Design of new white light LED module drive circuit

Due to the impact of the current greenhouse effect and energy crisis, people are paying more and more attention to energy-saving technology. LED lighting has the advantages of energy saving and long life. As a new green lighting technology, LED lighting technology is becoming more and more widely used. LED white light lighting is usually composed of multi-chip LED modules with three primary colors: blue, green, and red. Since the aging characteristics of each LED chip in this LED module are different, the color temperature will change with time and temperature. This brings limitations to the use of white light LEDs. This article analyzes the driving mode of self-illuminating LED modules and proposes a design scheme for a new white light LED module driving circuit.

1. Driving of white light LED modules

For the driving mode of traditional light sources, most of them exist in the form of constant voltage, which makes the driving of traditional light sources very simple. In the linear range, the luminous intensity of the LED is proportional to its driving current and forward voltage drop, and changes with temperature, which requires a constant current to drive the LED. In addition. There is an important indicator of white light LED modules, which is color temperature. Color temperature is expressed in absolute temperature K. When a light source has the same color as a black body, the absolute temperature of the black body at that time is called the color temperature of the light source. The color temperature of white light LED depends on the composition ratio of the three color components in the emitted white light.

In order to adjust the color temperature of the white light LED module light source, it is necessary to control the driving current of the blue, green and red LED chips in the white light LED module separately, which requires three independent constant current driving circuits. At present, the constant current driving circuit is generally implemented by a switch mode PWM controller, and the adjustment of the driving current is mainly divided into analog adjustment mode and PWM adjustment mode.

The analog regulation method uses a PWM controller to regulate the drive current to stabilize the LED drive current and obtain a stable output light intensity. Since the feedback control signal of the PWM controller is usually a voltage value, the analog regulation uses a measuring resistor to sample the output drive current and convert it into a feedback voltage signal. The PWM controller stabilizes the feedback voltage to achieve the purpose of stabilizing the output drive current. The circuit schematic is shown in Figure 1.

There is a big problem with the method of using analog methods to adjust the output current to adjust the LED output light intensity: since the output wavelength of the LED will change as the driving current of the LED changes, the color temperature of the white light LED module will change under different output light intensities.

In order to overcome the above-mentioned shortcomings, people have developed a PWM regulation method for LED drive current.

2 PWM regulation method of LED drive current

PWM regulation refers to a method of driving LEDs using a pulse drive waveform with a specific duty cycle within one cycle. The average drive current of the LED depends on the duty cycle of the pulse waveform and the rated drive current of the LED. When the rated drive current of the LED is fixed, the average drive current of the LED can be changed by changing the duty cycle, thereby changing the output light intensity of the LED. In order to avoid flickering effects, the frequency of the pulse waveform is usually greater than 200Hz.

Linear Technology's LED driver chip LT3474 is an LED constant current driver control chip with adjustable rated output current, and the maximum drive current can reach 1A. It provides two driving modes: analog regulation mode and PWM regulation mode. In the PWM regulation mode, by changing the duty cycle of the external PWM signal, a dimming ratio of 400:1 can be achieved. The typical application circuit of its PWM regulation mode is shown in Figure 2, where the external PWM control signal is applied by the PWM pin. [page]

Figure 3 shows the measurement results of the external PWM pulse drive signal and the output drive current under PWM regulation mode. It can be clearly observed from Figure 3 that the LED drive current is controlled by the external PWM signal. The output reference voltage of the REF pin is 1.25V. If the VADJ and REF pins are short-circuited, the output rated LED drive current is the maximum output current (1 A). The regulation of the rated drive current JLED of the LED is achieved by changing the voltage of the VADJ pin, which can be achieved by dividing the 1.25 V reference voltage output by REF. The rated current of the output LED is obtained by the following formula:

3 Design of white light driving circuit with adjustable color temperature

In order to stabilize the light intensity and color temperature of LED output light and realize the adjustment of light intensity and color temperature, a white light LED module driving circuit is designed, and its block diagram is shown in Figure 4. Light intensity detectors with blue, green and red filters are used to monitor the light intensity of the three LEDs constituting the white light LED module, and the duty cycle of the corresponding PWM control signal is changed through the control of the AVR microcontroller to stabilize the light intensity of the blue, green and red LEDs, thereby stabilizing the light intensity and color temperature of the white light LED module. LT3474 is used as the driving control chip in the driving circuit of each LED.

The adjustment of the output white light can be divided into light intensity and color temperature adjustment. If only the light intensity is changed, the duty cycle of the PWM signal in the three LED drive circuits is changed proportionally; if the color temperature needs to be changed, the blue, green and red LEDs are controlled with different duty cycles to adjust the proportion of the three color components in the white light and adjust the color temperature. Of course, the output light intensity and color temperature can also be adjusted at the same time. Through this circuit, the real-time control of the light intensity and color temperature of the white light LED module is truly realized. Moreover, due to the feedback control of the light intensity detector, the changes in light intensity and color temperature caused by LED aging are overcome, and the light intensity and color temperature of the LED lamps can be adjusted according to personal preferences, achieving a good lighting effect.

4 Conclusion

This article discusses two ways of regulating LED driving, introduces the circuit of PWM regulation, and proposes a driving control circuit for white light LED modules. By using a three-color light intensity detector, real-time regulation and control of light intensity and color temperature are achieved. This circuit can overcome the changes in light intensity and color temperature caused by LED aging, and can adjust the light intensity and color temperature of LED lamps according to personal preferences, achieving good lighting effects.