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Guide for Tech Beginners - Voltage Regulators

Voltage RegulatorsVoltage Regulators

A voltage regulator generates a fixed output voltage of a preset magnitude that remains constant regardless of changes to its input voltage or load conditions. There are two types of voltage regulators:

  •  Linear
  • Switching

The linear regulator's power dissipation is directly proportional to its output current for a given input and output voltage, so typical efficiencies can be 50% or even lower. Using the optimum components, a switching regulator can achieve efficiencies in the 90% range. 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.

Linear regulator

A linear regulator employs an active (BJT or MOSFET) pass device (series or shunt) controlled by a high gain differential amplifier. It compares the output voltage with a precise reference voltage and adjusts the pass device to maintain a constant output voltage.

This regulating device acts like a variable resistor and continuously adjusts the voltage divider network in order to maintain an output voltage which is constant. The difference between the input voltage and regulated voltage is continually dissipating as waste heat. Due to linear voltage regulators being often used in several electronic devices, linear regulators in integrated circuit (IC) form are very common. There are several different kinds of linear regulators.

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the dropout voltage. For example, a common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the input voltage remains above about 7V, before the output voltage begins sagging below the rated output. Its dropout voltage is therefore 7V − 5V = 2V. There are two types of linear regulator:

Fixed regulators

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of plus 3 V, and plus or minus 5 V, 6V, 9 V, 12 V, or 15 V, when the load is less than 1.5 A. The "78xx" series (7805, 7812, etc.) regulate positive voltages while the "79xx" series (7905, 7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the output voltage (e.g., a 7805 is a +5 V regulator, while a 7915 is a −15 V regulator). There are variants on the 78xx series ICs, such as 78L and 78S, some of which can supply up to 2 Amps.

Variable regulators

An adjustable regulator generates a fixed low nominal voltage between its output and its adjust terminal (equivalent to the ground terminal in a fixed regulator). This family of devices includes low power devices like LM723 and medium power devices like LM317 and L200. Some of the variable regulators are available in packages with more than three pins, including dual in-line packages. They offer the capability to adjust the output voltage by using external resistors of specific values.

The LM317 series (+1.25V) regulates positive voltages while the LM337 series (−1.25V) regulates negative voltages. The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

Commonly used linear voltage regulator

L7805 (Voltage Regulator - 5V): This is the basic L7805 voltage regulator, a three-terminal positive regulator with a 5V fixed output voltage. This fixed regulator provides a local regulation, internal current limiting, thermal shut-down control, and safe area protection for your project. Each one of these voltage regulators can output a max current of 1.5A.

L7812 (Voltage Regulator - 12V): This is the basic L7812 voltage regulator, a three-terminal positive regulator with a 12V fixed output voltage. This fixed regulator provides a local regulation, internal current limiting, thermal shut-down control, and safe area protection for your project. Each one of these voltage regulators can output a max current of 1.5A.

LM317 (Adjustable 1.25V to 37V): TheLM317 device is an adjustable three-terminal positive-voltage regulator capable of supplying more than1.5 A over an output-voltage range of 1.25 V to 37V. It requires only two external resistors to set the output voltage. The device features a typical line regulation of 0.01% and typical load regulation of 0.1%. It includes current limiting, thermal overload protection, and safe operating area protection.

Please check datasheet for details.

Switching regulator

A switching regulator converts the dc input voltage to a switched voltage applied to a power MOSFET or BJT switch. 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.

There are three common topologies: buck (step-down), boost (step-up) and buck-boost (step-up/stepdown). Other topologies include the flyback, SEPIC, Cuk, push-pull, forward, full-bridge, and half-bridge topologies.

Switching regulators require a means to vary their output voltage in response to input and output voltage changes. One approach is to use PWM that controls the input to the associated power switch, which controls its on and off time (duty cycle). In operation, the regulator's filtered output voltage is fed back to the PWM controller to control the duty cycle. If the filtered output tends to change, the feedback applied to the PWM controller varies the duty cycle to maintain a constant output voltage.

Boost converter

A boost converter (step-up converter) is a DC-to-DC power converter with an output voltage greater than its input voltage. Boost converters are used when it required higher voltage than available voltage from battery. Suppose you have a 3.7 V battery but you need 5 V for your device then you can use boost converter.

Because of the ease with which boost converters can supply large over voltages, they will almost always include some regulation to control the output voltage, and there are many I.Cs. manufactured for this purpose A typical example of an I.C. boost converter is the LM27313 from Texas Instruments. This chip is designed for use in low power systems such as PDAs, cameras, mobile phones, and GPS devices. Another common adjustable boost converter is LM2577.






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