IC power supply pin
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Almost all integrated circuits (ICs) have at least two pins which connect to the power rails of the circuit they are installed in. These are known as the IC's power supply pins. However, the labeling of the pins varies by IC family and manufacturer.
| Typical supply pin labeling | ||||
|---|---|---|---|---|
| BJT | FET | |||
| VCC | VDD | V+ | VS+ | Positive supply voltage |
| VEE | VSS | V− | VS− | Negative supply voltage |
The simplest labels are V+ and V−. However, internal design and historical tradition have led to a variety of other labels being used. Also, V+ and V− may be confused with the + and − voltage inputs to ICs like op amps.
Sometimes one of the power supply pins will be referred to as ground. In digital logic, this is nearly always the negative pin; in analog integrated circuits, it is most likely to be a pin intermediate in voltage between the most positive and most negative pins[citation needed].
Double subscript notation uses similar looking placeholders with subscripts. In that notation the subscripted letters denote two points.
[edit] History
VCC (note: lower case is often used instead of subscript, e.g. "Vcc") is an electronics designation that refers to voltage from a power supply connected to the "collector" terminal of a bipolar transistor. In an NPN bipolar junction transistor, it would be +VCC, while in a PNP transistor, it would be −VCC. There is debate over the origins of the double letter subscript naming convention. One proposal is that it originated as an abbreviation of the supply voltage for a common collector amplifier with the other power supply names mimicking this fashion. Double letters may also have been used to clearly indicate that a power supply voltage is being referred to.
In general, double letter subscripts refer to power supply voltages. For example, VEE refers to the "emitter" voltage as supplied by the power supply pin. In NMOS logic, VSS refers to the "source" voltage, and VDD is the "drain" voltage likewise.
Within a circuit, single letter subscripts refer to that voltage relative to ground; for example, VC is the "collector" voltage relative to ground. Two different letters indicate the voltage between two terminals; for example: VBE is the "base" to "emitter" voltage drop, while VCE is the "collector" to "emitter" voltage.
[edit] Modern use
CMOS ICs have generally borrowed the NMOS convention of VDD for positive and VSS for negative despite the fact that both positive and negative supply rails actually go to sources (positive supply goes to PMOS sources, negative supply to NMOS sources). ICs using BJTs have VCC (positive) and VEE (negative) power supply pins.
In single supply systems (e.g., most modern digital and analog circuits) the negative power supply pin is also commonly referred to as GND. In "split rail" supply systems (e.g., older analog circuits) positive, negative and ground power supply pins are used.
More advanced chips will often have pins carrying voltage levels for more specialized functions in or out of the chip and these are generally labeled with some abbreviation of their purpose. For example VBUS for the 5 volt supply needed for a bus-powered USB device or Vref for the reference voltage for an analog-to-digital converter.
