8-bit I/O expander for I2C and SMBus buses
The NXP PCA9557D,118 is an 8-bit I/O expander that communicates over I2C or SMBus at up to 400 kHz. It provides eight general-purpose I/O pins configurable as inputs or outputs, with open-drain or push-pull output drive. Each I/O can source 10 mA or sink 25 mA, enough to drive LEDs or small relays directly without a buffer transistor. A Power-On Reset (POR) circuit ensures the outputs start in a known high-impedance state, preventing glitches during power-up.
Supply voltage and I/O drive capability
Operating from 2.3 V to 5.5 V, the PCA9557D,118 covers the standard digital supply rails for microcontrollers and FPGAs. The 400 kHz I2C clock rate is compatible with most bus masters and allows daisy-chaining multiple expanders on the same two-wire bus. With 10 mA source and 25 mA sink per pin, the expander can drive indicator LEDs with a series resistor or switch small relays through a transistor. The open-drain output option allows wired-AND connections for shared interrupt lines.
Package and board-fit details
Housed in a 16-SOIC package with a 3.90 mm body width and 1.27 mm pin pitch, the PCA9557D,118 uses a standard SOIC-16 footprint. The surface-mount package is suitable for both reflow and hand-soldering in rework scenarios. The supplier device package is designated 16-SO, matching the common NXP SOIC-16 land pattern.