LED Dimming Engine: An 8-bit MCU-based
solution for a Switched-Mode Dimmable
Mark Pallones, Microchip Technology
Switched-mode dimmable LED drivers are known for their efficiency and precise
control of LED current. They can also provide dimming functionality which allows the
end user to create fantastic lighting effects while reducing their power consumption.
An 8-bit microcontroller (MCU) implementation can provide the necessary building
blocks to create solutions that enable communications, customizations and intelligent
control. Additionally, core independent peripheral integration provides significant flexi-
bility versus that of pure analog or ASIC implementation and enables innovation that
expands lighting product capabilities and provides product differentiation. Features
such as predictive failure and maintenance, energy monitoring, color and temperature
maintenance and remote communications and control, are just some of the advanced
capabilities that can make intelligent lighting solutions even more attractive.
Although LED drivers offer many advantages over previous lighting solutions, there
are also challenges in their implementation. But fear not, by the end of this article
youll learn how an 8-bit MCU can be used to alleviate design challenges and create
high-performance switched-mode LED driving solutions with capabilities beyond that
of traditional solutions.
An 8-bit microcontroller can be used to independently control up to four LED chan-
nels which is something most off-the-shelf LED driver controllers cannot provide. In
Figure 1, the LED dimming engines can be created out of the peripherals available in
the microcontroller. Each of these engines has an independent closed channel that
can control the switched-mode power converter with minimal to no central processing
unit (CPU) intervention. This leaves the CPU free to perform other important tasks
such as supervisory functions, communications or added intelligence in the system.
LED Dimming Engine
In Figure 2, the LED driver, which is based on the Current-Mode Boost converter, is
controlled by the LED dimming engine. The engine is mainly composed of core inde-
pendent peripherals (CIP) such as complementary output generator (COG), digital
signal modulator (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 when-
ever 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 an error amplifier (EA) with a Type II compen-
sator 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 CCPoutput
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 con-
troller does but it also has features that solve the typical problems that an LED driver
poses. Well now walk through these problems and how a LED dimming engine can
be used to avoid them.
Flickering is one of the challenges that typical switched-mode dimmable LED driv-
ers may have. While flickering can be a fun effect when its intentional, when LEDs
inadvertently flicker it can ruin the users 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 doesnt have a dampening effect, the driver must have enough dimming steps so
the eye does not perceive the changes. To meet this requirement, the LED dimming
engine employs PWM3 for controlling the dimming of the LED. The PWM3 is a 16-bit
resolution PWM that has 65 536 steps from 100% to 0% duty cycle, ensuring a smooth
LED Color Temperature Shifting
The LED driver can also shift the LEDs color temperature. Such color change can
be noticeable to the consumer and weaken claims made about the high-quality light-
ing experience of LEDs. Figure 3 shows a typical PWM LED dimming waveform.
When the LED is off, the LED current gradually diminishes due the slow discharge of
the output capacitor. This event can lead to color temperature shifting and higher
power dissipation of the LED.
Figure 1: Diagram of four LED strings being controlled by a Microchip
PIC16F1779 8-bit microcontroller
Figure 2. LED dimming engine