Atmel announces LED driver ICs for high-CRI color-mix SSL applications

Feb. 19, 2013
At the SIL conference, Atmel announced a driver IC family designed specifically for high-CRI lamps and luminaires that use a mix of phosphor-converted white LEDs and red or amber LEDs that combined deliver good light quality and high efficacy.

Atmel has announced the MSL202x family of LED driver ICs that can independently power two LED channels, enabling lamps and luminaires that deliver both high-CRI, warm-white light, and high efficacy. Launched at the Strategies in Light (SIL) conference, the ICs fit in with a prevalent SIL theme of the emergence of intelligent tunable lighting.

The concept of using a color mix to achieve high CRI and high efficacy in warm-white SSL products came originally from Cree in the True White products. More recently, Osram Opto Semiconductors has supported the concept with its Brilliant Mix technology.

In any case, the idea is to use red or amber LEDs to add spectral power in the visible red wavelengths in an efficient manner. Phosphors that can deliver such wavelengths are generally inefficient because they also generate energy that falls outside the human-visible spectrum, thereby wasting energy.

The use of two-channel driver electronics is a requisite in a color-mixed white SSL products. The white and red or amber LEDs have different forward voltage characteristics. Moreover, more white than color LEDs are generally needed. And the driver has to adjust the relative drive currents during dimming to achieve an incandescent-like, dim-to-warm-CCT experience.

In many cases today, SSL product developers must utilize two separate driver ICs to implement color-mixed designs. The Atmel MSL202x family is designed specifically for such applications.

The ICs include a linear controller used to drive a primary channel of white LEDs and a buck controller designed to drive a color LED string. The IC requires external MOSFETs. Atmel made the decision not to integrate MOSFETs to provide product developers freedom in supporting a range of string voltages.

Tushar Dhayagude, director of LED products at Atmel, said a typical design might include a 170V string of white LEDs and a 50V string of red LEDs. The external MOSFETs support such high voltages or lower-voltage implementations such as a 30V white string and a 12V color string that might be more typical of a retrofit lamp.

There are three versions of the new IC. The MSL2021 includes a temperature compensation feature that allows autonomous accurate control of the color-LED string. Red LEDs change in light output as temperatures rise at a different rate than white LEDs do, and therefore can change the appearance of the light. Some designs have utilized optical sensors to provide feedback and allow tuning of red brightness level to combat the change.

Thermal feedback can be used in a similar manner because the LED manufacturers provide LM-80 data with accurate projections of LED performance relative to temperature. Dhayagude said the temperature compensation offers performance "very close to the optical feedback." The MSL2021 stores color compensation data in an integrated non-volatile memory array.

The other ICs in the family include the MSL2023 and MSL2024, which are optimized for usage with a companion microcontroller that would perform the color-LED compensation. The MSL2023 includes an I2C serial interface to connect with a microcontroller and internal pulse-width-modulation (PWM) generators. The MSL2024 accepts PWM inputs from a microcontroller.

If power-factor correction (PFC) is required in an SSL design, driver designers will need to add one-or two-stage PFC ICs in front of the new Atmel driver ICs. The ICs can operate in isolated or non-isolated driver designs.

Atmel is also offering an evaluation system for product developers looking to use the new ICs. The kit includes an LED light engine with an appropriate heat sink. And the circuit board includes an Atmel AVR microcontroller. Development teams can use the kit to instantly begin prototyping work.