Backlight voltage - current driven per cathode
Hi all,
We are working on a design that includes the NHD-2.8-240320AF-CSXP-F display.
I would like the backlight to be dimmable, and since we already use a TLC59108, I'm wondering if we could also use it to drive the current display.
Since the TLC59108 is a Constant-Current LED Sink Driver with a maximum of 30mA per pin, I'm thinking of connecting each of the 4 cathodes to 1 input pin, and setting the current per pin to 25ma.
Adding up all 8 pins would then give a maximum current of 100mA, as per the datasheet of the display.
Would driving the cathodes individually like this be suitable? And if so, am I correct in thinking that the drive voltage connected to the anode of the backlight could be 5V since I'm limiting the current to stay within spec?
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Official comment
Hi,
I am unclear about what you meant in your connections for the anode and cathodes of the backlight, so I would like to clarify. The cathode pins of a diode are the negative terminal and should all be connected to ground, whereas the anode pin is the positive terminal and is connected to the supply. So pins 34 through 37 of the display (backlight cathode pins) should all be connected to ground, as seen in the image below.
With regards to using an external controller to drive/dim the backlight, this is fine to do. So long output of the driving controller is a pwm signal, it will be able to drive backlight with no issues. For your case with the TLC59108, tying 4 input pins together and connecting them directly to the backlight anode (pin 38 of the display) will work.
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Hi,
Thanks for your reaction. The TLC59108 led driver that I'm using is of a constant current sink type. This means that you normally connect the cathode's of the LED's to it, while connecting the anode straight to your voltage source.
As opposed to a constant current source type, where you would connect the anodes of the led's you're driving to the chips output pins.
When PWM'ing the pins of the TLC59108, they might not switch on at exactly the same time. So tying them together could cause to much current to briefly flow trough one of the pins. That is why I prefer connect a separate cathode to each of four pins, as shown here:
So this would be similar to this example, except that I would only connect 4 LED pins of course:
Would this be safe to do?0 -
Hi,
Yes, driving the cathode pins separately in this way is safe to do. So long as the voltage for each pin is the same.
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