36
6 Application Note
DC2100
Combination of Linear and Switching Drivers
This method combines the advantages of both method. A switching driver is used to
adjust the supply voltage to value near the LED forward bias. A following linear driver
generates a constant current with low ripple. The heat dissipation is reduced to a
minimum.
The disadvantage of this method is the use of more expensive components which
also need more space than conventional constant current linear or switching drivers.
6.2
Pulse Width Modulation
The Pulse Width Modulation is a modulation whereas the magnitude of the
parameter (e.g. electrical current) is switched between two values. The duty cycle is
modulated at a constant frequency. The demodulation is done with a low pass filter
in general. The resulting signal corresponds to the arithmetic mean value.
Figure 36
Pulse Width Modulation
If a LED is driven with a PWM the current through the LED is modulated. The period
T results from the frequency f (T = 1/f). The duty cycle is the ratio of 'On-Time' to
period (ton/T).
