Design of LED driver with constant illumination and adaptive dimming

1 System Introduction and Working Principle

The system consists of PIC16F690 microcontroller, TSL2561 light sensor, LED light source and LED driving circuit. Since the brightness of LED is proportional to the working current, the brightness of LED can be adjusted by adjusting the working current. At present, there are two main dimming methods: adjusting the forward current and pulse modulation dimming. Since pulse width modulation dimming has the advantages of no color shift, high dimming accuracy, combined with digital technology dimming, wide dimming range, no flicker, etc., this paper uses pulse modulation dimming.

The system mainly collects light intensity from TSL2561 and feeds it back to the PIC16F690 processing chip. After the PIC16F690 performs corresponding algorithm processing, it outputs a PWM waveform that changes with the light intensity. The driving circuit with a PWM interface drives the LED lamp to achieve dimming. The system block diagram is shown in Figure 1.

At the same time, human motion control and button control are added to this system, which can detect human motion and automatically turn off the LED lights when no one is around, further reducing energy waste; the buttons can adjust the frequency of the PWM waveform and set the maximum ambient light intensity, making the system more user-friendly.

1.1 PIC16F690 microcontroller

The PIC16F690 microcontroller has a high-performance RISC CPU, low power consumption, and abundant peripheral resources, which can meet the hardware resource requirements of this system. Since this microcontroller has abundant resources, it can meet the system requirements without wasting resources, thus reducing the controller cost.

1.2 TSL2561 light sensor

TSL2561 is a high-speed, low-power, wide-range, programmable and flexible light intensity digital conversion chip launched by TAOS. The application of this chip can provide the best display brightness and reduce power consumption. TSL2561 has the following features:

① The upper and lower thresholds of the permitted light intensity can be configured programmably, and an interrupt signal can be generated when the detected light intensity exceeds the threshold;

②The digital output complies with the standard SMBus and I2C bus protocols;

③ Programmable control of analog gain and digital output time;

④Ultra-small package and ultra-low power consumption;

⑤Automatically suppress 50 Hz/60 Hz light fluctuations.

The internal structure of TSL2561 is shown in Figure 2. It has two photodiode channels, namely channel 0 and channel 1. Channel 0 is sensitive to both visible light and infrared rays, while channel 1 is only sensitive to infrared rays. The current flowing through the photodiode is converted into a digital quantity by the integrating A/D converter, and the digital quantity is stored in the register inside the chip after the conversion. The integrating A/D converter will automatically perform the integration conversion process after an integration cycle is completed. TSL2561 can be controlled by setting its internal 16 registers, which can be accessed through the standard SMBus or I2C bus protocol.

2 Hardware Design

2.1 Control circuit design

The control circuit is mainly composed of the minimum system of PIC16F690, including reset circuit, input button circuit, download debugging circuit, power supply circuit, crystal oscillator circuit and corresponding signal input/output interface.

Since the research focus of this system is on dimming algorithm and the system is low power, the power supply circuit adopts a simple and practical resistor-capacitor circuit. The chip power supply voltage is 3.3 V, using ASM1117 voltage regulator chip.

The download debugging circuit is based on the information provided by Microchip, using the Microchip integrated emulator ICD3, and the interface circuit is designed according to the official information. The reset circuit and crystal oscillator circuit are designed according to the commonly used circuits. The overall hardware block diagram is shown in Figure 3.

2.2 Driving Circuit Design

Figure 4 shows an LED drive circuit based on the constant current LED control chip NCL30160. NCL30160 is an NFET hysteresis buck, constant current LED driver launched by ON Semiconductor. It increases the current to 1.5 A and is a new generation of high-efficiency solution with very low loss and small size, which can minimize space and cost. By utilizing the low on-resistance internal MOSFET of only 55 mΩ and the ability to operate at 100% duty cycle, it can provide a solution with energy efficiency up to 98%. The high switching frequency of up to 1.4 MHz allows designers to use smaller external components, minimize circuit board size and minimize cost.

Calculate the peripheral device parameters according to the data sheet and output requirements of NCL30160. The system uses 5 1 W LEDs in series with a constant current of 350 mA.

