The filtered power switch output voltage is fed back to a circuit that controls the power switch on and off times so that the output voltage remains constant regardless of input voltage or load current changes. Higher switching frequencies mean the voltage regulator can use smaller inductors and capacitors. It also means higher switching losses and greater noise in the circuit. Losses are also due to the energy needed to charge and discharge the capacitance of the MOSFET gate between the threshold voltage and gate voltage.
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. Output Voltage: The output voltage can be fixed or adjustable. If fixed, the voltage is set internally to the device and you purchase the specific part number for the output voltage that you want. If the regulator is the adjustable type, the voltage is usually set by a voltage divider made up of two resistors.
This offers some flexibility, but at the cost of extra components. Input Voltage: The minimum and maximum input voltage specified needs to be strictly adhered to.
Current Output: The maximum current the voltage regulator can provide is limited and is usually determined by the current carrying capability of the internal power transistor. All IC regulator solutions include a built-in current limit circuit to prevent damage. The amount of ripple in the output voltage is very important to consider since many types of circuits will be sensitive to any noise on their input supply. Linear regulators reject input ripple without adding additional ripple.
The higher the PSRR then the better the linear regulator is at rejecting any ripple on the input voltage. Switching regulators on the other hand create output ripple by their switching nature. The amount of ripple from a switching converter can be reduced by filtering and careful component selection.
A common design technique is to use a switching regulator to step down the supply voltage with minimal power dissipation, followed by a linear regulator to remove any ripple. Placing a capacitor of around 10nF on this pin to ground helps to filter out noise and ripple on the internal voltage reference and thus the output voltage.
Noise: Many electronic components, such as resistors and transistors, also produce a fundamental physical noise that is commonly confused with ripple. Noise will show as random fluctuations on the output voltage versus ripple will show up as a small periodic waveform. Although not related to ripple the same techniques that reduce output ripple also typically reduce noise — mainly the use of the noise-reduction capacitor.
Load Regulation: Load regulation refers to the ability of the regulator to keep the output voltage steady when the load current changes. This specification is often provided in the device datasheet as a plot of output voltage versus load current. Line Regulation: Variations in the voltage input to the regulator can cause variations in the output voltage, and line regulation is a measure of this variation.
Line Transient: This is a measure of how the output voltage responds to a sudden step change in the input voltage.
Regulators with a high-PSRR specification i. Voltage Drop-out: The drop-out voltage for classic linear regulators such as the LM or LM78xx series is about 2 volts.
This means that the input voltage must be at least 2 volts higher than the output voltage for the regulator to work. Low drop-out LDO regulators can work with an input to output voltage difference that is much smaller. For example the TPS family of low drop-out regulators has a voltage input range of 1. Efficiency: Efficiency is a measure of how much power is wasted by the regulator.
As previously mentioned a linear regulator wastes a lot more power than a switching regulator. This means a linear regulator has a much lower efficiency. Efficiency can be calculated by dividing the output power by the input power. This is the ideal, but unattainable scenario. The efficiency of a linear regulator varies with the ratio of the input voltage to the output voltage. This is because for a linear regulator the input current is always essentially identical to the output current.
Since power is equal to voltage times current the currents in the efficiency equation cancel out only leaving the voltages. This means the bigger the difference between the input voltage and the output voltage the worse the efficiency for a linear regulator. So for example, for a linear regulator with a 5VDC input voltage and a 3.
It is important to consider the estimated power dissipation of a linear regulator in application, since using larger input voltages results in high power dissipation that can overheat and damage components.
Another limitation of linear voltage regulators is that they are only capable of buck step-down conversion, in contrast to switching regulators, which also offer boost step-up and buck-boost conversion. Switching regulators are highly efficient, but some disadvantages include that they are generally less cost-effective than linear regulators, larger in size, more complex, and can create more noise if their external components are not carefully selected.
Noise can be very important for a given application, as noise can affect circuit operation and performance, as well as EMI performance.
There are various topologies for linear and switching regulators. Linear regulators often rely on low-dropout LDO topologies. For switching regulators, there are three common topologies: step-down converters, step-up converters, and buck-boost converters.
Each topology is described below:. One popular topology for linear regulators is a low-dropout LDO regulator. Linear regulators typically require the input voltage to be at least 2V above the output voltage. However, an LDO regulator is designed to operate with a very small voltage difference between input and output terminals, sometimes as low as mV. Step-down converters also called buck converters take a larger input voltage and produce a lower output voltage.
Conversely, step-up converters also called boost converters take a lower input voltage and produce a higher output voltage. A buck-boost converter is a single-stage converter that combines the functions of a buck and a boost converter to regulate the output over a wide range of input voltages that can be greater or less than the output voltage. The four fundamental components of a linear regulator are a pass transistor, error amplifier, voltage reference, and resistor feedback network.
One of the inputs to the error amplifier is set by two resistors R1 and R2 to monitor a percentage of the output voltage. The other input is a stable voltage reference VREF. Linear regulators typically only require an external input and output capacitor to operate, making them easy to implement.
On the other hand, a switching regulator requires more components to create the circuit. The power stage switches between VIN and ground to create charge packets to deliver to the output. Similar to a linear regulator, there is an operational amplifier that samples the DC output voltage from the feedback network and compares it to an internal voltage reference.
Then the error signal is amplified, compensated, and filtered. Electronics Basics What is a Transistor? What is a Diode? What are SiC Power Devices? What are SiC Semiconductors? What is IGBT? What are LEDs? What is a Photointerrupter? What is a laser diode?
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