TL431 is a precision reference voltage source integrated circuit that can produce a reference voltage of 2.5~37V. Its use is similar to that of a pressure regulator. When used, the current limiting resistor should be added, and the current flowing through the TL431 should be greater than 1mA, otherwise the voltage regulation effect will be lost.
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If the input voltage is lower than 2.5V, there is still voltage at both ends, but there is no voltage regulation at this time, and the voltage at both ends will change with the input voltage.