The following design examples illustrate how 3-terminal regulator integrated circuits can form the basis of higher-current, more complicated designs. Care must be taken, though, because all of the examples render the over temperature protection feature of the 3-terminal regulators useless.

Any overcurrent protection must now be added externally to the integrated circuit. The current-boosted regulator.

The design shown in Figure 2–6 adds just a resistor and a transistor to the 3- terminal regulator to yield a linear regulator that can provide more current to the load.

The current-boosted positive regulator is shown, but the same equations hold for the boosted negative regulator. For the negative regulators, the power transistor changes from a PNP to an NPN. Beware, there is no overcurrent or overtemperature protection in this particular design.

The current-boosted 3-terminal regulator with overcurrent protection This design adds the overcurrent protection externally to the IC. It employs the base-emitter (0.6 V) junction of a transistor to accomplish the overcurrent threshold and gain of the overcurrent stage.

For the negative voltage version of this, all the external transistors change from NPN to PNP and vice versa. These can be seen in Figures 2–7a and b.

Figure 2–6 Current-boosted 3-terminal regulator without overcurrent protection.

Figure 2–7 (a) Positive current-boosted 3-terminal regulator with current limiting. (b) Negative current-boosted 3-terminal regulator with current limiting.

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