Vcc, Vdd, Vee, Vss: Clearing Up The Confusion In Supply Voltage Naming Conventions

Deciphering Supply Voltage Labels

The naming conventions for supply voltages in electrical circuits can be confusing for those new to circuit design. Acronyms like VCC, VDD, VEE, and VSS are commonly used to denote supply rails in circuits, but the meaning behind these labels is not always clear. This article will explain the definitions and uses of these voltage labels to help demystify their purposes.

Defining VCC, VDD, VEE, and VSS

VCC, VDD, VEE, and VSS are acronyms used to identify the power supply voltage pins that provide current to integrated circuits and other electronic components. Here are the definitions of these commonly used supply voltage labels:

VCC stands for “Voltage at the Common Collector.” It represents the primary positive supply voltage in a circuit.
VDD means “Voltage Drain to Drain.” Like VCC, VDD also indicates a positive supply voltage connection.
VEE is “Voltage at Emitter End.” VEE denotes the primary negative supply voltage.
VSS, “Voltage at the Source Side,” refers to the ground reference voltage in a circuit.

VCC and VDD as Positive Supply Rails

VCC and VDD pins supply positive current to integrated circuits and other electronic components. Both terms refer to the primary positive power source used to provide operating current to devices in a circuit. The main difference between VCC and VDD is their points of reference:

Table of Contents

VCC measures voltage from the perspective of the common collector terminal of a transistor inside an IC.
VDD references the voltage from between the drain terminals of the transistors forming gates and other structures within an integrated circuit.

So VCC denotes the overall applied positive voltage to an IC or component, while VDD refers to the specific positive voltage used within internal transistor gates in digital logic. However, in practice VCC and VDD are often used interchangeably to represent the primary positive supply voltage.

VEE and VSS as Negative Supply Rails

The VEE and VSS pins provide the negative voltage and ground connections in electronic circuits. These supply rails serve vital functions:

VEE supplies the negative voltage that enables bipolar transistor operation in ICs and op-amps. VEE typically provides a negative DC voltage between -5V and -15V.
VSS connects components to ground in the circuit. Ground acts as the 0V reference all voltages are measured against. VSS may tie directly to the grounded chassis in some cases.

You can think of VEE and VSS as forming two sides of the power supply “battery” in a circuit – VEE as the negative terminal and VSS as the grounded terminal. Together with VCC/VDD, they provide the complete operating voltages needed for circuits and ICs to function.

Reasons for Multiple Power Supply Pins

Devices like microcontrollers, FPGAs, and complex ICs often require several different supply voltages for operation. They incorporate multiple VCC, VDD, VEE and VSS pins to accommodate this requirement. There are a few reasons why multiple power pins with various labels are necessary:

Separate voltages may be needed for different internal logic blocks and I/O banks.
Having independent pins allows noise isolation on sensitive voltage planes.
More power pins help distribute high supply currents across the device.
Separate analog and digital power pins prevent coupling between sections.

For example, a microcontroller would typically have a primary VDD pin that provides voltage to the digital core logic, along with a separate analog VDDA pin to power internal analog peripherals. The usage of these multiply-labeled voltage inputs is device-specific, so always check the IC datasheet when connecting power.

Example Circuits Using VCC, VDD, VEE, and VSS

Here are two example circuits demonstrating how VCC, VDD, VEE and VSS pins are used to provide supply voltages:

Microcontroller Circuit

This circuit shows a typical power supply configuration for a microcontroller such as an AVR or PIC MCU. The positive supply rail is labelled VCC, providing 5V to the chip from an external source. The VSS pin connects to ground. Note the analog AVCC pin, used to supply a separate analog section inside the microcontroller.

[Diagram of microcontroller with VCC, AVCC, VSS pins]

Op-Amp Circuit

This op-amp circuit utilizes split +/- supplies, with VCC and VEE providing symmetrical positive and negative voltages to power the amplifier. VEE typically provides a negative voltage between -5V to -15V. The VSS pin ties the op-amp ground reference to the system ground. Together, VCC, VEE and VSS form the complete power supply for the op-amp.

[Diagram of op-amp with VCC, VEE, VSS]

Choosing Supply Voltage Labels for Your Design

When working on your own PCB or IC designs, you as the designer must choose appropriate names for the supply voltage pins. Here are some guidelines on selecting naming conventions for power rails in a robust, readable way:

Use VCC for the primary positive voltage supply, assume VCC = 5V if unspecified.
Label additional positive supplies for special functions as VDDx (e.g. VDDA, VDD5V).
Use VEE for negative supplies from -5V to -15V as commonly needed.
Always label the ground reference VSS for clarity.
Add descriptive labels if necessary e.g. VCC_MICRO for the microcontroller positive supply.

Adhering to standard naming practices for supply voltages makes designs more portable and easier to understand for those not intimately familiar with the intricacies of your specific circuit or PCB layout.

Following Industry Naming Conventions

Beyond VCC, VDD, VEE and VSS, there are other supply voltage naming conventions used across various electronics industries. Here are some examples of other prefixes frequently used:

VA, VB, VC – Often used for external power supplies such as AC-DC converters and power modules.
VIN, VOUT – Typical naming for input and output power pins on voltage regulator ICs.
V+ , V- – Used when positive and negative supplies need to be clearly differentiated.
VP, VN – Supplies in analog circuits are sometimes labelled VP (positive) and VN (negative).
HV/LV – High-voltage and low-voltage supplies are prefixed HV and LV respectively in some applications.

These examples illustrate the variety of standard designations used for supply voltages across different product spheres. Familiarity with the common naming schemes used in your application helps improve comprehension of new designs encountered.

When to Use Custom Power Pins

Occasionally, a custom or product-specific voltage pin naming scheme might be warranted when working on proprietary or specialized ICs and hardware. Reasons to deviate from accepted naming conventions may include:

Insufficient available pins on the package to append descriptive labels.
Widely understood specific naming in an established proprietary product family.
Requires compatibility with labeling used on prior generation ICs or modules.

However, bending the standards too far risks confusion, so should be undertaken cautiously. Aim to reuse existing pin designations if possible, and provide abundant documentation on any deviations. By starting from accepted vocabulary though, even esoteric products can build understanding effectively.