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The Low dropout linear regulator or LDO can be used in applications where you need to drop a higher input voltage to a lower output voltage at relatively moderate power levels. They are especially suitable in applications which require low noise, low current and which have a small difference between the input and output voltage.
LDOs regulate the output voltage by controlling the conduction of the pass element in a linear regulation. This linear regulation provides accurate, noise free output voltage which can quickly respond to load changes on the output. The key advantage of an LDO is its simplicity and low-cost design with low noise and fast voltage conversion.
However, compared to a switching regulator, an LDO has relatively low efficiency in applications with high VIN / VOUT ratio and power dissipation is more critical. The linear regulation means that the voltage difference between input and output times the average load current is dissipated in the LDO pass element. Power Dissipation can be calculated as PD = (VIN - VOUT) * ILOAD, so big difference between VIN and VOUT with high load currents would lead to excessive dissipation.
Higher power dissipation requires LDOs in larger packages, which increases cost, PCB space and application heat. When LDO power dissipation exceeds ~ 0.8W, it is wise to look for alternatives such as Buck converters.
When selecting an LDO, one should first consider the input and output voltage range, the LDO current capability and package dissipation capability. LDO dropout voltage is the minimum VIN - VOUT voltage where the device can regulate.
In micro power applications, (i.e. applications that need to run from a battery for several years) the LDO quiescent current IQ needs to be low to avoid unwanted battery drain. These applications need special low IQ LDOs.
In applications that require a very clean and low noise output voltage, low noise and good PSRR will be the LDO key selection criteria.