NHD-4.3-480800EF-CTXP backlight

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

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Comments

9 comments

Date Votes
  • 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. 

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  • 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.

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  • 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.

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  • dnmeid

    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..

    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.

    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?

    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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  • dnmeid

    Investigating the topic a little more now I definitely think the BLPWM pin only makes sense when it is internally connected to the PWM_OUT pin of the HX3869-A02.
    This would mean it gives the PWM signal for controlling an external LED driver circuitry.
    The high voltage should be IOVDD and the frequency should be adjustable via registers between 15MHz and 117kHz. If you adjust the master clock you can even go lower.
    The duty cycle and hence the display brightness can be controlled by registers and may be automatically adjusted when you use the content aware brightness control.

    The only weak point is: this is only my theory. I still don't have the display and need somebody to confirm this.
    MT5555, were you able to meter the pin?

    Dorian

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  • dnmeid

    And another backlight question to Newhaven (I hope somebody from Newhaven kicks in soon!):
    according to the datasheet the 8 LEDs of the backlight are parallel. In the shown circuit I don't see internal resistors and with 120mA in total it makes 15mA and 3.2V per LED, which is not unusual.
    My question is: why are the LEDs parallel? How do you guarantee the uniformity of light distribution? Usually there are tolerances between the single LEDs and this will lead to some brighter and some darker LEDs. And what about LED lifetime? Usually the LED with the smallest forward voltage gets the largest current and is the brightest. It also gets the hottest and that leads to an even smaller resistance and thermal race.
    The datasheet doesn't say anything about the light distribution or backlight lifetime. Can you please specify that.

    Dorian

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  • Michael_L

    I greatly apologize for the delay in response here, most of last week was spent moving to our new location and getting everything up and running.  As we are now settled in, I have had a chance to investigate this and have found that the BLPWM pin is in fact a no connect, and is incorrectly labeled in the specification.  I will update this in our datasheet right away.
    If you wish to PWM the backlight, you would need to do so either on the anode or cathode pin of the LED backlight circuit directly.

    As for your question about the uniformity, there are layers inside the display module (more specifically the backlight unit) that disperse and "guide" the light evenly across the panel.
    The lifetime of the led backlight is rated at 50,000 hours until half-brightness.

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  • dnmeid

    I have had a chance to investigate this and have found that the BLPWM pin is in fact a no connect, and is incorrectly labeled in the specification.

    Oh really, what a pity. Why didn't you at least connect it internally to the PWM_OUT pin of the HX3869-A02?

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  • Michael_L

    Since the backlight circuit does not have an LED driver IC, you would not be able to use this PWM_OUT directly because it can't source the current needed.  If using an external LED driver IC, customers will typically use their own PWM modules since they are typically more configurable, and more familiar to the designer.

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