The AT9531 is an 8-bit I2C-bus and SMBus I/O expander optimized for dimming LEDs in 256 discrete steps for Red/Green/Blue (RGB) color mixing and back light applications. The AT9531 contains an internal oscillator with two user programmable blink rates and duty cycles coupled to the output PWM. The LED brightness is controlled by setting the blink rate high enough (> 100 Hz) that the blinking cannot be seen and then using the duty cycle to vary the amount of time the LED is on and thus the average current through the LED. The initial setup sequence programs the two blink rates/duty cycles for each individual PWM. From then on, only one command from the bus master is required to turn individual LEDs ON, OFF, BLINK RATE 1 or BLINK RATE 2. Based on the programmed frequency and duty cycle, BLINK RATE 1 and BLINK RATE 2 will cause the LEDs to appear at a different brightness or blink at periods up to 1.69 second. The open-drain outputs directly drive the LEDs with maximum output sink current of 25 mA per bit and 100 mA per package. To blink LEDs at periods greater than 1.69 second the bus master (MCU, MPU, DSP, chip set, etc.) must send repeated commands to turn the LED on and off as is currently done when using normal I/O expanders like the NXP Semiconductors AT8574 or AT9554. Any bits not used for controlling the LEDs can be used for General Purpose parallel Input/Output (GPIO) expansion, which provides a simple solution when additional I/O is needed for ACPI power switches, sensors, push-buttons, alarm monitoring, fans, etc. The active LOW hardware reset pin (RESET) and Power-On Reset (POR) initializes the registers to their default state causing the bits to be set HIGH (LED off). Three hardware address pins on the AT9531 allow eight devices to operate on the same bus.


  • Eight LED drivers (on, off, flashing at a programmable rate)

  • Two selectable, fully programmable blink rates (frequency and duty cycle) between 0.59 Hz and 152 Hz (1.69 second and 6.58 milliseconds)

  • 256 brightness steps

  • Input/outputs not used as LED drivers can be used as regular GPIOs

  • Internal oscillator requires no external components

  • I2C-bus interface logic compatible with SMBus

  • Internal power-on reset

  • Noise filter on SCL/SDA inputs

  • Active LOW reset input

  • Eight open-drain outputs directly drive LEDs to 25 mA

  • Edge rate control on outputs

  • No glitch on power-up

  • Supports hot insertion

  • Low standby current

  • Operating power supply voltage range of 2.3 V to 5.5 V

  • 0 Hz to 400 kHz clock frequency

Block diagram

Typical Application Circuit