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Pulse positioning for multi-channel PWM signals Disclosure Number: IPCOM000010469D
Publication Date: 2002-Dec-05
Document File: 1 page(s) / 21K

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LED-based backlights can be used for Backlight LCD displays for TVs, Monitors and other displays. The electronic driver circuit for the LEDs controls the light output in color and/or intensity. Known principles for light control are Amplitude Modulation (AM) and Pulse Width Modulation (PWM).

White Light can be generated by combining the light of Red, Green and Blue LEDs. However, the control algorithm for an electronic driver for three color LEDs can be rather complex. That is why it is advantageous to use micro controller or a DSP in the control loop for such driver.

A drawback of AM control of the LED current is the need for a D/A converter stage. Another drawback is thatwith low amplitudes of the LED current non ‑ uniformity’s in LED light output occur.

A drawback of PWM control of the LED current is that the LEDs are always driven for a certain amount of time on the maximum current amplitude. This is a disadvantage for LED lifetime.� Furthermore a square wave pulse (like with PWM) generates more EMI.

A micro-controller controlled PWM current generation has freedom of pulse positioning. By use of an appropriate PWM pulse-positioning scheme, the maximum current amplitude of the driver can be reduced significantly. If used in combination with AM, the duty factor can be made as high as possible, thus eliminating short periods of overlap and current spikes.

In fig.1 the waveforms are shown present in a system utilizing a ba...