Three-channel constant-current LED driver with PWM brightness control for large display applications

22nd October 2007
Posted By : ES Admin
Three-channel constant-current LED driver with PWM brightness control for large display applications

The new A6281 from Allegro MicroSystems Europe is a three-channel constant-current LED driver IC with an on-chip oscillator for simple programmable brightness control.

The new device is ideally suited to driving clusters of red/green/blue (RGB) LEDs in applications where the cluster forms one pixel in a large display or a lighting fixture. It precisely controls LED brightness 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 150 mA per channel and operates from a supply voltage of up to 17 V, which allows 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 allows large numbers (hundreds, depending on clock frequency and pixel spacing) of devices to be daisy-chained. Pixel spacing can range up to 3 metres.

Output current is programmable in three ways: a single external resistor sets the maximum current that all outputs can drive; three 7-bit 'dot correction' registers individually set the output currents for the three channels; and three 10-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 thus 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 aging.

The PWM registers typically control the brightness of the LEDs at high refresh rates to display text, graphics and video. The three 10-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 synchronize pixels at up to 5 MHz 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 3 mm QFN package, allow it to be used in compact board-level designs for applications such as large video displays and signs and architectural and decorative lighting.

The A6281 is available in the 'E' extended temperature range of -40°C to +85°C.

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