There are many advantages to digital control in lighting applications including improved performance, improved efficiency, added protection for reliability and added features that cannot easily be implemented with full analog control. However, traditional approaches require and expensive DSP or high performance microcontroller. A newly developed platform of digital controllers for lighting, the STLUXTM, uses dedicated hardware, the SMED, too implement the PWM function. The first member of this family STLUX385A provides digital LED lighting control with easy design at a much lower cost than the traditional methods.
What is a SMED
SMED stands for State Machine Event Driven. A SMED is a software configurable PWM generator that enables true real-time processing without the intervention of a DSP or µP. The SMED is essentially a state machine that can be triggered from the internal timer or from external events. When coupled with the on chip comparators, the SMED can respond to analog events such as reaching a desired peak current, over current detection or detection of a voltage or current zero crossing.
The analog sensing capability allows the SMED implement a constant off time peak current detecting buck converter without any intervention from the core processor. The internal timer sets the off time and also sets up minimum on time, to ignore switching noise, and a maximum on time that insures a nice soft start. A PI control loop in the software can easily set the value of the desired peak current to perform the output regulation.
In LLC resonant controllers the analog sensing is used to sense the zero crossing of the current at the end of the off time to start the next cycle and also for over current sensing and protection.
The combination of using the internal timer and external digital or analog events makes the SMED a very powerful PEM controller.
For half bridge configurations, the two gate drive signals with inserted dead time can easily be implemented by coupling two SMED cells together.
The STLUX385 has 6 SMED cells and 4 analog comparators plus on board A/D converters and opamps for other measurement requirements.
Digital Control in LED Lighting
In LED lighting applications the SMEDs can be used to implement the PFC control, control of the LC or LLC resonant converter, buck or inverse buck current control, and many other topologies. Since the STLUX385 has multiple SMED it is possible to implement multiple configurations on a single STLUX385.
Single String Configuration
For a high power, single string configuration the control for the PFC and the control for the half bridge LC conversion stage can both be implemented in a single STLUX385 replacing the PFC controller IC and the LLC resonant controller IC. Two SMED cells and two of the analog comparators are used for the PFC stage and three SMED cells and two analog comparators are used for the half bridge LC converter.
Multiple String Configuration
In a multiple string configuration, the SMED cells can be configured to control multiple buck or inverse buck circuits. A single SMED can be used to independently control the current in each string of LEDs.