FIGURE 2.
The practical
implementation
of the LM317
adjustable regulator
requires a few
additional
components
for optimal
performance and
reliability. The
text describes the
additional parts
and their function.
be higher than the
output voltage for proper operation.
For the standard three-terminal voltage regulators, this voltage is about
1-2.5 V, depending on temperature
and current drawn. (However, for
proper design purposes the 2.5V
“drop-out” voltage should always be
employed.)
If the desired input-output voltage
difference is less than this 2. 5 V,
the circuit will no longer have the
headroom to regulate properly
and drop out of regulation. (There
are other “low-dropout” or LDO
regulators available that can operate
with much less input-output differential — some as low as 0.1 V. However,
care must be taken to provide proper
capacitive loading or else they may
not function properly.)
The third point is that it takes
some small amount of time for an
output change to propagate through
the op-amp and transistor (loop
delay). During this time, the output
is not being properly regulated.
Obviously, the faster the op-amp and
transistor, the shorter this transient
will be. However, manufacturing
fast, high-power transistors is not easy
or cheap.
Additionally, the faster the transistor and op-amp circuit is, the more
prone to oscillation it is. Having your
power supply oscillate is not a good
thing. Most three-terminal regulators
have a transient response (closed-loop
delay time) of about 30 μs.
The last point concerns adjustable
voltage regulators (Figure 1 shows a
fixed-voltage regulator). Without
getting into a long technical discussion, it is difficult to provide a stable
and cheap reference voltage below
1.2 V. This limits the minimum output
voltage of adjustable regulators.
However, we’ll see a cheap and easy
solution to this problem, later.
Additional Components
Needed
For the fixed voltage regulator,
only an input and output bypass
capacitor (connected to ground) are
required (typically 0.1 to 1.0 μF).
These are to stabilize the op-amp
in the regulator. If the large filter
capacitor is in close physical proximity
(less than 6”) to the regulator input,
then the input bypass capacitor can
be omitted.
In theory, the output capacitor
can also be omitted. But if your circuit
to be powered just happens to have
a load capacitance of 500 pF to
5,000 pF, then the regulator might
oscillate. Using a 1.0 μF output bypass
capacitor forces the op-amp into a
stable operating mode. It is always a
good idea to spend a few pennies on
this to be sure you have a reliable
power supply.
Figure 2 shows the proper design
for an adjustable regulator. The input
and output bypass capacitors (Ci
and Co) are the same as in the fixed
regulator implementation.
The two resistors (Rb and Radj)
are required to choose the output
voltage. Typically, Rp is set at 240
ohms to provide a proper current
(typically about 50 μA) into the
feedback of the op-amp. Radj is
used to vary the output to the
desired value. The capacitor connected to the adjustment pin (Cbyp)
is used to improve ripple/noise
rejection. This is especially useful if
the raw DC input is coming from a
FIGURE 3. By
substituting the load
for the adjustment
resistor, the LM317
becomes a current
regulator (the voltage
changes according
to the load). This is a
very useful circuit
that is not often used.
switching power supply which is
typically quite noisy.
The diodes are used to protect
the regulator against unexpected
voltage reversals. If the input should
be shorted to ground, capacitors on
the output and/or the adjustment pin
will still maintain their voltage. They
will then discharge backwards through
the respective pins towards the input.
This can destroy the device.
If there is no adjustment bypass
capacitor used (Cbyp), then the Dadj
can be omitted. If you are sure that
the device being powered will never
have 10 μF or more load capacitance,
then Do can be omitted, as well.
Adjustment Calculations
It is important to remember that
(for adjustable regulators) the
adjustment pin acts as a non-inverting
feedback point to the internal op-amp.
As such, the op-amp will do whatever
it can to maintain the voltage on this
pin (which is specified to be 1.25 volts
below the output). So if we apply a
voltage (through a resistor) to this pin,
the output will go up. If we short this
pin to ground, then the output will fall
to about 1.25 volts. If a negative 1.25
volts is applied, then the output will
go all the way to zero volts.
Generally, a voltage divider is
used to pass some of the output
voltage back into the regulator to set
the output voltage (as shown in Figure
2). The equation is Vout = Vref (1 +
Rp/Radj) + (Iadj x Radj). Vref is 1.25 V
as noted above. The second term (Iadj
x Radj) is the error correction for the
1.25 V reference. Generally this is
quite small and is often ignored. If this
is ignored and the typical value of 240
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