Linear Voltage Regulators
Linear Voltage regulators are easy
voltage regulator circuits usually utilized in electronics. This paper shortly
discusses however linear regulators work, their benefits and disadvantages,
variations on the linear regulator, and necessary datasheet parameters.
How Linear regulators work
Linear Voltage regulators use a closed feedback circuit to
bias a pass component to keep up a continuing voltage across its output
terminals. The op-amp drives the base of Q1 to ensure that the voltage at its
inverting input will be equal to the voltage reference at its non-inverting
input.
The op-amp in this circuit has a
small load, the base current, and minimal capacitive loading. Consequently, it
can respond to changes in load very quickly.
Two things can be observed from this schematic:
1.) Linear regulators are reduction
converters, meaning that the output voltage will always be less than the input
voltage. In fact, there's a minimum voltage distinction between VIN and VOUT
which will permit the linear regulator to work. In datasheets, this value is
called the drop-out voltage. If the VOUT > VIN - VDROPOUT, then the linear
regulator can’t regulate the output voltage at the desired voltage.
2.) Power is dissipated in the pass
transistor. The amount of power is P= (VIN-VOUT)*ILOAD. This power is wasted
heat. This heat causes the regulator to get warm.
Advantages of linear regulators
Linear Voltage regulators are usually
highly integrated, including the pass element and the feedback loop. Some
linear regulators, just like the LM317, are adjustable once used with an
external resistor divider.
Disadvantages of linear regulators
Linear regulators have the following advantages:
Simple.
Cheap.
Power supply rejection ratio. Linear Voltage regulators respond quickly to changes in input
voltage, producing an output voltage that is mostly free of any ripple on the
input.
Respond quickly to changes in load
voltage.
No switching noise. Other voltage
conversion circuits, known as DC-DC converters, have high-frequency switching
noise. Linear regulators don’t have this characteristic.
The main disadvantage of linear Voltage
regulators is that they are inefficient. This is because of the voltage drop
across the pass element. This inefficiency will cause the linear regulator to
be hot. Pay attention to the expected heat dissipation for your application,
and be sure to use adequate heats inking or a copper fill to manage the
temperature rise. If high power, efficiency, or an increase convertor is
desired, use a DC-DC convertor.
Variations on the linear regulator
Linear Voltage regulators come in many varieties. Some linear
regulators have fixed outputs. Some have outputs programmable by resistor
divider. Some regulate negative voltages. Low-dropout regulators, known as
LDOs, have a small dropout voltage. Some linear regulators include battery
charging smarts. Some are complex, programmable chips used in automated test
equipment. It is common to linear regulators to have over-temperature
closedown.
Important datasheet parameters
Maximum input voltage: This is the maximum voltage that can be applied at the input
terminal without damaging or destroying the part.
Input voltage-output voltage differential: Some adjustable linear regulators
have a most input-output voltage differential rating.
Current rating:
the most current the linear regulator will source. This is dependent on other
factors such as input-output voltage differential, ambient temperature, and
heats inking. The power rating of the package indicates how much power the
package can dissipate; this may be dependent on heats inking and layout
requirements.
Drop-out voltage: This is the minimum input-output voltage differential the device can
accept and produce the regulated voltage.
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