Unlike AC circuits where a simple transformer can change the incoming voltage to a different level, DC circuits need an active device to change the voltage to the desired level. In general, there are two types of circuits to do this—switching and linear. Switching regulators are highly efficient and work on buck, boost, or buck-boost technology to change the voltage level. On the other hand, linear regulators such as LDOs are ideal for powering very low power devices or applications where the difference between the input and output voltages is small. Compared to switching regulators, linear regulators generate lower noise, are simple and cheap, but inefficient.
Linear Regulators (Low-Dropout Regulators)
Using linear circuits and non-linear techniques, linear regulators regulate the voltage output from the input supply. The resistance of the regulator varies according to the load and this creates a constant output voltage.
Irrespective of their make and design, all linear regulators must have their input voltage at least some minimum amount higher than the desired output voltage. Engineers call this minimum amount as the dropout voltage. An LDO regulator or low-dropout regulator is a DC linear regulator that is able to regulate the output voltage even for very low differences between the input and output voltages.
Therefore, applications that need an input voltage very close to the supply voltage and consume low power are ideal for linear regulators. As the product of the load current and difference of the input and output voltages governs the power dissipated by a linear regulator, a smaller difference means the regulator can handle higher power or allow a higher load current.
Although the linear regulators or low-dropout regulators offer a simple and cheap solution, these devices are notoriously inefficient as they dissipate heat based on the difference between the input voltage and the regulated output voltage. Most low-dropout regulators are low-current devices, offering well-regulate outputs, and require very few external components. They usually come in small packages, have fast transient response, and are highly accurate.
Most solutions for power management today require low power consumption under various load conditions, ability to operate in small spaces, offer high reliability, and the capability of withstanding wide input voltages. Therefore, a broad range of applications is moving towards highly efficient, wide input, low quiescent current switching regulators.
Switching regulators work by switching a series element on and off very rapidly. The series element can be either synchronous or non-synchronous FET switches. Usually, an associated inductor stores the input energy temporarily, and releases the energy subsequently to the output circuit at a different voltage level. The duty cycle of the switch determines the amount of charge transferred to the load.
Switching regulators operate efficiently, as their switching element dissipates almost no power, because the element is either switched off or fully conducting. Unlike linear regulators, switching regulators can generate output voltages higher than the input voltage or of the opposite polarity.
Therefore, switching regulators offer wide input and output voltage ranges, integrated series elements, pin-to-pin compatible parts, internal compensation, and light load efficiency modes, while being simple and easy to use.