We are often asked how to drive a backlight in our displays. There are three main ways to do so: with an LED driver, a current generating resistor, or a current generating resistor and a transistor. We will explore the benefits and drawbacks of these choices.
It’s important to remember that display backlights are typically LED arrays and, like all diodes, LEDs are all about current. Every datasheet will list the necessary current for the backlight, typically called IF (forward current) or ILED.
Method 1: LED Driver
Using an LED driver is the ideal way to drive an LED backlight. An LED driver is current controlled so the backlight brightness is consistent. If a constant voltage is used instead of a constant current, the backlight brightness can change with the LED temperature. The brightness can also be controlled via pulse width modulation (PWM), which allows designs to be usable in a wider variety of lighting situations.
For more information about LED drivers, check out this helpful blog post.
It’s important to note that an LED driver will not work in all situations. Some display backlights include an internal diode with a forward voltage of 3.3v. If this is the case, the drive will be unable to turn off the backlight. Additionally, some backlights have a forward voltage below 3.3v. In this case, a different method will have to be used or a series resistor will be needed so the voltage is above the forward voltage.
Method 2: Resistor in series with constant voltage source
This method is less efficient than an LED driver, but is flexible and uses common parts. Plus, any forward voltage on the LEDs can use this method. However, this method leaves the brightness open to change with LED temperature and does not allow for brightness adjustment..
To calculate the value of resistor to use, find the supply or forward voltage and current specifications for the backlight from the datasheet. Determine what your voltage source and voltage value will be. Calculate the voltage drop between the voltage source and the forward voltage spec of the LED. Then use Ohm’s Law to calculate the resistor value (V=IR so R=V/I).
Method 3: Resistor in series with the LED and a transistor with the emitter connected to GND
The final method is to use a resistor in series with the backlight and a transistor with the emitter tied to ground. This method is similar to method 2, but with the added bonus of being able to control the backlight brightness. Connect a fairly large resistor between a PWM signal and the base of the transistor. This makes the voltage drop between the collector and emitter close to zero.
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