Design of LED driver circuit for intelligent lighting system

　　In recent years, semiconductor light sources are gradually entering the lighting field as a new type of solid light source. According to the principle of solid light emitting physics, the luminous efficiency of LED can be close to 100%, with the advantages of low operating voltage, low power consumption, short response time, high luminous efficiency, impact resistance, long service life, pure light color, stable and reliable performance and low cost. With the continuous reduction of LED prices and the continuous improvement of luminous brightness, semiconductor light sources have shown a wide range of application prospects in the field of lighting. The volt-ampere characteristics of LED are the same as those of ordinary diodes. A small fluctuation in forward voltage will cause a sharp change in forward current. The magnitude of LED forward current will change with the ambient temperature. When the environment reaches a certain temperature, the allowable forward current of LED will drop sharply; in this case, if a large current is still passed, it is easy to cause LED aging and shorten the service life. Therefore, LED needs an LED drive system with constant temperature and constant current control and reliable protection function during application. This article introduces a design method for an intelligent LED drive system.

　　Constant current drive circuit

　　Under certain voltage and temperature changes, the constant current source produces a current change close to zero, with a constant current value and a very high dynamic output resistance. Generally, the constant current drive circuit is composed of electron tubes, transistors, constant current devices, integrated circuits, integrated voltage regulators and other components. In order to be suitable for the application of LED lamps, the constant current source must not only have high stability and current output accuracy, but also the output current of the constant current drive circuit is designed to be adjustable. In order to ensure the accuracy of the output current, this design uses a single-chip microcomputer system D/A to convert the output voltage and adjust the output current of the constant current source. The schematic diagram is shown in the figure.

　　This constant current drive circuit belongs to the topology of current series negative feedback, in which LED is the load and R6 is the sampling resistor. In this design, in order to realize the control of adjustable constant current source, the adjustable voltage signal Vs output by the single-chip system D/A is introduced at the in-phase input of the operational amplifier to make it a controlled constant current source, that is, the reference voltage. The sampling resistor R6 is connected to the reverse input. The operational amplifier works in a deep negative feedback state. It cooperates with the power MOS tube to follow the input reference voltage Vs through feedback. The power MOS tube is connected to the base of the operational amplifier to increase the driving current. When the in-phase input voltage of the operational amplifier is constant, the output voltage is guaranteed to be constant due to the existence of negative feedback, so that the current flowing through the LED load is a constant current. The output current of the constant current source directly depends on the ratio of the output voltage of the D/A and the sampling resistor R6. Since the operational amplifier is used in the feedback link, the loop gain of the feedback loop is increased, the feedback depth is increased, and the output impedance of the constant current drive circuit is large, which meets the use requirements.

　　The output current is directly related to the input control voltage VS. The offset of the op amp has the same effect on the output current as VS. Therefore, the stability of the output current depends on the stability of the controlled voltage VS and the offset drift of the op amp. Selecting a high-stability reference voltage source as the control voltage and a low-drift operational amplifier is an important way to improve the output current stability of this circuit. The integrated operational amplifier LM358 used in the circuit includes  two independent, high-gain, internal frequency-compensated operational amplifiers, which have the advantages of high gain and small offset voltage influence.

　　MCU constant current control

　　In order to obtain a stable driving current and improve the light stability of the LED, this system changes the peripheral voltage of the constant current source and uses the change in voltage to control the output current. The control part of this system is based on the single-chip microcomputer system, combined with button input and LED digital display. The A/D sampling voltage is compared with the input voltage through closed-loop feedback, and corresponding adjustments are made. Finally, the analog voltage output by the D/A conversion is used as the reference input voltage of the constant current source.

　　MCU Hardware System

　　The single-chip microcomputer system mainly consists of AT89C51 , ADC0809 , DAC0800 , digital tube, buttons and other parts. The schematic diagram of the single-chip microcomputer system is shown in Figure 5.

　　The sampled analog voltage is input to the input of ADC0809, and after conversion by ADC0809  , an 8-bit binary number is output to the microcontroller port. The microcontroller converts the obtained 8-bit binary number into a 3-bit decimal number and displays it on the digital tube. At the same time, the current value is compared with the reference value, and the software system makes corresponding adjustments and controls. The microcontroller system software calculates and outputs an 8-bit binary value, which is converted into D/A by DAC0800 and output to the DA1 port. The voltage of the DA1 port is input to the constant current drive circuit, and the reference voltage VS is adjusted to realize the constant current drive circuit output current design as an adjustable output.