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INDUSTRIAL MARKET

4

SEE ELECTRONICS

LED Dimming Engine: An 8-bit MCU-based

solution for a Switched-Mode Dimmable

LED driver

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

you’ll 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. 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 driv-

ers 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. 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

lighting-level transition.

LED Color Temperature Shifting

The LED driver can also shift the LED’s 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