Three Terminal Regulators

Three-terminal IC regulators have been especially useful to the designer of small, regulated power supplies or on-card regulators. Three-terminal regulators are popular because they are small and require a minimum number of external components.

STABILIZATION

Mounting and using three-terminal regulators usually presents no problem, however, there are several precautions that should be observed. Positive regulators, in general, use npn emitter follower output stages whereas negative regulators use npn common-emitter stages with the load connected to the collector. The emitter follower output stage configuration is not used in negative regulators because monolithic pnp series-pass transistors are more difficult to make. Due to their output stage configuration, positive regulators are more stable than negative regulators. Therefore, the practice of bypassing positive regulators may be omitted in some applications. It is good practice, however, to use bypass capacitors at all times.

For a positive regulator, a 0.33µF bypass capacitor should be used on the input terminals. While not necessary for stability, an output capacitor of 0.1µF may be used to improve the transient response of the regulator. These capacitors should be on or as near as possible to the regulator terminals. See Figure 5-36.

When using a negative regulator, bypass capacitors are a must on both the input and output. Recommended values are 2µF on the input and 1µF on the output. It is considered good practice to include a 0.1µF capacitor on the output to improve the transient response (Figure 5-37). These capacitors may be mylar, ceramic, or tantalum, provided that they have good high frequency characteristics.

Figure 5-36. Positive Regulator
Figure 5-37. Negative Regulator