Figure 3.23 Forward Current Through LED
Emitters as a Function of Applied Voltage
for Resistive Limited Circuits Shown in
Figure 3.22.
Figure 3.24 Block Diagram of LED Signal Lamp Design
Using Series-Pass Constant-Current Regulator.
The key elements of the series pass constant-
current regulator are shown in Figure 3.24. For
series strings of three forward voltage category
3 HPWT-DH00 emitters, the forward voltage of
the string is about 7.10 V at 50 mA. Assuming
a voltage drop across the sense resistor of 0.25
V, then at an input voltage of 9 V, the drop-out
voltage of the regulator would be (9 V – 7.1 V –
0.25 V), or 1.65 V. Since there are 10 strings of
LED emitters, the total LED array current would
be 50 mA times 10, or 500 mA. Thus, the sense
resistor would be (0.25 V / 0.500 A), or 0.5
ohms. Then over an input voltage range of 7
to 18 volts, the total load current of the circuit
would vary as shown in Figure 3.25. As
Figure 3.25 Forward Current Through LED Emitters
as a Function of Applied Voltage for Series-Pass
Constant-Current Regulator Circuit Shown in Figure
3.24.
The key elements of the switching constant-
current regulator are shown in Figure 3.26. There
are a number of different types of switching
regulators. Buck or Down Converters are
designed, this circuit maintains a constant
current through the LED array at input voltages
greater than 9 V. Suppose that the minimum
compliance voltage of the circuit is designed to
be 10 V, then an additional volt can be dropped
across the load or series pass regulator.
designed to generate a regulated output voltage
that is always less than the input voltage. Boost
or Up Converters are capable of generating a
regulated output voltage that is always higher
than the input voltage. Buck/Boost or Up/Down
Converters can generate a regulated output
voltage using any input voltage. By comparison,
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