1) Main features: Three-terminal adjustable voltage regulator can be divided into positive pressure output and negative pressure output.
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1Easy to use, only need two external resistors to determine the output voltage within a certain range.
2 performance indicators are better than three-terminal fixed voltage into a voltage regulator.
3 with full overload protection, including current limit, overheating and safety zone protection. All protection circuits are still active even if the regulation terminal is left floating.
In short, the three-terminal adjustable voltage regulator is a convenient and widely used voltage regulator integrated circuit.
2) Working principle: The LM317 adjustable voltage regulator is taken as an example to briefly introduce the working principle of the three-terminal adjustable voltage regulator.
The figure shows the outline and internal block diagram of the LM317. In the figure, the DC voltage input is the voltage output, and the ADJ is the adjustment terminal. Compared with the 78×× series fixed three-terminal regulator, the LM317 redirects the common terminal of the internal error amplifier and the constant current source of the bias circuit to the output terminal. Therefore, it has no ground terminal. The 1.25V reference voltage inside the LM317 is set between the non-inverting input of the error amplifier and the regulated terminal of the regulator. A constant current source supplies a constant regulation current IADJ of 50μA, which flows from the regulation terminal (ADJ). The ROSP is an overcurrent detection resistor set inside the chip.
Figure 3 The shape and internal principle of the adjustable voltage regulator
a) TO-220FF package b) TO-3 package c) Internal principle
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