• The operational amplifier adds some additional tolerances, such as VOS and additional noise terms.

• The output voltage is easily adjusted or trimmed, by resistors, during manufacture.

Voltage regulators are three terminal devices that include a voltage reference, which can be a reference diode type device or a band-gap voltage reference, an error amplifier, and a power stage. Voltage regulators often include additional functions such as current limit, soft-start, and over-temperature protection, to name a few.

The typical characteristics of a voltage regulator are summarized as follows:

• The tolerance of the voltage reference is generally not as good as that of a reference.

• The output noise is generally much higher than that of a reference.

• The voltage regulator can generally source current, but not sink current.

• The voltage regulator is very sensitive to load capacitance and can be easily destabilized or made to oscillate due to selection of output filter components. The stability criteria are often neglected from datasheets and many devices do not have external compensation capability.

• The stability is dependent on operating conditions, such as input voltage and load current.

• The voltage regulator is capable of much greater output currents than a reference.

Reference diodes, voltage references, and voltage regulators are not functionally interchangeable. Each device has benefits and drawbacks that must be evaluated on a case-by-case basis. Different disciplines require different performance characteristics, while component manufacturers tend to provide a “one-sizefits- all” solution. The end user often adds elaborate filtering and external circuits in order to obtain “improved” performance. Owing to insufficient data and a less than complete understanding of the intricacies of the devices, intended improvements often create performance issues that are difficult, and sometimes impossible, to correct.

Reference
1. Cecil Kent Walters, United States Patent 3,549,988, “Temperature Compensated Reference Voltage Circuitry Employing Current Limiters and Reference Voltage Diodes,” filed Jan. 2, 1968, granted Dec. 22, 1970.

STEVE SANDLER, AEi Systems LLC, 5933 West Century Blvd., Suite 1100, Los Angeles, CA 90045; (310) 216-1144, FAX: (310) 499-0554, e-mail: info@aeng.com, Internet: www.aeng.com.

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