R1=200 mV/ILED=200 mV/350 mA≈0.56 Ω (1)

In the formula, ILED is the LED string current. C5 in Figure 4 uses the value recommended in the official data sheet. Inductor L1 and ROT are obtained by equations (2) to (4). In the formula, the values ​​of toff, ton, △I, and RDS(oN) refer to the data sheet; VIN is taken as 24 V here, and DCRL is the inductor resistance, which is taken as 0 Ω here.

3 Software Design

Software design includes four parts: main program design, I2C bus communication program design, dimming algorithm design, and motion control and key input program design. It is the core of realizing intelligent control of the system.

3.1 Main program design

Figure 5 is the main program flow chart. The main program is used to select whether to change the PWM frequency and the upper limit of illumination, the unattended mode and the occupied dimming mode. The constant illumination algorithm is implemented in the timer interrupt handler.

3.2 Dimming Algorithm Design

The dimming algorithm mainly realizes the acquisition of light intensity signals, data processing operations, and real-time tracking and compensation of illumination through corresponding algorithms, thereby achieving smooth dimming. The digital output of the TSL2561 light intensity sensor complies with the I2C bus standard protocol. TSL2561 communication must be implemented to realize light intensity acquisition. The reading and writing methods of the I2C bus standard protocol mainly include hardware implementation and software simulation. Since the PIC16F690 controller does not have its own I2C bus interface, this article adopts the software simulation implementation method.

The collected data is calculated according to the data specified in the TSL2561 standard to obtain the illuminance. The collected indoor illuminance is compared with the set standard value to calculate the illuminance value that the LED needs to compensate, and converted into the value of the PWM register in the processor, which is recorded as the current PWM value. The previous PWM register value is recorded as the original PWM value. The original PWM value is continuously compared with the current PWM value to increase or decrease, so that the PWM output tracks the current PWM value with a very small step size, thereby achieving smooth dimming. This algorithm can also solve the startup jump problem, so that the PWM changes slowly from zero to the current PWM value when the system starts. The dimming algorithm program flow chart is shown in Figure 6.

3.3 Motion Control and Key Input Programming

The motion control uses an infrared thermoelectric human motion sensor, which can detect whether there is someone in the room and select the corresponding lighting mode by judging the human activity. If there is someone, the dimming mode is selected, and if there is no one, the LED light is turned off to further save energy.

The key input mainly includes mode switch key and increase/decrease key. The mode switch key is mainly used to switch the frequency or change the maximum setting value of the illuminance. Considering the hardware resources of the microcontroller and the reasonable dimming requirements, the clock pre-division ratio is set to 4, the frequency variation range is 10-100 kHz, and the maximum setting value of the illuminance is selected according to the appropriate adjustment range of the illuminance measured in each time period multiple times, so the maximum setting value of the illuminance is in the range of 350-500.

4 Experimental Results

This experiment uses the TSL2561 light sensor, control circuit, and drive circuit. The LED load is 5 1 W LEDs connected in series, with a constant current of 350 mA. The system's working state can be divided into occupied and unoccupied. When the sensor detects that there is no one, the PWM output is turned off, and the LED light string is off; otherwise, the brightness of the LED light string is automatically adjusted according to the indoor light intensity. Due to experimental conditions, indoor light intensity is achieved by using blackout curtains to block the strong light from the windows. If the system data and waveform are consistent with the theoretical analysis, it means that the system design is correct. The theoretical calculation formula is as follows:

Table 1 is the experimental data measured under cloudy weather conditions. The system adjusts the PWM output waveform according to different indoor illumination to change the LED brightness. Due to the deviation between the indoor illuminance meter probe position and the TSL2561 sensor, the data has a certain error. That is, when there are people, the PWM duty cycle calculated according to formula (5) is basically consistent with the waveform displayed on the oscilloscope; when there are no people, no matter what the indoor light illumination value is, the PWM duty cycle is zero, and the LED is not illuminated. Comparing the experimental data with the theoretical value, considering a certain error, it is basically consistent. The output PWM waveform is shown in Figure 7, which is the PWM waveform when the indoor light is 22.82 lx and 317.08 lx respectively.

Conclusion

Experiments have shown that the system can perform constant illumination control well according to the program design; the motion control is automatically turned off when no one is around, and constant illumination control is immediately performed once human activity is detected. At the same time, the system is set to adjust the dimming frequency and the maximum illumination value, which can be changed appropriately according to different requirements. Constant illumination adaptive dimming and human motion control effectively achieve energy saving requirements.