current-limiting resistors and series-pass
input voltages, the efficiency of the switching
regulator is better than the series-pass regulator
and the resistor-limited circuits. Assuming a
0.25 V drop across the sense resistor, then for
the ten-string circuit, RSENSE would be equal to
(0.25 V / 0.500 A), or 0.5 ohms. Assuming a
power conversion efficiency of 80% and an input
voltage range of 7 to 18 volts, then the input
current and total load current of the circuit would
vary as shown in Figure 3.27.
regulators can only reduce the output voltage to
a lower value than the input voltage. Thus for
some types of switching regulator circuits the
number of LED emitters per string can be larger
than the number of LED emitters per string for a
resistor-limited or series-pass regulator circuit.
In general, the switching regulator converts the
average input power (VIN times IIN) to the desired
output power (VLOAD times ILOAD) with a relatively
fixed power conversion efficiency. At higher
Figure 3.26 Block Diagram
of LED Signal Lamp
Design Using Switching
Constant-Current
Regulator.
Figure 3.27 Forward Current Through LED
Emitters as a Function of Applied Voltage for
Switching Constant-Current Regulator Circuit
Shown in Figure 3.26.
Figure 3.28 Comparison of Total Supply
Current versus Applied Voltage for Circuit
Designs Shown in Figures 3.22, 3.24 and 3.26.
The total power consumption for the four
different LED signal lamp designs is shown in
Figure 3.28. The series pass and switching
regulator designs provide substantial power
savings compared to the resistor-controlled
circuits during over-voltage conditions. Note
that at an input voltage of 18 V, both resistor
limited circuits have an overall power consumption
of 15 W. The series pass current regulator circuit
has an overall power consumption of 9 W. The
switching current regulator has an overall power
consumption of 5 W.
36
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
