5 volt variable power supply circuit

5 volt variable power supply circuit 

Circuit Description

Although linear-regulator !Cs are frequently used in variable power-supply applications, they might noLalways be the best choice. At low output voltages, power losses in these regulators can cause headaches for designers. For example, if an output current of 1.25 A is required at 1.25 V from an input of 8 V. a regulator (such as the LT317) dissipates more than 10 W. The figure depicts a de-de converter that functionally replaces a linear regulator in the just described application. The converter not only eliminates the problem of power loss, but it can be adjusted for output voltages (as low as 25 mY) while delivering an output current of 1.5 A. The circuit uses a basic positive-buck topology with one exception. A control voltage is applied through R4 to the feedback summing node at pin 1 of the LT1076 regulator JC, making it possible to adjust the output from 0 V to approximately 6 V. This range encompasses the 3.3-V and 5-V logic supply voltages for portable and desktop equipment, as well as battery-pack combinations of one to four cells. As R4 is driven from 0 to 5 V by the buffer (Ul), more or less current is required from R2 to satisfy the loop's desire to hold the feedback swmning point at 2.37 V. This forces the converter's output to swing over the range of 0 to 6 V. The LT1076 is capable of 1.75-A guaranteed output current in this application, ;md 2 A is typical. If more current is required, the LTl 07 4 can be substituted for the 1:1'1076.