What Are Linear Voltage Regulators?
In electronic devices, power management chips act like energy managers, directly affecting the device's battery life, performance, and stability. Among them, linear voltage regulators play an indispensable role in many electronic systems due to their simplicity and low-noise characteristics. Whether in smartphones, medical devices, or automotive electronics, linear voltage regulators are crucial for providing clean and stable voltage to sensitive circuits. This article will take you through the working principle of linear voltage regulators, their features, differences from DC-DC converters, and their applications.
What Are Linear Voltage Regulators?
A linear voltage regulator is a power management device that uses a transistor or a field-effect transistor (FET) operating in its linear region to subtract excess voltage from the applied input, producing a regulated output voltage. The dropout voltage refers to the minimum difference between the input voltage and the output voltage required for the regulator to maintain the output within 100 mV of its rated value.
Linear regulators typically consist of input, output, and ground pins. Adjustable versions include a feedback pin to monitor output voltage. Internally, they are mainly composed of an error amplifier (for error detection using an operational amplifier), a reference voltage source, and an output transistor. The pass transistor can be a P-channel or N-channel MOSFET, or a bipolar PNP or NPN transistor.
II. Work Principles
The working principle of a linear regulator is to control the voltage drop across the pass transistor (the dropout voltage) via a control circuit composed of the error amplifier and feedback, achieving a stable output voltage.
When the input voltage fluctuates or the load current changes, causing a variation in output voltage, the feedback and control circuit adjusts the voltage drop of the pass transistor to keep the output voltage essentially constant. Specifically, the error amplifier detects the output voltage (through a feedback resistor divider), compares it with the internal reference voltage (for example, 1.25 V), and then adjusts the conduction of the pass transistor. By changing the transistor's voltage drop (VIN - VOUT), the output voltage remains stable.
All regulators use a negative feedback loop to maintain output stability. The feedback signal experiences gain and phase changes as it passes through the loop. To keep the circuit stable, the total phase shift of the feedback signal around the loop must have at least a 20-degree phase margin. This prevents the negative feedback from turning into positive feedback, which could destabilize the output.
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