In LED lighting applications it is desirable or necessary to control the light intensity of the LEDs for overall dimming, color mixing, and compensation for aging of the components. There are basically two competing methods to control LED light intensity:
1. Linear mixing system - LMS
2. Pulse width modulation - PWM
In the case of the linear mixing system, the LED current (dc level) is varied to directly control the steady-state current in the LED device. In the case of pulse width modulation, the duty cycle of the LED current (at maximum level Is varied to change the average current in the LED device.
The basic concept is to vary the duty cycle of the LED "on time" to achieve a luminance output that is proportional to the applied duty cycle.
However, as the duty cycle changes there are also temperature changes in the devices that will change the luminance and shift the color. In the case of color mixing, these changes must be detected and compensated to achieve optimal performance.
Also, if optical efficiency is important, it may be better to drive the LED in a more optimal current range to minimize the power lost as heat. LMS needs to maintain accurate current control to be effective. With proper integrated solutions, this can be readily Achieved so that the inherent advantages of the LMS method can be realized.
LMS operates at the lowest forward current and voltage point to achieve a given light output with optimum efficiency from the LED. Outside of the LMS controller, the circuitry required for the LMS approach operates at a much lower frequency range requiring simpler, low bandwidth circuits. This results in lower cost and less EMI.
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