Basics of digital LED control

Most engineers are quite familiar with typical low-power-indicator LEDs, whether they be surface-mount or the classical through-hole packages. All that's needed to use them is a voltage source and a series resistor of the right value. The resistor, of course, keeps the current of the LED within spec — typically less than 5 mA. Tie this to a GPIO pin on a microcontroller and you get one of the world's most common demonstrations — a blinking LED.

A typical LED luminous flux vs. forward current curve shows how luminous flux is nearly linearly proportional to the diode forward current.

However, all of the simplicity goes out the window when you move to a high-brightness, high-current LED with a forward current of well over 350 mA. This is particularly true when you put 10 of them together in a string.

The first issue with a high-brightness LED is the complicated process of efficiently maintaining a high constant current. LED brightness and color both change as a function of current. The accompanying figure shows the luminous flux of the LED — effectively the measurement of the amount of visible light it emits — is a function of the forward current through the LED. This shows the need to maintain a constant forward current, I_F, through the LED to get a consistent color and light output.

Changes in the diode forward voltage affect the forward current.

Consider the case of a simple resistor, R, in series with the LED. The diode forward current is determined by I_F = (V_Source-V_F)/R where V_F is the LED forward voltage. As the source voltage, V_Source, varies, the forward current I_F will also fluctuate, varying the amount of light the LED emits. This clearly shows that the LED must be driven by a power supply that actively regulates I_F.

In general, one important characteristic of LEDs and diodes is that their V_F rises with temperature — even with a constant and regulated forward current. This is another example of the need to properly regulate I_F rather than V_F.

The next major challenge is heat. High-power LEDs get extremely hot. Excessive heat will significantly reduce LED lifespan and possibly cause premature failure. Active control of the LED forward current gives designers the ability to determine the necessary heat-sinking based upon the target forward current and estimated forward voltage. The use of temperature sensors also provides the option of monitoring for possible over-temperature situations. Additionally, there are other issues with high-brightness LEDs that must be addressed. But the intelligence of a DSC (digital signal controller) lets designers handle these issues through the power of software-based control.

LEDs have the amazing ability to change their light outputs almost instantly. This makes them candidates for use in color light-fixtures that provide rapid color change. It's possible to make any color of the rainbow by simply stringing together red, green and blue LEDs and then adjusting the brightness of the appropriate devices.

But dimming each LED becomes a design challenge in this scheme. Because the forward current of the LED dictates the brightness, the obvious approach is to simply raise or lower each LED's forward current. However, this creates a problem, as the color of the LED will also change slightly when its forward current changes. A varying LED color is usually undesirable.

The usual solution is to pulse the forward current rather than directly lowering or raising it. The effect on dimming is the same as a reduced forward current.

Use of digital control greatly simplifies the process of pulsed current dimming. Many DSCs have advanced PWM (pulse-width modulation) modules able to control the power stage of the LED. These PWM modules have override inputs that can quickly and precisely shut off the PWM outputs, thereby sending a precisely pulsed current to the LED.