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Electronic Design

Voltage Regulator ICs

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What is the function of a voltage regulator IC?
A voltage regulator generates a fixed output voltage of a preset magnitude that remains constant regardless of changes to its input voltage or load conditions. There are two types of voltage regulators: linear and switching.

What is a linear voltage regulator?
A linear regulator employs an active (bipolar junction transistor (BJT) or MOSFET) pass device (series or shunt) controlled by a high-gain differential amplifier. It compares the output voltage with a precise reference voltage and adjusts the pass device to maintain a constant output voltage.

What is a switching regulator?
In a switching regulator, the input voltage is applied to an LC filter and is controlled by a power switch (MOSFET or BJT). The output voltage is fed back to the switching regulator controller that varies the length of time the power switch is on (duty cycle) to keep the output voltage constant.

What are some of the switching regulator topologies?
There are three main topologies: buck (step-down), boost (stepup), and buck-boost (stepup/step-down). Other topologies include flyback, SEPIC, Cuk, push-pull, forward, full-bridge, and half-bridge.

How does switching frequency impact regulator designs?
Higher switching frequencies mean the voltage regulator can use smaller inductors and capacitors. They also mean higher switching losses and greater noise in the circuit.

What losses occur with the switching regulator?
Power losses occur as a result of the power needed to turn on and off the MOSFET. The MOSFET driver must bear this loss. Also, the MOSFET takes a finite time to switch to/from a conduction state to non-conduction state. Hence, power loss will be associated with this activity in the MOSFET itself. These losses are dominated by the MOSFET gate charge and the capability of the drive, in effect the energy needed to charge and discharge the capacitance of the MOSFET gate between the threshold voltage and gate voltage.

What are the usual applications for linear and switching regulators?
The linear regulator's power dissipation is directly proportional to its output current for a given input and output voltage, so typical efficiencies can be 50% or even lower. Using the optimum components, a switching regulator can achieve efficiencies in the 90% range. However, the noise output from a linear regulator is much lower than a switching regulator with the same output voltage and current requirements. Typically, the switching regulator can drive higher current loads than a linear regulator.

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