3An Introduction to the Linear Regulator
on. ” The negative feedback loop determines the degree of conductivity the pass unit
should assume to maintain the output v
The heart of the negative feedback loop is a high-gain operational ampliﬁ er called a voltage
error ampliﬁ er. Its purpose is to continuously compare the difference between a very stable
voltage reference and the output voltage. If the output differs by mere millivolts, then a
correction to the pass unit ’ s conductivity is made. A stable voltage reference is placed on
the noninverting input and is usually lower than the output voltage. The output voltage is
divided down to the level of the voltage reference. This divided output voltage is placed into
the inverting input of the operational ampliﬁ er. So at the rated output voltage, the center
node of the output voltage divider is identical to the reference voltage.
The gain of the error ampliﬁ er produces a voltage that represents the greatly ampliﬁ ed
difference between the reference and the output voltage (error voltage). The error
voltage directly controls the conductivity of the pass unit thus maintaining the rated
output voltage. If the load increases, the output voltage will fall. This will then increase
the ampliﬁ er ’ s output, thus providing more current to the load. Similarly, if the load
decreases, the output voltage will rise, thus making the error ampliﬁ er respond by
decreasing pass unit current to the load.
The speed by which the error ampliﬁ er responds to any changes on the output and how
accurately the output voltage is maintained depends on the error ampliﬁ er ’ s feedback loop
compensation . The feedback compensation is controlled by the placement of elements
within the voltage divider and between the negative input and the output of the error
ampliﬁ er. Its design dictates how much gain at DC is exhibited, which dictates how
accurate output voltage will be. It also dictates how much gain at a higher frequency and
bandwidth the ampliﬁ er exhibits, which dictates the time it takes to respond to output
load changes or transient response time.
The operation of a linear regulator is very simple. The very same circuitry exists in the
heart of all regulators, including the more complicated switching regulators. The voltage
feedback loop performs the ultimate function of the power supply—the maintaining of
the output voltage.
1.2 General Linear Regulator Considerations
The majority of linear regulator applications today are board-level, low-power
applications that are easily satisﬁ ed through the use of highly integrated three-terminal
regulator integrated circuits. Occasionally, though, the application calls for either a higher
output current or greater functionality than the three-terminal regulators can provide.
There are design considerations that are common to both approaches and those that are
only applicable to the nonintegrated, custom designs. These considerations deﬁ ne the