Linear Regulated Power Supply: Principles, Advantages, and Disadvantages
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Linear regulators come in two main types: standard linear regulators and low-dropout regulators (LDO). The key difference between them is that standard linear regulators, such as the popular 78 series three-terminal regulators, require a significant voltage difference between the input and output (typically 2 to 3V), which leads to higher power consumption. On the other hand, LDOs operate with a much smaller voltage difference, often below 1V, resulting in lower power consumption.
1. Basic Working Principle of Linear Regulators
A linear regulator functions through a control circuit that includes output voltage feedback and error amplifiers. The control circuit adjusts the voltage drop (VDD) across the regulating element, thereby maintaining a stable output voltage. The principle is that the input voltage (VIN) must always be higher than the output voltage (VOUT), and the regulating element operates in its linear region, which is where the name “linear regulator” comes from. When there are changes in the input voltage or load current, the control circuit modifies the voltage drop (VDO) to ensure the output voltage remains stable.
Although the basic working principle of standard linear regulators and LDOs is the same, the structural difference in the regulating element allows LDOs to achieve a smaller voltage difference and lower power consumption.
Certain linear regulators, particularly those used in liquid crystal displays (LCDs), have an output control terminal. This control terminal allows the output voltage to be regulated through an external signal. The internal block diagram of such a regulator typically includes an EN (or SHDN) pin, which enables or disables the output when triggered by a microprocessor. When the power is off, the current is reduced to approximately 1µA.
2. Characteristics of Linear Regulators
Linear regulators offer several advantages, such as low cost, compact size, minimal external components, and low noise. They are available in a variety of package types and are especially well-suited for use in liquid crystal displays. For fixed voltage output applications, only 2 or 3 small capacitors are typically needed for the complete circuit.
The biggest advantage of linear regulators is their ultra-low output voltage noise. The ripple in the output voltage is typically less than 35µV (RMS), giving them a high signal-to-noise rejection ratio. This makes them ideal for powering sensitive circuits, such as small-signal processing devices. Moreover, linear regulators do not generate electromagnetic interference (EMI) due to large current changes, making them easier to design.
However, linear regulators are less efficient and can only be used in step-down applications. Their efficiency is dependent on the ratio of output to input voltage. For instance, with a 5V input and a 2.5V output, a standard linear regulator would operate at just 50% efficiency, meaning half of the electrical energy is wasted as heat. This is why linear regulators tend to heat up during operation. LDOs, with their smaller voltage drop, are more efficient—for example, at 76% efficiency with a 3.3V input and a 2.5V output. Due to this, LDOs are more commonly used in applications where energy efficiency is critical, such as in LCDs.
Pros and Cons of Linear Regulators
Each type of linear regulator has its own set of advantages and disadvantages, and the choice depends on specific design requirements, such as dropout voltage, ground current, and stability compensation. These factors determine which type of regulator is best suited for a particular application.
The main characteristics of a linear regulator, such as voltage difference and ground current, are determined by its pass element. Below are the common types of pass elements used in different linear regulators:
- Standard NPN Regulator: This type has a stable ground current that is equal to the base current of the PNP transistor, making it stable even without an output capacitor. It is suitable for devices with higher dropout voltage requirements but is less ideal for embedded devices where low dropout voltage is necessary.
- NPN Pass Transistor Regulator: These regulators offer low dropout voltage and are commonly used in embedded systems. However, they are not suitable for battery-powered devices requiring very low dropout voltages.
- PNP Bypass Transistor Regulator: This is a low-dropout regulator where the pass element is a PNP transistor. With a typical voltage difference between 0.3V and 0.7V, this regulator is ideal for battery-powered devices. However, its high ground current can shorten battery life, and its low gain can lead to unstable ground currents.
- P-Channel FET Regulator: These regulators are commonly used in battery-operated devices due to their low dropout voltage and ground current. The pass element is a P-channel FET, which allows for low voltage drop and low gate current. However, the large gate capacitance requires the use of external capacitors with specific ESR values for stable operation.
- N-Channel FET Regulator: Ideal for applications requiring low dropout voltage, low ground current, and high load current, these regulators use an N-channel FET as the pass element. Although they also require external capacitors, the required capacitance is smaller, and the ESR is less critical. However, they require a charge pump to establish the gate bias voltage, making the circuit more complex.
In summary, the choice of a linear regulator depends on the specific requirements of the application, including the need for low dropout voltage, ground current, and efficiency. Each regulator type offers unique advantages for different device applications.