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I am implementing a LED driver for a LCD that has the particular arrangement of the backlight LEDs shown in the picture bellow, 4 parallel LEDs. It seems quite inconvenient not to have them in a series string as the typical voltage needed over the string is now only 3V, bellow what a typical boost constant current LED driver implementation can achieve with standard 3.3V input voltage. Is there an common appropriate way to do this that enables PWM dimming ?

LCD Parallel LEDs

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  • \$\begingroup\$ Depends what voltage supplies you have available. Often these sort of displays are powered from 5V which makes supplying the LEDs very simple. \$\endgroup\$ – Finbarr Sep 6 '17 at 9:06
  • \$\begingroup\$ 3.3V and 5V are available. But any solution should support PWM dimming, i added that detail to my question. \$\endgroup\$ – Stonie Sep 6 '17 at 9:52
  • \$\begingroup\$ Then power it from 5V using PWM, what exactly are you having difficulty with? \$\endgroup\$ – Finbarr Sep 6 '17 at 10:25
  • \$\begingroup\$ I am looking for good industry practices to solve this problem, what are the good solutions here?. A solution needs to constant current control and support for dimming through PWM. Obviously it is possible to solve this with a constant current LDO and PWM'ing the enable pin but that would output a PWM waveform out of the board directly (EMC hazard) instead of a regulated waveform out as for example the tps61169 would do for a string of LEDs with a voltage high enough to warrant its use. \$\endgroup\$ – Stonie Sep 6 '17 at 10:56
  • \$\begingroup\$ Why do you think you need a constant current driver? Unless your supply voltage or LED characteristics are going to vary widely, just set the maximum current with a resistor. \$\endgroup\$ – Finbarr Sep 6 '17 at 11:48
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Treat the whole backlight as a single LED, and go with the manufacturer specs of voltage and current. The fact that it happens to be implemented with four LEDs inside is irrelevant, and there is nothing you can do about it anyway.

If you want to adjust the brightness via PWM, then drive it in pulses. The simplest way is to arrange for the maximum current when the switch is on. The duty cycle then becomes the fraction of maximum brightness.

Keep in mind that humans perceive light intensity logarithmically, not linearly. Therefore, brightness levels of 1, 1/2, 1/4, 1/8, etc, will appear to be about equal steps.

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  • \$\begingroup\$ Thanks for the answer, the parallel LEDs surely seems odd. What do you think about the EMC aspect of outputing a PWM wave directly out of the board to the LCD. I did consider using a LDO in CC configuration and PWM'ing the enable. The fundamental surely will be low at some hundreds of hertz but i guess the risk is a hazard nevertheless without a ferrite perhaps. \$\endgroup\$ – Stonie Sep 6 '17 at 11:38
  • \$\begingroup\$ Parallel LEDs are not so odd. It let's you drive the LCD backlight with 5V and a resistor. 5V is obviously super common and a resistor is less than a penny. No boost circuit needed. Add an NPN for PWM driving costs another 2 cents. \$\endgroup\$ – Vince Patron Sep 6 '17 at 16:25

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