There are two major approaches to dimming LEDs: PWM and analog. Both approaches have advantages and disadvantages. PWM dimming greatly reduces color changes in the LED with varying brightness levels, because the LED essentially runs at a constant current when it is on and at no current when it is off. However, this advantage comes at the expense of additional logic to create the PWM waveforms.
Analog dimming can use a simpler circuit, but the variable current supplied to the LED means that the regulator supplying the current to the LED must soak up any power not supplied to the LED. This additional power arises from the difference between the raw supply voltage powering the LED/regulator subsystem and the voltage across the LED. That power is wasted as heat. In addition, analog dimming may be inappropriate for applications that require a constant color temperature. An LED’s color will change depending on the current driven through the device.
Where does LED light come from?
The average forward LED current determines the brightness of an LED. The relationship between forward LED current versus lumens of optical output for a given LED is remarkably linear over usable ranges of forward current (IF). Optical conversion nonlinearity increased with IF and photonic output efficiency drops as the LED’s operating current exceeds its linear range. Operation beyond the linear range results in output power converted to waste heat in the LED. This wasted heat complicates thermal design, shortens the LED’s usable life, and wastes energy, which translates into unnecessary operating costs.
An LED’s color temperature metric describes the color of the light emitted by the LED. The color temperature of a given LED shifts with variances in IF, junction temperature, and the LED’s age. “Warm” LED temperatures are more red-yellow, and “cool” LED color temperatures are more blue-green. Many datasheets for colored LEDs specify a dominant emitted-light wavelength instead of color temperature. These, too, are subject to wavelength shifts.
PWM dimming switches the LED on and off at a high rate. The effective IF becomes the time-based average of IF when the LED is on and when it is off. When using the PWM method of LED dimming, the on/off frequency must be faster than the human eye can detect to avoid visible flicker. PWM frequencies of 200 Hz or greater usually avoid flicker problems. Many current PWM LED drivers feature a specialized PWM dimming pin that accepts a wide range of PWM frequencies and amplitudes, allowing a simple interface to external control logic. Often, these LED-dimming driver chips provide several ways to dim an LED.
For example, Texas Instruments’ LM3409 LED driver provides several ways to modulate an LED’s IF using PWM techniques. Many vendors, including Allegro Microsystems, Analog Devices, Linear Technology, and Texas Instruments offer PWN LED drivers.
TRIAC Dimming: TI’s LM3445
In general lighting situations, especially where LEDs are replacing incandescent bulbs, there is a need to permit the use of TRIAC controls, because TRIAC dimming controls have become commonplace over the last few decades. Here, the TRIAC dimmer supplies time-sliced sections of sinusoidal AC voltage to the LED driver. The driver must then translate this chopped AC power into something the LED can use. Texas Instruments’ LM3445 TRIAC-dimmable LED drivers offer 100:1 full range dimming capability, going from nearly imperceptible light to full on in a continuous range. TI’s LM3445 maintains a constant current to large strings of LEDs driven in series off of a standard line voltage. TI’s TRIAC dimmable LED driver allows master-slave operation control in multi-chip solutions, which enables a single TRIAC dimmer to control multiple strings of LEDs with smooth, consistent, flicker-free LED dimming.
Note: This article is based on Texas Instruments' Power Designer newsletter, Number 126.