Low-Dropout Linear Regulators (Part-1)

Low-Dropout Linear Regulators (Part-1)

The Linear Regulators are simple-to-use devices that appear in nearly every type of electronic equipment, where they produce a clean, accurate output voltage which can be used by sensitive components. These low dropout voltages guarantee the component to get a clean, well regulated supply that quickly reacts to induced load changes as well as input supply variations. Let us first break down a typical Linear Regulator and see how it basically works and get onto to parameters that has to be looked upon while selecting a LDO.

A typical Linear regulator's working can be explained using the Figure Fig. 1.
   

A simple explanation for the above circuit is as follows. A pass transistor is controlled by an operational amplifier which compares the output voltage to a reference. As the output voltage drops, the op-amp increases drive to the pass element, which increases output voltage. Conversely, if the output rises above the desired set point, the op amp reduces drive. These corrections are performed continuously with the reaction time limited only by the speed of the op amp and output transistor loop.

Where to Use a LDO?

When there is a need for selecting the power supplies, there will be usually two ways. One is to use a SMPS and another is to use a LDO. While the usage of SMPS will be detailed in the next blog, the usage of LDO has some considerations that has to be made with a SMPS. SMPS are converters that can be used while there is a need for regulation  when the input and output voltage differences are considerably large. But when a LDO is used in the case which involves larger input and output voltage difference, there will be an excessive amount of heat released. So people usually go for LDO where there is very small input-output voltage differences. So most LDOs are used as “Post-regulators” for a SMPS circuit.

Basic Design Parameters:

The below mentioned parameters will help designers for selecting the power supply (LDO)

⦁ Output voltage is an important parameter, as this is the reason most designers purchase a regulator. Linear regulators are available in both fixed output voltage and adjustable configurations. Fixed voltage regulators offer enhanced ease-ofuse, with their output voltages accurately trimmed at the factory—but only if your application uses an available voltage. Adjustables allow using a voltage custom-tailored for your circuit.

⦁ Maximum output current is the parameter generally used to group regulators. Larger maximum output currents require larger, more expensive regulators.

⦁ Dropout voltage is the next major parameter. This is the minimum additional voltage on the input that still produces a regulated output.

⦁ Ground current is the supply current used by the regulator that does not pass into the load. An ideal regulator will minimize its ground current. This parameter is sometimes called quiescent current, but this usage is incorrect for PNP-pass element regulators.


In the next blog, a detailed explanation of LDO and its course to SMPS designing will be discussed.

Author

Santhosh Raju

Hardware Engineer