Overview: 1, a single TL431 constant current circuit
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As LED lighting is getting hotter and hotter, as the life pillar of LEDs, LED driver power is getting more and more attention.
People say that LED power supply is a special power supply, which is very different from ordinary power supply, so do LED power supply to find professional LED power supply engineers. This kind of statement casts a mysterious veil on the LED power supply, but as a power supply professional, we know that the LED power supply is actually nothing special. It is characterized by the need for constant current limiting, and the long-term work is under full load, so The efficiency requirements are relatively high, and some power supplies have height requirements due to structural size limitations.
Below I will try to summarize some common methods of secondary constant current for the small and medium power LED lighting power supply. It is not necessarily comprehensive, and it is not necessarily very deep, but I can finally have some engineers who are new to the line. help.
It is no exaggeration to say that the LED driver power supply will directly determine the reliability and life of the LED lamp; as a power supply engineer, we know that the characteristics of the LED require constant current drive to ensure uniform brightness and long-term reliable illumination.
Let's talk about several constant current methods of the popular TL431. 
As shown above, the schematic principle of using a single TL431 constant current is:
This circuit is very simple, using the 431's 2.495V reference for constant current, which also limits the voltage drop across the LED, but the advantages and disadvantages are equally obvious.
advantage:
The circuit is simple, the components are few, and the cost is low. Because the reference voltage precision of TL431 is high, R12 and T13 only need high precision resistance, and the constant current precision is relatively high.
Since the TL431 is a 2.5V reference, the loss of the constant current sampling circuit is extremely large, and it is not suitable for a power supply with an excessive output current. The fatal defect of this circuit is that it cannot be idling, so it is not suitable for an external LED power supply to improve the defect. Continue to post the improved circuit.
The constant current point calculation of this circuit is believed to be known to everyone: ID=2.495/(R12//R13)
The power of the sampling resistors R12 and R13 is PR=2.495*2.495/R13). For small power supplies, the loss of this power is considerable, so it is not recommended to use this circuit for products with current greater than 200mA (please read the PDF for details) )