The output voltage of the three-terminal voltage regulator is adjusted steadily through the coordination of internal reference voltage, error amplifier, adjusting tube and protecting circuit. Here's an example of how LM317 works:
Reference voltage Formation: When the input end has a normal power supply voltage input, this voltage not only supplies power to the regulator (adjustment tube), but also to the reference voltage amplifier through a constant current source, thus generating reference voltage. When reference voltage is added to the in-phase (+) input of the error amplifier, the error amplifier provides a conduction voltage to the regulator, causing the regulator to start output voltage. When the voltage is output through the output terminal, it powers the load.
Error amplification and feedback regulation: The output voltage is sampled through the voltage divider resistor and fed back to the inverting (-) input of the error amplifier. When input voltage increases or the load becomes lighter, causing the output voltage to increase, the inverted input voltage of the error error amplifier, the voltage input provided by the error amplifier amplifier to the regulator decreases, the output voltage of the regulator decreases, and the final output voltage decreases to the specified value. When the output voltage drops, the voltage regulation control process is reversed. In this way, the circuit control ensures that voltage output of voltage voltage regulator does not change with the change of supply voltage voltage and load.
Adjustment tube control: adjustment tube adjusts its conduction degree according to the output signal of the error amplifier, thus controlling the output voltage. Take LM317 for example and connect its 1.25V reference voltage generator to the regulator terminal ADJ. Changing the ADJ terminal voltage can change the size of the LM317 output voltage. For example, by adjusting the control circuit to increase the ADJ terminal voltage, the voltage output of the reference voltage generator increases, while the voltage output of the error amplifier increases with the increase of the cophase input terminal voltage. When this voltage is added to the regulator, the output voltage of the regulator increases and the voltage provided by the regulator to the load increases. The voltage provided by the voltage regulator to the load is reduced by adjusting the control circuit to reduce the ADJ terminal voltage.
Protective function: when the load abnormality causes the regulator to overflow, after being detected by the overcurrent protection circuit, the regulator stops working, so as to avoid the overflow damage to the regulator, thus achieving the purpose overcurrent protection. In addition, when the regulator overflows, temperatures can rise sharply. When detected by the overheating protection circuit in the chip, the regulator will stop working to avoid overheating and damage the regulator, thus achieving overheating protection.
