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I'm working on a handheld gaming project using a LS020B1DD01 LCD panel and am looking for a way to control the LCD's brightness via two buttons. From my understanding, the backlight on this panel is just a set of 3 LEDs (datasheet isn't exactly clear)? And from my basic knowledge and research, brightness of LEDs can be controlled by increasing/decreasing resistance or by using a microcontroller and PWM.

I'm leaning towards using a ATtiny85 and PWM since I only want 4 levels of brightness which I think is possible to define on the software side.

Questions:

  • How do I interpret the backlight driving conditions in the datasheet to figure out the current limiting resistor?
  • From other examples of PWM brightness control, a transistor was used (Control LCD backlight using PWM). What does a transistor do in that application? I'm assuming that I'll need to use this configuration since I'm supplying the LCD with 10V from an external source and not the ATtiny85?

Feel free to ask clarifying questions as I'm sure I haven't explained everything clearly. Thank you!

Backlight Driving Conditions

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The middle column tells you that nominally if you pass 10.8mA through the chain of LEDs then you will see a voltage drop of 10.4V.

The other columns show you how that will very with tolerances. You want a control scheme that will let you set the current.

One way to set the current is to control the PWM on/off ratio. The more on it is, the more the average current flow. For sizing the series resistor you should take your max supply voltage, subtract the nominal forward voltage and then divide that by the nominal forward current.

For example (12v - 10.4v)/10.8mA = 1.6/0.0108 = 148 ohms so use 150 ohms. Then if you drive the transistor at 50% duty cycle you would get half brightness.

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  • \$\begingroup\$ Thanks for the information! This has helped me connect a few dots. Just a few clarifying questions: - If I'm supplying 10V to the LCD backlight, if I want to have the brightest setting, I would set the series resistor to ~185 ohms (10V-7.8V)/12.0mA =~183 ohms. Then drive the transistor at 50% duty cycle via PWM to halve that brightness? \$\endgroup\$
    – user346278
    Commented Mar 8 at 21:44
  • \$\begingroup\$ 10v isn't enough to drive the backlight you included in your question. That aside 50% duty cycle will give you 50% current but the human eye might not perceive that as 'half as bright'. Quite often human responses are logarithmic not linear. \$\endgroup\$
    – Robin Iddon
    Commented Mar 9 at 19:37

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