backlight

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dnmeid:
Hello,
we want to use the mentioned display but I'm not exactly sure how to design the supply for the backlight.
The datasheet shows eight LEDs in parallel between pins 2 and 3 and a PWM input on pin 39.
How should this be understood?
I guess I have to connect a constant current source with 120mA to LED-A and LED-K and am able to switch the backlight with a 3.3V logic level signal on the PWM input.
So the PWM signal is fed into the display and not delivered by the display.
Is this right or did I got it wrong?
What is the recommended way of connecting the backlight?

Dorian

MT5555:
Could someone answer the above question.

Because I have the same question.

I've connected :

pin 2 to 3V
pin 3 to GND
pin 39 to a PWM going at 10kHz

I see the LEDs in the frame light up on power up.  But I cannot seem to control their brightness (as far as my eyes can see) by varying the PWM frequency. 

Is the backlight in this LCD fixed to a certain level of brightness with the BLPWM (pin 39) being of no real use.  Please confirm. 

dnmeid:
MT5555, while I still have not an answer to my original question, I just want to make sure that you are trying to adjust the pulse width of the PWM, not the PWM frequency. The PWM frequency should not matter as long as it doesn't lead to too short on or off times.

MT5555:
Hi,

I'm trying both, adjusting the duty cycle and the frequency.  Doesn't seem to make a difference as far as my eyes can see.

How about you? 

My guess is that the whole purpose is to reduce the average ON time per second of the LCD.  This in turn affects its overall brightness.  Whether that's done via duty cycle or frequency in theory should not matter too much.  But hey, what do i know..

Have you got anything displaying on screen yet. 

If you're using the same display as I am NHD-4.3-480800EF-CTXP, would you like to stay in touch via email?

Let me know please.  Thanks.


dnmeid:

--- Quote from: MT5555 on July 23, 2014, 01:41:54 PM ---My guess is that the whole purpose is to reduce the average ON time per second of the LCD.  This in turn affects its overall brightness.  Whether that's done via duty cycle or frequency in theory should not matter too much.  But hey, what do i know..

--- End quote ---
With the duty cycle you adjust the ratio of the on-time to off-time this will affect the brightness. You are right, with the frequency you are adusting the on-time per second too, but at the same time you are adjusting the off-time likewise.
If you have 50% duty cycle you have 50% power at 100Hz and the same at 10kHz.

The only important caveats with frequency are, that you have to choose it high enough so you don't see flickering (typically >100Hz) and and low enough so your smallest desired on-time and off-time can be switched properly. For example if you have a PWM frequency with 10kHz one period is 0,1ms. If your desired resolution is 8 Bit, the smallest step is 0,1ms/256 = 390ns. So you have to make sure that the switch you are using is able to go from full on to full off within at least that time and vice versa. To minimize nonlinearities in the far ends of your range it is better to have a switch witch even only uses less then half of that time.


--- Quote from: MT5555 on July 23, 2014, 01:41:54 PM ---Have you got anything displaying on screen yet. 
If you're using the same display as I am NHD-4.3-480800EF-CTXP, would you like to stay in touch via email?

--- End quote ---

Sure, but I don't have the screen yet in the lab. I'm still waiting for the order.
But I'm supposed to do some PCB in the meantime, so I'd really like to know how the control of the backlight is meant to be.

If the described control for you doesn't work, did you try to check if pin 39 maybe is an output from the displays integrated controller. This would be the second thing I could think of. That this pin provides the PWM signal for an external switch. The HX8369-A02 built into the display at least has a PWM output for what Himax calls "content aware brightness control". Look at page 166 of the HX8369-A02 datasheet.
The longer I think of it the more likely it seems, that the PWM is an output and not an input like said in the datasheet (or maybe I've just seen it from the wrong direction).

MT5555, can you please check that?

Dorian

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