Linear Voltage Regulators - Components for Maintaining Voltage Stability
Linear voltage regulators, also known as linear stabilizers or regulators, are components designed to maintain a stable voltage output. The resistance within these regulators adjusts in response to changes in load and input voltage, ensuring a constant output voltage.
They function akin to a continuously auto-adjusting variable resistor, maintaining a fixed voltage division with the load. The excess energy, resulting from the difference between output and input voltage, is dissipated as heat.
In contrast, switching regulators rapidly toggle between on and off states, maintaining the desired average output voltage. For linear regulators to effectively reduce voltage and stabilize output, the input voltage must exceed the output voltage, which inherently limits their efficiency.
Linear voltage regulators can be configured in series (series regulators) or parallel (shunt regulators) with the load. Simple linear regulators may consist of just a Zener diode and a series resistor, while more complex ones include multiple components: a reference voltage, an error amplifier, and a power transfer element.
Due to their widespread application, linear regulators are often integrated into circuits but can also be constructed from discrete solid-state devices or vacuum tubes.
78xx: A common type of linear regulator.
These regulators control the current flowing through the load using transistors (or other devices), stabilizing the output voltage. By comparing the output voltage with an internal reference voltage, a differential signal is generated to control the transistor, creating a negative feedback loop.
With appropriate compensation, the output voltage is adjusted to the target level, unaffected by changes in input voltage or load, and remains reasonably stable.
Linear regulators require a minimum voltage difference between input and output to stabilize at the target voltage. This minimum voltage difference is known as the dropout voltage.
For instance, the popular 7805 model maintains a 5V output, but requires at least 7V input, resulting in a dropout voltage of 2V (7V - 5V). Due to this necessary voltage difference, linear regulators are generally inefficient, as they dissipate excess energy as heat within the transistor.
This inefficiency is particularly pronounced at low output voltages. For applications requiring low dropout voltages, Low Dropout Regulators (LDOs) with dropout voltages typically below 300mV are used, commonly in microprocessor power supplies.
Switching power supplies can achieve similar functions more efficiently but are costlier and generate switching noise. In scenarios with light loads, higher output voltages, or when the output voltage is close to the input voltage, linear regulators can be more efficient and thus a suitable choice.
Their advantages include the absence of magnetic components (inductors or transformers), which can be relatively expensive or bulky, simpler design, and lower noise. Some linear regulator designs, using only transistors, diodes, and resistors, are easier to integrate into circuits, further reducing their size, footprint on PCBs, and cost.
Linear regulators are primarily categorized into two types: series and shunt regulators.
The more common form, these operate by providing a path from the source voltage to the load through a variable resistor (mainly the transistor in the upper half of the divider). The power dissipated by the regulating device equals the output circuit voltage multiplied by the voltage drop across the device.
These operate by providing a path from the source voltage to ground through a variable resistor (mainly the transistor in the lower half of the divider). They divert the current flowing through the shunt regulator away from the load and to the ground, making them less efficient than series regulators.
However, they are simpler, sometimes consisting only of a reference voltage diode, and are used in low-power circuits where the wasted current is negligible. This form is often used in voltage reference circuits.
To prevent permanent damage to linear voltage regulators when loads exceed their designed capacity, these regulators may include, but are not necessarily limited to, the following protection features:
- Current limiting, such as constant current limiting and foldback current limiting.
- Over-temperature protection (also known as thermal shutdown).
- Safe operating area protection, which limits the power transistor to operate within predefined current and voltage conditions, ensuring normal operation and preventing damage. These conditions include maximum voltage, current, temperature, and power under continuous and transient pulse states.