﻿ What is the Dropout Voltage of a Regulator? # What is the Dropout Voltage of a Regulator? The Dropout Voltage of a regulator is the amount of voltage that a regulator needs to be fed above its rated output voltage to maintain the output voltage.

Just as a brief review, a voltage regulator is a device which accepts an input voltage which it regulates down to the output voltage which it is rated for. In order for a voltage regulator to output its rated voltage, it must be fed a voltage that is higher than its rated voltage. The amount of voltage higher that the input voltage must be than the output voltage is the dropout voltage of the regulator.

For example, a LM7805 voltage regulator is a regulator that outputs 5 volts. However, in order to give out 5 volts, it needs to be fed a higher voltage as its input. The LM7805 needs at least 2 volts higher than the 5 volts to do provide the 5 volts. Therefore, it needs 7 volts as its input. Thus, its dropout voltage is 2 volts, since 5 volts + 2 volts= 7 volts. Although voltage regulators need higher voltages as their inputs, higher is not better above a certain value. Normally the voltage that is provided as the input should be limited to 3 volts higher than the output voltage. So for a 5-volt regulator, no more than 8 volts should be applied as the input voltage. Although voltage regulators can withstand much higher voltages, the difference between the input and output voltage appears as heat. The greater the difference between the input and output voltage, the more heat is generated. If too much heat is generated, through high input voltage, the regulator can overheat. If the regulator does not have a heat sink to dissipate this heat, it can be destroyed and malfunction. Thus, it is recommended to follow the dropout voltage given by the manufacturer for the regulator on the datasheet. Too low a voltage as the input and the regulator won't output its rated voltage or too high and the voltage regulator may dissipate too much heat. The dropout voltage, therefore, is crucial for getting the input voltage in the right range.

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