IC power supply pin

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.

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 Fact|date=February 2007.

Double subscript notation uses similar looking placeholders with subscripts. In that notation the subscripted letters denote two points.

History

In circuit diagrams and circuit analysis, there are long-standing conventions regarding the naming of voltages, currents and some components.cite book |title=Electronic Engineering |last=Alley |first=Charles L |coauthors= Atwood, Kenneth W |year=1973 |publisher=John Wiley & Sons, Inc |edition=Third edition |location=New York and London |isbn=0-471-02450-3 ] In the analysis of a bipolar junction transistor, for example in a common emitter configuration, the DC voltage, with regard to ground, at the collector, emitter and base may be written as "V_C", "V_E" and "V_B" respectively. Resistors associated with these transistor terminals may be designated "R_C", "R_E" and "R_B". In order to create the DC voltages, the furthest voltage, beyond these resistors or other components if present, was often referred to as "V_CC", "V_EE" and "V_BB". In practice "V_CC" and "V_EE" then refer to the plus and minus supply lines respectively in common NPN circuits. Note that "V_CC" would be negative and "V_EE" would be positive in equivalent PNP circuits.

Exactly analogous conventions were applied to field-effect transistors with their drain, source and gate terminals. This led to "V_D" and "V_S" being created by supply voltages designated "V_DD" and "V_SS" in the more common circuit configurations. In equivalence to the difference between NPN and PNP bipolars, "V_DD" is positive with regard to "V_SS" in the case of "n"-channel FETs and MOSFETs and negative for circuits based on "p"-channel FETs and MOSFETs.

Although still in relatively common use, there is limited relevance of these device-specific power supply designations in circuits that use a mixture of bipolar and FET elements, or in those the employ either both NPN and PNP transistors or both "n"- and "p"-channel FETs. This latter case is very common in modern chips, which are often based on CMOS technology, where the "C" stands for "complementary" meaning that complementary pairs of "n"- and "p"-channel devices are common throughout.

These naming conventions were part of a bigger picture where, to continue with bipolar transistor examples although the FET remains entirely analogous, DC or bias currents into or out of each terminal may be written "I_C", "I_E" and "I_B". Apart from DC or bias conditions, many transistor circuits also process a smaller audio-, video- or radio-frequency signal that is superimposed on the bias at the terminals. Lower case letters and subscripts are used to refer to these signal levels at the terminals, either peak-to-peak or rms as required. So we see "v_c", "v_e" and "v_b" as well as "i_c", "i_e" and "i_b". Using these conventions, in a common emitter amplifier, the ratio "v_c"/"v_b" represents the small-signal voltage gain at the transistor and "v_c"/"i_b" the small-signal "trans-resistance" from which the name "transistor" is derived by contraction. In this convention, "v_i" and "v_o" usually refer to the external input and output voltages of the circuit or stage.

Similar conventions were applied to circuits involving vacuum tubes or "thermionic valves" as they were known outside of the U.S. Therefore we see "V_P", "V_K" and "V_G" referring to plate (or "anode" outside of the U.S.), cathode (note "K", not "C") and grid voltages in analyses of vacuum triode, tetrode and pentode circuits.

Modern use

CMOS ICs have generally borrowed the NMOS convention of V_DD for positive and V_SS for negative despite the fact that both positive and negative supply rails actually go to source terminals (positive supply goes to PMOS sources, negative supply to NMOS sources). ICs using bipolar transistors have V_CC (positive) and V_EE (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 V_BUS for the 5 volt supply needed for a bus-powered USB device or V_ref for the reference voltage for an analog-to-digital converter.

References

External links

* [http://www.maxim-ic.com/glossary/index.cfm/Ac/V/ID/943/Tm/VEE/ln/en Electrical Engineering Glossary Definition for Vcc]
* [http://encyclobeamia.solarbotics.net/articles/vxx.html Vcc, Vdd, Vss, etc.]