What are Linear Voltage Regulators?
A linear voltage regulator is an integrated circuit that maintains a constant output voltage level by continually adjusting the voltage applied to its control terminal.
Basic Functioning
Linear voltage regulators work by comparing the output voltage to an internal reference voltage. If the output voltage rises above the reference level, circuitry inside the regulator dumps excess current to ground. If the output voltage falls below the reference level, the regulator source current to maintain the proper output level. This constant feedback and correction mechanism allows the regulator to keep the output voltage level steady, within its specified tolerance, regardless of changes in load current or variations in input voltage within the regulator's rating.
Advantages of Linear Regulators
- Simple Design: The design of a linear regulator is relatively simple compared to switching regulators. They require fewer external components.
- Low noise: Since Linear Voltage Regulators operate by continually adjusting voltage, they produce very little output noise. This makes them suitable for noise-sensitive applications.
- Fast response: Due to their operational nature, linear regulators can respond quickly to changes in load conditions, keeping output voltage regulation tight.
Disadvantages of Linear Regulators
- Low efficiency: The "burn off" of excess voltage as heat means 70-80% of the input power is dissipated as heat in many linear regulator designs. This limits their use to low-power applications.
- Heat generation: Significant amounts of heat are generated inside the regulator, requiring heat sinking for reliability in all but tiny output currents. Overheating leads to failure.
- Higher input-output voltage difference required: A minimum input-output differential of around 1.5-3V is required for stable operation, restricting uses requiring very low output voltages.
Basics of Linear Regulator Circuits
Most linear regulators contain the following key components in their operational circuit:
- Input and output pins: The IN pin receives the supply/input voltage, while the OUT pin provides the regulated output.
- Reference voltage source: A precision reference voltage source establishes the target output voltage level.
- Error amplifier: This high-gain differential amplifier compares the output voltage to the internal reference and amplifies any difference as error signal.
- Pass element: Usually a transistor, it acts to either source more current from input to output to increase voltage if it falls, or dumps excess to ground if voltage rises.
- Feedback: Part of the output voltage is fed back to the inverting input of the error amp, closing the feedback loop. This ensures the pass element appropriately adjusts the output voltage to continuously match the reference.
- Protection circuitry: Overcurrent, overvoltage and thermal protection ensure safe and reliable operation under all load and input conditions.
Major Linear Regulator IC Types
78xx Series
The ubiquitous 78xx series, with the LM78xx being a common variant, are generic 3-pin positive voltage regulators available as standard voltages like 5V, 9V, 12V etc. Their simple design is suitable for low-current applications below 1A.
LM317 Adjustable Regulator
The LM317 is a very versatile adjustable linear regulator capable of output voltages between 1.2V and 37V by varying a single resistor. It provides better thermal performance than fixed regulators and is used in a variety of adjustable power supplies.
LDO (Low Dropout) Regulators
LDO regulators can operate with very small input-output voltage differential (100mV-200mV), enabling regulation even when supply voltage sags close to output level. This makes them popular for powering digital ICs from batteries where voltage may dip during operation. Common types are LP2950, TPS7A49.
Other Specialized Regulators
There are also linear regulators tailored for very low voltage applications like the NCV7381, current mode regulators like LM350, and heat-sink optimized heavy duty versions like LM1085. Each serve specific needs beyond the capabilities of standard regulators.
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