TABLE OF CONTENTS It is sometimes necessary to provide multiple voltage references in an application. In some cases you may have to generate the same voltage to multiple loads, whereas in others several different voltages may be required. Over the course of the past 25 years, a few monolithic reference devices have been introduced that have provided two different simultaneous voltages. Few of these devices are still available though. For one thing, producing two different reference voltages on the same chip is expensive for manufacturers. In addition, testing and trimming two or more reference voltages to very high accuracy may result in one reference passing, whereas the other fails.
For most designers, creating a custom circuit using two or more individual references has been the only possible solution. One can choose different references from the same family and manufacturer or choose a particular reference from supplier A, another from supplier B, and yet another from supplier C. They can work constantly and independently of one another (as shown earlier in Figure 15.29) or be individually selectable, so they only work some of the time. Using different voltage devices from the same family makes it relatively easy in one's design, because many of the same basic specifications will be the same, such as tempco, dropout voltage, noise level, line/load regulation, I Q, thermal hysteresis, turn-on settling time, power-up drift, and so on. Using different voltage devices (perhaps even using different topologies) and from...
