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.
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 VREF represents the IC internal LED current reference voltage and RSNS represents the current sense resistor.
In some cases, the LED current can be adjusted by changing VREF, but it is worth noting that not all current reference voltages in the LED driver IC are adjustable. For those tunable ICs, there are usually two ways to do this: one is to apply an analog voltage to the reference voltage regulation pin (one example is Texas Instruments' LM3409), and the other is to adjust the reference using a digital communication interface such as I2C. Voltage (example is Texas Instruments' TPS92660, where the I2C interface allows the user to adjust the LED current reference voltage through I2C commands).
For analog dimming by adjusting the LED current sense voltage, since the current sense resistors in most applications are less than 1 Ohm, it is not practical to use a potentiometer to change this value. Instead, the CS (current sense) voltage of the IC can be changed by injecting an external DC voltage. Figure 2 shows a typical analog dimming circuit that operates on the principle of changing the current sense voltage. The voltage at the CS pin is determined by the following equation:
At steady state, the CS pin voltage is equal to the reference voltage. The adjustment of the LED current can be achieved by adjusting the value of the external DC voltage VADJ or the variable resistor R2.
However, in color mixing applications, analog dimming has one drawback. The color temperature of the LED changes with the current, and the brightness and color of the LED may change during analog dimming, especially when the current varies greatly. Therefore, under these conditions, the system may not be able to generate the desired color.
PWM dimming
PWM dimming actually turns the LEDs on and off at a fixed duty cycle and frequency. Assuming that the switching or multiplex speed is fast enough (usually 200 Hz or higher), the human eye barely notices the on/off switching of the LED. The LED current with 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 accepts the PWM dimming input signal from the microcontroller. In general, the driver IC turns off the MOSFET driver only when the PWM dimming signal is weak. The MOSFET driver will turn on when the signal is strong. During the PWM dimming off period, the internal circuitry is fully operational; this prevents the IC from restarting, which in turn causes a delay in the PWM dimming rising edge.
For switching power LED drivers, a capacitor for filtering high frequency switching noise is usually installed in the LED string. This capacitor can slow the rising and falling edges of the PWM dimming LED current, so this capacitor can be omitted in high frequency, low duty cycle PWM dimming applications. Figure 3 shows the LED Barker regulator PWM dimming waveform with no output capacitors.
Analog-PWM dimming
Some LED driver ICs also feature analog-PWM dimming: The IC's dimming pin receives the analog signal and converts it to a PWM dimming signal. The PWM dimming frequency is fixed while the PWM dimming duty cycle is proportional to the input analog signal level.
Analog-PWM dimming is especially useful for lighting applications that are not suitable for use with microcontrollers, as well as for thermal protection when the LED current is dimmed due to dimming of the LED panel temperature above the set point.
Shunt FETPWM dimming
Shunt FET (Field Effect Transistor) PWM dimming is typically used for very high frequency LEDPWM dimming. Figure 4 shows a shunt regulator based shunt FET PWM dimming circuit. The external shunt FET is mounted in parallel with the LED string to quickly bypass (short-circuit) the output current of the converter. When the shunt FET is turned on, the LED string is turned off; when the shunt FET is turned off, the LED string is turned on. Therefore, PWM dimming can be effectively realized by this shunt FET. Some LED driver ICs also integrate a MOSFET driver for shunt FETPWM dimming, eliminating the need for an external MOSFET driver.
During shunt FETPWM dimming, the switching regulator's inductive circuitry remains continuous, so there is no delay due to increasing or decreasing inductor capacitance. With a powerful MOSFET driver, the shunt FET can be turned on or off at an extremely fast rate. As a result, the PWM dimmed LED current has very sharp rising and falling edges. Shunt FETPWM dimming is ideal for high frequency PWM dimming applications.
To achieve the desired color and brightness in an LED color mixing application, dimming is critical. Among the two methods of analog dimming and PWM dimming, analog dimming can be realized by a relatively simple circuit, and its cost is low, and it is suitable for a system without a microcontroller. But it is not suitable for applications that require a constant color temperature, while PWM dimming can achieve very accurate color temperatures by reducing the color variations associated with LED current levels. PWM dimming usually requires input digital signals from the microcontroller to control, so the system cost is relatively higher.
July 30, 2023
