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5 Appendix
17
Combination of Linear and Switching Drivers
This method combines the advantages of both principles. A switching driver is used to adjust
the supply voltage to a value close to the LED forward bias. It is followed by a linear driver, gen-
erating constant current with low ripple. In such a design the heat dissipation is reduced to a
minimum. Thorlabs DC2100 LED Driver is based on this approach.
The disadvantage of this method is the use of more, often expensive components and the re-
quirement for more space than conventional constant current linear or switching drivers.
5.2.2 Pulse Width Modulation
The change of the duty cycle of a pulse train, having a constant amplitude and pulse frequency,
is called Pulse Width Modulation. The magnitude of the parameter (e.g. electrical current) is
switched between two values. The duty cycle is varied (modulated) while maintaining a con-
stant frequency. The demodulation is done usually by a low pass filter.
Pulse Width Modulation
When a LED is pulse-width modulated, the amplitude
I
max
of the current is constant, while the
ratio of "On"-time (
t
on
) to "Cycle"-time (period
T
of the pulse train) is varying. The ratio
t
on
/
T
is
know as "duty cycle". By varying the duty cycle, the brightness of the LED can be changed, it
corresponds to the arithmetic mean current value.
5.2.3 LED Connector
A custom LED connection cable (one end open) is supplied with each T-Cube LED driver. The
figure below shows the pin-out of the male connector of this CAB-LEDD1. It is a standard M8x1
sensor connector. Pin 1 and 2 are the connections to the LED. Pin 3 and 4 must not be connec-
ted.
Male Connector of the CAB-LEDD1