Three-Terminal Linear Regulator

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The following circuit (Fig. 4.1) is a typical application configuration for an LM117 circuit. The input voltage is 22 V DC. Resistors R1 and R2 set the regulated output voltage at 16.7 V. 

One interesting measurement that can be made on this circuit would be the stability. In order to measure the stability in the lab, a special test configuration is used. This test set up is shown in Fig. 4.2. The injection signal must be kept very small (700 /xV is suggested), and the measurement probes should be placed at R and A on the diagram in 

Using the test setup of Fig. 4.2, several configurations were measured in the lab. The cases will be considered one at a time, with comparisons to the SPICE results in each case. The first case is the recommended operational use by the Linear Databook (Linear Technology 1990, pp. 

4-137), which recommends a 1 /U.F tantalum input bypass capacitor, a 1 pF capacitor at the output, and a 10 //F capacitor at the adjustment pin. The recommended type of capacitor is a solid tantalum. The SPICE configuration for testing stability is shown in Fig. 4.3. 

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Figure 4.1 Typical application for an LM117 three-terminal linear regulator.

Three-terminal regulators are used in the majority of board-level regulator applieations. They exeel in eost and ease of use for these applieations. They ean also, with eare, be used as the basis or higher funetionality linear regulators. 

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