《A Matter Of Light Part4》.pdfVIP

A Matter of Light Part 4: PWM Dimming Introduction Whether an LED is driven by a buck, boost, buck-boost or linear regulator, the common thread that links every drive circuit is the need to control the light output. A few applications are as simple as ON and OFF, but most need control in-between 0 and 100%, often with fine resolution. Two main choices for light control exist: adjust the LED current linearly (analog dimming) or flipping back and forth between 0 and a target drive current at a frequency high enough for the eye to average the light output (digital dimming). Using PWM to set the period and duty cycle is perhaps the easiest way to accomplish digital dimming, given that the same technique controls most switching converters to begin with. IF = 1A VIN CB 10 nF L1 5V/DIV VIN BOOT SW VDIM 33 H CIN RON D1 4.7 F 68.1 k RON LM3404 CS VDIM RSNS 0.22  0.5A/DIV DIM GND VCC CF IF 0.1 F 400 kHz 4μs/DIV Figure 1: LED Driver Using PWM Dimming with Waveforms PWM Dimming Preferred Analog dimming is often simpler to implement. The output current of the LED