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I need to design a backlight unit that will change its duty cycle and constant LED current via a microcontroller. My supply is a SMPS with 12V - 20A output. I need to drive 16 * 16 = 256 LEDs(seperate 16 LED strings each combined of 16 LED). LEDs have 2.1 forward voltage and typical current is 60 mA.

Most of the LED Driver chip work in the same way. To output a reference voltage and let us to adjust constant current via an external resistor to flow from. Like this below:

led driver symbol

However, once you put the resistor there you are not able to change the current. So, I draw this circuit:

schematic

driver chip is lm3410 and analog switch is NX3L1G3157G.

PCB

You see, I thought that I can switch between two external resistors via a deMUX this way I will be able to have at least two current levels. And then I can use few of the same configuration to get what I need.(Red circles are external resistors and the switch)

The thing is, I couldn't find any other switch that supports more than 350 mA and 100m Ohm resistance except NX3L1G3157G. And it has only 2 switch. Also, probably I will have to change the LEDs to another one with higher luminance and higher current flow so I will need a switch which can support up to 500 mA. Also, I need 5 current levels like 100% DT(Duty Cycle) - X mA, 50% DT - 2X mA, 33% DT - 3X mA, 25% DT - 4X mA and 20% DT - 5X mA. I could still have managed to do that if we don't need to care for the PCB area, but we do!

Also, I am aware of some LED driver chips provides limited current adjustment register like MC34844 but they do not let more than 30 mA to flow per each channel so useless for me.

So, since I had all my hidden answers from this forum, again, I ask help from other experienced engineers. Do you have any suggestion for my trouble? I need to adjust the current flow through LEDs to 500 mA (20% DT), 400 mA (25% DT), 300 mA (33% DT), 200 mA (50%DT) and 100 mA continuous. Regards.

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    \$\begingroup\$ The top circuit that you believe you cannot change the current has the ability to have the LEDs dimmed by the input called "PWM dimming". This will adjust the average current flow and do what you want so, because this seems so obvious you must mean something else. Please clarify why PWM dimming doesn't work for you and, as an aside explain the relevance of the circuit you have drawn - it appears to contribute nothing to the question and is difficult to understand. \$\endgroup\$
    – Andy aka
    Jan 9, 2016 at 18:21
  • \$\begingroup\$ PWM dimming works like this: You always have a stable current flow(say 10 mA) but when you do dimming it decreases the duty cycle. This is how to do brightness adjustment. So actually, avarage current decrease this way, but I want avarage current to be same. I want to decrease the duty cycle but increase the current with reverse proportional to decrease of the duty cycle. That I can not do. \$\endgroup\$
    – Alper91
    Jan 9, 2016 at 18:27
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    \$\begingroup\$ If you want average current to remain the same then I don't understand what this question is about. \$\endgroup\$
    – Andy aka
    Jan 9, 2016 at 18:30
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    \$\begingroup\$ It would help anyone answering to understand why you would want to change the duty cycle if you want the current to be the same? I don't understand the application - I can see how to fix it but it makes no sense to do this (yet). \$\endgroup\$
    – Andy aka
    Jan 9, 2016 at 18:51
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    \$\begingroup\$ @Andyaka - It appears that he's trying to make a constant-brightness strobe with variable flash duration. So he wants to decrease the light duration (PWM duty cycle) at the same time he increases the instantaneous brightness so as to maintain constant average light output. \$\endgroup\$
    – WhatRoughBeast
    Jan 9, 2016 at 19:03

1 Answer 1

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If I understand your text you are asking how to control LEDs in the following scenarios:

  • Continuous backlight dimmable.
  • Strobed backlight where LEDs strobe in-synch with 3D lenses, also dimmable but at same average brightness as continuous mode.

You seem to think that you will have to use a different peak current for the two situations despite using PWM control. This is incorrect. What you do is set the max current of your system so that at, say, 40% PWM the LEDs are giving the required brightness. When you switch to 3D mode you pulse at 80% PWM but modulate the LEDs at the required strobe rate. Effectively you have a high-speed PWM controlling the average current to the LEDs and a low speed strobe synchronised with your camera.

enter image description here

Upper trace shows LED PWM current waveform during strobed mode. Note long on-time. Lower trace shows continuous LED lighting at half-current thus maintaining same average brightness on a longer time scale.

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  • \$\begingroup\$ That was genius! So all I have to do is to find a LED driver with high power and high dimming frequency. Thanks! \$\endgroup\$
    – Alper91
    Jan 9, 2016 at 20:08
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    \$\begingroup\$ Yes, but just watch the absolute max current ratings of the LEDs. You don't say what the strobe rate is but I assume 25 - 30 Hz and this should be fast enough that the LEDs don't get too hot during one strobe. Someone else may be more familiar with that aspect. I would imagine a PWM frequency of 10 to 100 times the camera frame rate would be sufficient but there may be other factors such as exposure time, etc., that could cause discernible flicker in the video. \$\endgroup\$
    – Transistor
    Jan 9, 2016 at 20:12

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