24-Output PWM LED Drivers for Message Boards

The MAX6974/MAX6975 precision current-sinking, 24-output PWM LED drivers drive red, green, and blue LEDs for full-color graphic message boards and video displays. Each output has an individual 12-bit (MAX6974) or 14-bit (MAX6975) PWM-intensity (hue) control and 7-bit (MAX6974) or 5-bit (MAX6975) global PWM intensity (luminance) control. The MAX6974/MAX6975 feature a high-speed, fully buffered cascadable serial interface, open-circuit LED fault detection circuitry, as well as a watchdog timer.

The driver has three banks of eight outputs, with each bank intended to drive a different color in RGB applications. The full-scale current for each bank of eight outputs is adjustable from 6mA to 30mA in 256 steps (0.3125% per step) to calibrate each color.

The MAX6974/MAX6975 can optionally multiplex by using outputs active-low MUX0 and active-low MUX1, which each drive an external pnp transistor. Multiplexing doubles the MAX6974/MAX6975 drive capability to 48 LEDs.

The MAX6974/MAX6975 operate from a 3.0V to 3.6V power supply. The LED power supply can range from 3V to 7V. The LED drivers require only 0.8V headroom above the LEDs' forward-voltage drop. Using a separate LED supply voltage for each LED minimizes power consumption.

The serial interface uses differential signaling for the high-speed clock and data signals to reduce EMI and improve signal integrity. The MAX6974/MAX6975 buffer all interface signals to simplify cascading devices in modules that use a large number of drivers.

An internal watchdog timer, when enabled, automatically clears the pixel-data registers and blanks the display if any of the signal inputs fail to toggle within 40ms.

The MAX6974/MAX6975 are available in 40-pin TQFN packages and operate over the -40°C to +125°C temperature range.

Refer to the MAX6972/MAX6973 data sheet for a 16-output, 11mA to 55mA software-compatible device.

Applications

* Graphic Panels * LED Message Boards * LED Video Display Panels * Signs * Variable Message Signs (VMS)

* 24 LED Current Sink Outputs (Three Banks of Eight Outputs)

* 48 LED Drive Option When Multiplexing

* 33MHz Clock Supports Up to 63 Frames per Second of Video

* Constant Output Current Calibration from 6mA to 30mA in 256 Steps

* EZCascade™ Interface Simplifies Multiple Driver Cascading Without External Buffers

* 12-Bit or 14-Bit Individual PWM LED Intensity Controls

* 7-Bit or 5-Bit Panel PWM-Intensity Control

* +3V to +7V LED Power Supply

* +3.0V to +3.6V Logic Supply

* Open-Circuit LED Fault Detection

* Optional Watchdog Timer Blanks Display if Interface Fails

* Standard -40°C to +125°C Operating Temperature Range