In many solid-state lighting (SSL) applications, such as architectural, area, and spotlighting, color accuracy is crucial. Currently, many products on the market allow for color adjustment during lighting, enabling the setting of white points or CCTs. LEDs are an ideal light source for achieving precise color lighting; color changes are achieved by mixing different colored LEDs, such as red, blue, and green monochromatic lights. When mixing LED colors, the brightness of one or more LED strings needs to be adjusted to achieve the desired color blend. There are many methods for adjusting LED illumination using color mixing, which will be analyzed in detail below.
A single LED chip can only emit monochromatic light. To obtain multiple colors, three primary color LEDs (red, green, and blue, RGB) can be used simultaneously to achieve color mixing. Simply switching the red, green, and blue LED channels can generate seven basic colors: red, green, blue, yellow, purple, white, and light green. To generate even more colors, the brightness of each LED channel can be adjusted, which can be achieved 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 LEDs by controlling the average current flowing through each LED string, and both can be applied to switching power supplies or linear LED drivers. Figure 1 shows a dual-string LED driver based on the TPS92660, which includes a Barker switcher and a linear regulator. Both LED strings can be dimmed using either analog or PWM technology, each with its own 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 sensing voltage outside the IC. For most LED drivers (including switching regulators and linear regulators), the LED current is determined by the following equation:
VREF represents the internal LED current reference voltage of the IC, and RSNS represents the current sensing resistor.
In some cases, the LED current can be adjusted by changing VREF, but it's important to note that not all LED driver ICs have an adjustable current reference voltage. For those that are adjustable, there are generally two ways to achieve this: one is to apply an analog voltage to the reference voltage adjustment pin (an example being Texas Instruments' LM3409), and the other is to adjust the reference voltage using a digital communication interface such as I2C (an example being Texas Instruments' TPS92660, where the I2C interface allows users to adjust the LED current reference voltage via I2C commands).
For analog dimming methods that adjust the LED current sensing voltage, using a potentiometer to change this value is impractical since the current sensing resistor in most applications is less than 1 Ohm. Instead, the IC's CS (current sensing) voltage can be changed by injecting an external DC voltage. Figure 2 shows a typical analog dimming circuit that operates by changing the current sensing voltage; the voltage at the CS pin is determined by the following equation:
Under steady-state conditions, the voltage at the CS pin equals the reference voltage. The LED current can be adjusted by changing the external DC voltage VADJ or the value of the variable resistor R2.
However, analog dimming has a drawback in color mixing applications. The LED color temperature changes with the current, and the brightness and color of the LED may also change during analog dimming, especially when the current change is large. Therefore, under these conditions, the system may not be able to generate the desired color.
PWM dimming
PWM dimming essentially turns the LED on and off at a fixed duty cycle and frequency. Assuming the switching or multiplexing speed is fast enough (typically 200Hz or higher), the human eye can barely perceive the LED's on/off switching. The LED current in PWM dimming is determined by the following equation:
Where IDIM is the dimmed LED current, D is the continuous duty cycle of the PWM dimming signal, and ILED is the constant current supplied to the switched LED string.
Many LED driver ICs are equipped with a PWM dimming input pin that receives a PWM dimming input signal from a microcontroller. Typically, the driver IC only shuts down the MOSFET driver when the PWM dimming signal is weak, while the MOSFET driver turns on when the signal is strong. During the PWM dimming off cycle, the internal circuitry operates normally; this prevents the IC from restarting, which would cause a delay in the rising edge of the PWM dimming signal.
For switching power supply LED drivers, a capacitor is typically installed in the LED string to filter out high-frequency switching noise. This capacitor slows down the rise and fall edges of the PWM dimming LED current, so it can be omitted in high-frequency, low-duration PWM dimming applications. Figure 3 shows the PWM dimming waveform of an LED Barker regulator without an output capacitor.
Analog-PWM dimming
Some LED driver ICs also feature analog-PWM dimming: the IC's dimming pin receives an analog signal and converts it into a PWM dimming signal. The PWM dimming frequency is fixed, while the PWM dimming duty cycle is proportional to the level of the input analog signal.
Analog-PWM dimming is particularly useful for lighting applications that are not suitable for using microcontrollers. It can also be used to implement a thermal protection mechanism when the LED current decreases due to dimming when the LED board temperature is higher than the set point.
Shunt FETPWM dimming
Shunt FET (Field-Effect Transistor) PWM dimming is commonly used for extremely high-frequency LED PWM dimming. Figure 4 shows a shunt FET PWM dimming circuit based on a Barker regulator. An external shunt FET is mounted in parallel with the LED string to quickly bypass (short-circuit) the converter's output current. When the shunt FET is on, the LED string is off; when the shunt FET is off, the LED string is on. Therefore, PWM dimming can be effectively achieved through this shunt FET. Some LED driver ICs also integrate a MOSFET driver for shunt FET PWM dimming, eliminating the need for an external MOSFET driver.
During shunt FET PWM dimming, the switching regulator's inductor circuitry remains continuous, thus eliminating delays caused by increasing or decreasing inductor capacitance. Utilizing a powerful MOSFET driver, the shunt FET can be turned on or off extremely quickly. This results in very sharp rise and fall edges for the LED current in PWM dimming. Shunt FET PWM dimming is ideal for high-frequency PWM dimming applications.
Dimming functionality is crucial for achieving the desired color and brightness in LED color mixing applications. Of the two main methods, analog dimming and PWM dimming, analog dimming can be implemented with relatively simple circuitry and is less expensive, making it suitable for systems without microcontrollers. However, it is not suitable for applications requiring a constant color temperature. PWM dimming, on the other hand, can achieve very precise color temperature by reducing color variations related to LED current levels. PWM dimming typically requires control via input digital signals from a microcontroller, thus resulting in a relatively higher system cost.
