Voltage regulators and voltage references are essential to most electronic circuits, whether analog, digital, or a combination of the two. Although these circuits can appear deceivingly simple, much experience in worst-case analysis has shown them to also be the source of a large percentage of problems or issues as part of larger circuit designs. Often, these issues are due in part to a lack of understanding of the actual complexities of these seemingly simple circuits. Another major contributor is a general lack of data from integrated-circuit (IC) manufacturers—data that is critical to completing a successful and robust design.

First, there is some confusion about the types and descriptions of different devices commonly used to provide a stable voltage source. A reference diode is a two-port element that does not provide a voltage by itself. The reference diode is either a Zener diode or bandgap shunt regulator, requiring an external current source. There is no significant difference between a reference diode and a Zener diode, other than the fact that a reference diode is generally a temperaturecompensated device and is often available with tighter initial tolerances than Zener diodes. A Zener diode allows current flow not only in the normal forward direction but also in the reverse direction when the applied voltage is larger than the breakdown voltage (or Zener knee voltage). A conventional diode will not allow significant current flow if it is reverse biased below its reverse breakdown voltage.

A voltage reference is a three-port device that provides a precision output voltage when an appropriate input voltage is connected. The reference is typically capable of either sourcing or sinking current, often to approximately 10 to 20 mA. Adding to the confusion, there are devices that offer operation as either a two-port reference diode or as a threeport reference device.

A linear voltage regulator is a threeport device that provides a regulated output voltage when an appropriate input voltage is connected. The regulator is generally capable of sourcing but not sinking current, and is usually designed for much higher output current than a reference. Typical current capabilities are from 500 mA to several amperes, though some regulators have been developed with capability exceeding 100 A.

The key performance metrics for a regulated voltage source are:

• ripple and noise,
• power supply rejection ratio (PSRR),
• absolute voltage accuracy,
• temperature coefficient,
• output current capability,
• input voltage range,
• control loop stability,
• output impedance, and
• sink current and source current.

It may be obvious upon inspection that a subset of this list may be of interest to any one discipline, but no single discipline will be concerned with the entire list. For example, a designer of battery-powered equipment will be concerned about operating current but is not likely to be concerned with ripple rejection. An analogto- digital-converter (ADC) designer is concerned with noise, absolute accuracy and, depending on the application, PSRR. An RF circuit designer is not generally concerned with absolute accuracy, but is very concerned about output noise and ripple rejection. Many of these designers may be concerned with output impedance or the manifestations resulting from the output impedance variations. For example, a logic designer needs to be concerned with the impact of large dynamic currents. ADCs often present dynamic current loads on the reference, although much smaller than in highly integrated logic systems or computers. The performance requirements for each of these regulated sources are dependent on the discipline to which they are applied. There is not a “one size fits all” solution.

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