Linear Regulators
The linear regulator
is the basic building block of nearly every power supply used in
electronics. A voltage
regulator provides this
constant DC output voltage and
contains circuitry that continuously holds the output voltage at the design
value regardless of changes in load current or input voltage.
Why do we need Regulators ?
Let’s start with a
simple example. In an embedded system, a 12V bus rail is available from the
front-end power supply. On the system board, a 3.3V voltage is needed to power
an operational amplifier (op amp). The simplest approach to generate the 3.3V
is to use a resistor divider from the 12V bus.
Does it work well? The answer is usually no.
The circuit load
current current may vary under different operating conditions. If a fixed
resistor divider is used, the IC VCC
voltage varies with load. Besides, the 12V bus input may not be well regulated.
There may be many other loads in
the same system sharing the 12V rail. Because of the bus impedance, the 12V bus
voltage varies with the bus
loading conditions. As a result, a resistor divider cannot provide a regulated
3.3V to the op amp to ensure its proper
operation. Therefore, a dedicated volt-age regulation loop is needed.
That’s is the reason
we need a regulated power supply to a circuit for proper operation.
The Basic Linear Regulator
A linear regulator
operates by using a voltage-controlled current
source to force a
fixed voltage to
appear at the regulator output terminal.
The control
circuitry must monitor (sense) the output voltage, and adjust the current
source (as required by the load) to hold the output voltage at the desired
value. The design limit
of the current
source defines the
maximum load current
the regulator can source and
still maintain regulation.
The output voltage
is controlled using a feedback loop, which requires some type of
compensation to assure
loop
stability. Most linear
regulators have built-in compensation, and
are completely stable
without external components.
Some regulators (like Low-Dropout
types), do require
some external capacitance connected from the output lead to
ground to assure regulator stability.
Another
characteristic of any linear regulator is that it requires a finite amount of
time to "correct" the output voltage after a change in load current
demand. This "time lag"
defines the characteristic called transient
response, which is a measure of how fast the regulator returns to
steady-state conditions after a load change.
Why use Linear Regulators ?
In addition to their
simplicity of use, linear regulators have other performance advantages. Power
management suppliers have
developed many integrated linear regulators. A typical integrated linear
regulator needs only VIN, VOUT,FB and optional GND
pins.
Drawback ?
A major drawback of
using linear regulators can be the excessive power dissipation of its series
transistor operating in a
linear mode. As explained previously, a linear regulator transistor is
conceptually a variable resistor. Since all the load
current must pass through the series transistor, its power dissipation is PLoss
= (VIN – VO) x IO.
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