In recent years, semiconductor light sources are gradually entering the field of illumination with the role of new solid-state light sources. According to the principle of solid-state luminescence, LED luminous efficiency can be close to 100%, with low working 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. Etc. As the price of LEDs continues to decrease and the brightness of illumination continues to increase, semiconductor light sources have shown broad application prospects in the field of illumination. The volt-ampere characteristics of LEDs are the same as those of ordinary diodes, and small fluctuations in forward voltage result in sharp changes in forward current. The magnitude of the LED forward current changes with the ambient temperature. When the environment reaches a certain temperature, the LED allows the forward current to drop sharply. In this case, if the current is still passed, it is easy to cause the LED to age and shorten the service life. In the application process, an LED driving system with constant temperature and constant current control and reliable protection is needed. This article describes the design of a smart LED drive system.
Constant current drive circuit
The constant current source produces a current change close to zero under a certain voltage and temperature change, with a constant current value and a high dynamic output resistance. Generally, a constant current driving circuit is composed of a tube, a transistor, a constant current device, an integrated circuit, an integrated voltage regulator, and other components. In order to be suitable for the application of LED lamps, the constant current source must not only have higher stability and current output accuracy, but also the output current of the constant current driving circuit is designed as an adjustable output. In order to ensure the accuracy of the output current, this design uses the D / A conversion output voltage of the single-chip system to adjust the output current of the constant current source. The schematic diagram is shown in the figure.
Intelligent LED lighting system drive circuit design
This constant current drive circuit belongs to the topology of current series negative feedback, where LED is the load and R6 is the sampling resistor. In this design, in order to realize the adjustable constant current source control, the adjustable voltage signal Vs outputted by the single chip system D / A is introduced at the non-inverting input terminal of the operational amplifier to make it a controlled constant current source, that is, a reference voltage. Connect the sampling resistor R6 to the inverting input. The operational amplifier operates in a deep negative feedback state. It cooperates with the power MOS transistor to follow the input reference voltage Vs through feedback. The power MOS transistor is connected to the base of the operational amplifier to increase the drive current. When the input voltage of the non-inverting terminal of the operational amplifier is constant, the output voltage is kept 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 is directly dependent on the ratio of the output voltage of D / A and the sampling resistor R6. Since the operational amplifier is used in the feedback loop, the loop gain of the feedback loop is increased, the feedback depth is increased, and the output impedance of the constant current driving circuit is large, which satisfies the use requirements.
The output current is directly related to the input control voltage VS. The op amp offset has the same effect as the VS on the output current. 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 high stability. The reference voltage source is used as the control voltage, and the low-drift operational amplifier is selected, which is an important way to improve the stability of the output current of the circuit. The integrated operational amplifier LM358 used in the circuit, the LM358 internally includes two independent, high-gain, internal frequency-compensated op amps with high gain and small offset voltage effects.
Single chip constant current control
In order to obtain a stable driving current, the light stability of the LED is improved. The system controls the output current by changing the peripheral voltage of the constant current source and using the voltage change. The control part of the system is controlled by the single-chip microcomputer system. Combined with the button input and LED digital display, the A/D sampling voltage is compared with the input voltage through closed-loop feedback, and the corresponding adjustment is made. Finally, the analog voltage output by D/A is converted as constant. The reference input voltage of the flow source.
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