Vdd

Vdd is the nominal positive supply voltage used by many digital integrated circuits, especially those based on MOSFET technology. It represents the higher rail that powers transistors in a circuit, with the complementary rail commonly designated as Vss (ground) or Vee in some analog contexts. In CMOS and NMOS/PMOS designs, Vdd is the positive supply, while Vss is the reference ground.

Common values vary by technology. TTL-era logic typically used Vdd around 5 volts, while many modern CMOS devices run at 3.3 volts, 2.5 volts, or lower. Some processors use multiple rails, such as a core Vdd near 1.0–1.2 V and an I/O Vdd near 1.8–3.3 V. The actual voltage is specified in device data sheets and may tolerate a percentage variation around a nominal value.

Electrical design considerations: Vdd and ground define logic levels and thresholds; fluctuation can cause errors or timing changes. Designers use decoupling capacitors and proper voltage regulators to maintain stable Vdd, and consider noise, supply droop, and electrostatic discharge. Dynamic power consumption scales approximately with Vdd squared, so reducing Vdd can reduce power but may require re-timed or re-architected circuits to maintain performance.

Terminology: Vdd is contrasted with Vcc (historically the supply for bipolar transistors) and with Vee or Vss (negative rail or ground). The exact naming conventions can vary by company or technology, but Vdd remains a standard label for the main positive supply in many MOS-based circuits.