Posted to News on 23rd Oct 2007, 15:28
Three-channel LED driver suits large display applications
Allegro MicroSystems is launching the A6281, a three-channel constant-current LED driver IC with an on-chip oscillator for simple programmable brightness control. It is suitable for driving clusters of red/green/blue (RGB) LEDs in applications where the cluster forms one pixel in a large display or a lighting fixture. LED brightness is controlled precisely via 10-bit pulse-width modulation (PWM) on each channel, complemented by 7-bit analogue current control on each channel to adjust colour balance.
The A6281 drives up to 150mA per channel and operates from a supply voltage of up to 17V, which enables series strings of four or more LEDs to be used on each output. An on-chip voltage regulator supplies the internal logic and reduces the number of external components and connections.
Control data is loaded serially, which minimises the number of pins on the device and the package size. Only four control signals are needed: clock, serial data, latch and output enable. These signals are buffered on the chip to drive the next pixel in a daisy-chained cascade. An innovative clock regeneration scheme enables large numbers (hundreds, depending on clock frequency and pixel spacing) of devices to be daisy-chained. Pixel spacing can range up to 3m.
Output current is programmable in three ways: a single external resistor sets the maximum current that all outputs can drive; three seven-bit 'dot correction' registers individually set the output currents for the three channels; and three ten-bit PWM registers individually set the duty cycles of the three outputs.
The dot-correction registers typically adjust output currents to compensate for differences in efficiency of the LEDs on each output and thereby tune the white balance of the display. The dot correction registers can also be used to compensate for long-term changes in LED brightness with temperature and ageing.
One billion colours
The PWM registers typically control the brightness of the LEDs at high refresh rates to display text, graphics and video. The three ten-bit PWM registers allow a colour depth of 30 bits, giving the capability to produce over one billion colours.
The A6281 requires only three external components to operate: a resistor to set the maximum output currents and decoupling capacitors for the on-chip voltage regulator and the LED supply voltage.
Flexible clocking options are available to tailor the device to the application. To simplify system design, the A6281 can use its internal clock for PWM control and an external clock for loading data; alternatively, the A6281 can use the external data clock as its PWM clock to synchronise pixels at up to 5MHz in larger systems.
Thermal shutdown and undervoltage lockout are provided for self-protection and predictable operation under extreme conditions and faults.
The high level of integration of the new device, coupled with the tiny 3 x 3mm QFN package, enable it to be used in compact board-level designs for applications such as large video displays and signs, as well as architectural and decorative lighting.
Allegro's A6281 is available in the 'E' extended temperature range of -40 to +85degC.
