New-Tech Europe | March 2017 | Digital Edition

Automotive Special Edition

an error amplifier (EA) with a Type II compensator configuration. The FVR is used as the DAC input to provide voltage reference to the OPA non- inverting input based on the LED constant current specification. In order to achieve dimming, the PWM3 is used as a modulator of the CCP output while driving the MOSFET Q2 to rapidly cycle the LED ON and OFF. The modulation is made possible through the DSM module and the modulated output signal is fed to the COG. PWM3 provides pulse with variable duty cycle which controls the average current of the driver and in effect controls the brightness of the LED. The LED dimming engine can not only accomplish what the typical LED driver controller does but it also has features that solve the typical problems that an LED driver poses. We’ll now walk through these problems and how a LED dimming engine can be used to avoid them. Flickering Flickering is one of the challenges that typical switched-mode dimmable LED drivers may have. While flickering can be a fun effect when it’s intentional, when LEDs inadvertently flicker it can ruin the user’s desired lighting design. In order to avoid flickering and provide a smooth dimming experience, the driver should perform the dimming step from 100% light output all the way down to its low-end light level with a continuously fluid effect. Since the LED responds instantaneously to current changes and doesn’t have a dampening effect, the driver must have enough dimming steps so the eye does not perceive the changes.

Figure 1: Diagram of four LED strings being controlled by a Microchip PIC16F1779 8-bit microcontroller

(DSM), comparator, programmable ramp generator (PRG), op amp (OPA), and pulse-width modulator 3 (PWM3). Combining these CIPs with other on-chip peripherals, such as fixed-voltage regulators (FVR), digital-to-analog converters (DAC) and Capture/Compare/PWM (CCP), completes the whole engine. The COG provided the high frequency switching pulse to MOSFET Q1 to allow the transfer of energy and supply current to the LED string. The switching period of the COG output

is set by the CCP and the duty cycle, which maintains the LED constant current and is dictated by the comparator output. The comparator produces an output pulse whenever the voltage across Rsense1 exceeds the output of PRG module. The PRG, whose input is derived from OPA output in the feedback circuit, is configured as a slope compensator to counteract the effect of inherent subharmonic oscillation when the duty cycle is greater than 50%. The OPA module is implemented as

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