As shown in Figure 3.21, there are three basic
types of regulator circuits. The circuits shown in
Figures 3.21a and 3.21d are called “shunt”
regulators. They use a dynamic load in parallel
with the load being regulated that shunts some
of the supply current around the load. Shunt
regulators also have a power resistor in series
with both loads. The value of the power resistor
has been selected such that at the minimum
input voltage and maximum load condition, the
current through the dynamic load goes to zero.
At higher input voltages or smaller loads, the
current through the dynamic load is increased,
which increases the voltage drop across the
power resistor to keep the load current or
voltage constant. In this way, the shunt
due to the large variations in input voltage and
will not be covered further in this section.
The circuits shown in Figures 3.21b and 3.21e
are called “series-pass” regulators. They use a
dynamic load in series with the load being
regulated. At minimum input voltages the voltage
drop across the dynamic load goes to a
minimum value. This minimum voltage drop is
called the “drop-out” voltage. At higher input
voltages, the voltage drop across the dynamic
load increases so as to maintain either a fixed
current or voltage across the load. At voltages
below the drop-out voltage, the dynamic load
can no longer regulate the output voltage or
current. Thus, for proper voltage or current
regulation, the input voltage needs to be higher
than the sum of the drop-out voltage, the voltage
across the load, and the voltage drop across the
sense resistor (if applicable).
regulator maintains either a fixed current or
voltage across the load. Shunt regulators are
not very practical for automotive signal lamps
Figure 3.21 Block Diagrams of Several Active Drive Circuits for LED Signal Lamps.
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