Controlled dimming of tunable white and colour lighting LEDs

September 09, 2015 // By Graham Prophet
Aimed at smart connected lighting applications that use tunable white and colour LED bulbs, the AL1791/2/3/4 from Diodes Inc. are single-, dual-, triple- and quad-channel DC to constant current (DC-CC) LED driver ICs that provide both analogue and PWM dimming control.

When combined with an AC-DC power conversion block and a microcontroller, these current-ratio-optimised regulators provide a cost-effective, scalable and easy-to-implement solution for smart lighting. Operating from a 6.5V to 30V input, the constant-current regulators can drive up to four independent LED channels with a maximum of 500 mA per channel or 1.5A total drive current for the AL1792/3/4, or 1A maximum for the AL1791. Each channel can be individually controlled down to 0.125% with 500 Hz to 10 kHz flicker-free PWM dimming or with analogue dimming down to 1.25%. Using the same pin for both inputs ensures the appropriate analogue or PWM dimming response.

Support for deep dimming (to below 1%) is important because the human eye perceives light strength as the square root of the measured light, which is proportional to the dimming current. Similarly, while the human eye only notices flicker below 120 Hz, the ability to operate PWM dimming above 1 kHz avoids problems with e-Flicker, which is the flicker picked up by cameras such as those found in smart phones that can result in dark bands on still images. The AL179X is capable of high frequency PWM for deep dimming applications: 10 kHz at 1.0%, 4 kHz at 0.4% and 1 kHz at 0.1%.

The AL179x LED drivers all feature an enable pin for low standby power, internal protection for LED string open/short-circuit and under voltage lockout, over-temperature protection with thermal shutdown and auto-recovery, and full fault reporting as well as LED power-good reporting. An integrated low-side current sink topology allows for either lower cost common-anode LED connections or different anode voltages for improved energy-efficiency. All devices operate over an ambient temperature range of -40C to +125C and are packaged in the same 14-pin UDFN-4030 package, which allows for easy scalability of LED driver design in smart, connected-lighting applications.

Unit pricing is from $0.64 for the single-channel AL1791AFE-13 (1000).