The Regulator The 78XX series of linear voltage regulators provide positive fixed output v...

The Regulator The 78XX series of linear voltage regulators provide positive fixed output voltages for a range of values. The last two digits in the part number indicate the output voltage. The 7812 provides a 12 V regulated output. The change in output voltage for a specified change in input voltage is called the line regulation. The change in output voltage for a specified change in load current is called the load regulation. These parameters are specified on the datasheet. It is recommended by the manufacturer that a 0.33µF capacitor be connected from the input terminal to ground and a 0.1µF connected from the output terminal to ground, as shown in Figure 1 to prevent high-frequency oscillations and improve the performance. You may wonder about putting a small-value capacitor in parallel with a large one; the reason is that the large filter capacitor has an internal equivalent series resistance, which affects the high frequency response of the system. The effect is cancelled with the small capacitor.

Figure 1

A partial datasheet for a 7812 is shown in Figure 2 Notice that there is a range of nominal output voltages, but it is typically 12 V. The line and load regulation specify how much the output can vary about the nominal output value. For example, the typical 12 V output will change no more than 11 mV (typical) as the load current changes from 5 mA to 1. 5 A. Package configurations are shown in part (b).

Figure 2 Partial datasheet and packages for a 7812 regulator. You can view an entire datasheet at www.faitchildsemiconductor.com. Copyright Fairchild Semiconductor Corporation. Used by permission.

From the datasheet, determine the maximum.output voltage if the input voltage to the regulator increases to 22 V, assuming a nominal output of 12 V