Simplified 5V tolerance circuit for 3.3V I/O design

摘要

A low voltage interface circuit with a high voltage tolerance enables devices with different power supply levels to be efficiently coupled together without significant leakage current or damage to the circuits. The interface circuit includes an impedance control circuit, an output buffer, an input buffer, an isolation circuit, and a pullup protection circuit. The output buffer includes a pullup transistor and a pulldown transistor for applying an output signal to an I/O pad. When a high voltage (i.e., higher than the internal voltage of the interface circuit) is applied to the I/O pad, the pullup protection circuit drives the gate of the pullup transistor to the high I/O pad voltage to ensure that no current flows to the positive supply voltage. Also, the isolation circuit couples the high I/O pad voltage to the body (well) of the pullup transistor to prevent leakage current through parasitic diodes formed by the pullup transistor.