What is voltage dropout?
Dropout is the smallest difference between a regulator’s input and its output voltage, which is required to maintain regulation and enable the regulator to provide rated voltage and current. It is important to have a low dropout voltage for increased efficiency and minimal heat dissipation.
Linear voltage regulators require inputs which are higher than the rated output voltages. As the input voltage decreases towards the desired output voltage, it leads to condition of insufficient voltage which causes the regulator to drop out and provide unregulated output.
As an example, consider a 5v regulator with a 2V dropout voltage, for this to give a regulated output, the input voltage must be least equal to the output voltage (5V) plus the dropout voltage (2), which is 7V, any input below 7V will result into unregulated voltage output.
Linear regulators have much bigger heat losses and are less efficient due to larger dropout voltages. The excess voltage is usually dropped across the regulator and dissipated as heat. On the other hand the switching regulators are more efficient since they deliver power to the load by rapidly switching on and off.
In design processes the dropout voltage is crucial and it is important to follow manufacturer’s recommended dropout voltage provided in the datasheets. This enables one to select the right input voltage and avoid situations such as having a low input in which the regulator will be unable to provide the rated voltage, or too high input resulting in too much heat dissipation and possibly destroying the regulator.
In practical applications, the dropout is said to have been reached when the output voltage is drops below the design value by 100mV. The dropout voltage is sometimes affected by the junction temperature and the load current and manufactures usually specify the range of load current and operating temperature in which the dropout is applicable.
The dropout voltage in linear regulators is dependent on the load and is higher at high loads since more voltage is dropped across the regulators circuitry internal resistance. The low-dropout regulators however do not have this problem and maintains the rated output voltage over a wide range of load currents and input voltage, in addition to having low dropout voltages with typical values of 80mv at 2A.
More efficiency is achieved with the LDOs which have small voltage dropouts and able to provide stable voltage irrespective of load and line variations, temperature changes and time.