NXP – PCA9955ATW: 16-Channel Fm+ I2C-Bus 57mA/20V Constant Current LED Driver

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Electronic content in automobiles increases each year. LEDs find new uses for exterior and interior lighting and as status indicators for driver and passenger control panels. New LED driver ICs – that do more than simply turn the LED on and off – are now available in automotive grade.

Automotive LED applications can be broadly divided into two types: illumination and signaling. While high power LEDs are applied to exterior and interior lighting, adding smaller LEDs to driver and passenger control panels still requires careful design and the addition of safety related features. For example, in critical applications in brakes and engine health monitors, the LED itself must report opens or shorts that might otherwise cause false operation.

The latest from NXP (PCA9955ATW/Q900) requires only two IO port pins over the ubiquitous I2C or SPI buses built into most MCU ICs. Up to sixteen channels of LEDs can be controlled for ON-OFF or brightness (using PWM), and these channels can also be assigned to groups for global dimming, making it ideal for automotive applications in sunlight and moonlight.

A recent addition to these clever LED drivers is the ability to perform gradation – an automatic cycle that controls ramp up and ramp down of LED brightness. This is commonly used to indicate a sleep state, when powered down, with only a tell-tale LED ‘breathing’. Registers in this LED driver are easily programmed with ‘ramp up’, ‘hold’, ’ramp down’ and ‘off’ durations, so that LED gradation (blinking and breathing) gives the system architect unique signaling options with no burden on the MCU overhead.

For backlighting with a string of LEDs, these LED drivers provide constant current (for better LED matching) and up to 20V needed for longer strings.

Each channel can sink over 50mA with no additional components needed, not even traditional LED ballast resistors. LEDs connect directly to this LED driver IC, which only requires a single global maximum current setting resistor.

These devices are now available in automotive grade AEC-Q100 defined by the Automotive Electronics Council (AEC).

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PCA9955A: Simplified Block Diagram

PCA9955A: Simplified Block Diagram

Feature Benefit
Gradation control Automatic “breathing effect”
Fault detector Real-time reporting of LED open, short, or over-temp
Constant current drive Improved matching (multi-LED strings) and eliminates LED ballast resistors
57mA/20V LED drivers Drive strings with up to six LEDs in series
4% channel to channel current match Simplifies LED brightness matching
8-bit (256 step) PWM control Wide range and smooth dimming
HTSSOP28 package Small footprint (with thermal pad)

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