[Technical Analysis] How to achieve illuminator color adjustment by LED light string dimming

In many solid-state lighting (SSL) applications, such as buildings, areas, and spotlights, color accuracy is important. There are also many products on the market that can be tinted during the lighting process, which can be used to set white point or CCT. LED is an ideal light source for precise color illumination. The color conversion can be realized by mixing different colors of LEDs, such as red, blue and green. When mixing LED colors, the brightness of one or more LED strings needs to be adjusted to achieve the desired color mixing. There are many ways to adjust the LED illumination with color mixing, which will be analyzed one by one below.

A single LED die can only emit monochromatic light; to obtain multiple colors, three main color (red, green, blue, RGB) LEDs can be used simultaneously to achieve color mixing. Simply switch between the red, green, and blue LED channels to create seven basic colors, red, green, blue, yellow, purple, white, and light green. To generate more colors, you can adjust the brightness of each LED channel, which in turn can be adjusted by adjusting the current flowing through each LED string.

Fundamentally, there are two ways to achieve LED dimming: analog/linear current control, and pulse width modulation (PWM). Both adjust the brightness of the LED by controlling the average current flowing through each LED string, and both can be applied to a switching power supply or a linear LED driver . Figure 1 shows a TPS92660-based dual-lamp LED driver with a Barker switch and a linear regulator. Both LED strings can be dimmed using analog or PWM techniques, each of which has advantages and disadvantages. In most applications, the choice of dimming method is generally based on color mixing performance requirements.

Analog dimming

Analog dimming is achieved by adjusting the LED current reference voltage inside the IC or by adjusting the LED current sense voltage outside the IC. For most LED drivers (including switching regulators and linear regulators), the LED current is determined by the following equation:

Where V _REF represents the IC internal LED current reference voltage and R _SNS represents the current sense resistor.