LevelShifter

A level shifter, or voltage level translator, is a circuit that converts digital signals from one voltage domain to another. It allows devices operating at different supply voltages, such as 3.3V and 5V logic, to communicate without exceeding component limits. Level shifters are used in microcontroller interfaces, sensors, buses, and other interconnections.

There are several approaches. For bidirectional signaling on open-drain buses such as I2C, a MOSFET plus pull-up resistors on each side provides safe translation and allows signals to travel in either direction. Dedicated translator ICs provide either unidirectional or bidirectional translation with defined speed and load specifications. Simpler resistor networks or capacitive methods exist for certain unidirectional, fast signals, but they are less robust for general digital interfaces.

Operating principle (high level): In a MOSFET-based bidirectional shifter, each side is pulled up to its respective voltage. When one side pulls low, the line goes low and the MOSFET conducts, pulling the other side low as well. When both sides are high, the transistor remains off and the voltages stay within their rails. This arrangement supports automatic direction sensing and helps preserve signal integrity across voltage domains.

Considerations include voltage ranges, required bandwidth and timing, directionality, drive capability, leakage and parasitics, and the need for isolation or ESD protection. Some devices require unidirectional translators for push-pull interfaces, while open-drain buses benefit from bidirectional shifters. Always respect the maximum ratings of each device.

Applications include interfacing microcontrollers with sensors, memory, or peripheral ICs, bridging I2C, SPI with level-shifted lines, and multi-voltage development boards.