Linear regulator ICs are commonly used in variable power supplies. Common types such as the 317 can be adjusted as low as 1.25 V in single-supply applications. At low output voltages power losses in these regulators can be a problem. For example, if an output current of 1.5 A is required at 1.25 V from an input of 8 V, the regulator dissipates more than 10 W. Figure 1 shows a DC/DC converter that functionally replaces a linear regulator in this application. The converter not only eliminates power loss as a concern, but can be adjusted for output voltages as low as 25 mV while still delivering an output current of 1.5 A.
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| Figure 1. | Adjustable LT1074/LT1076 0 V to 5 V power supply. |
The circuit of Figure 1 employs 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 switching regulator IC, allowing the output to be adjusted from 0 V to approximately 6 V. This encompasses the 3.3 V and 5 V logic supply ranges as well as battery pack combinations of one to four D cells.
As R4 is driven from 0 V to 5 V by the buffer (U1) more or less current is required from R2 to satisfy the loop’s desire to hold the feedback summing point at 2.21 V. This forces the converter’s output to swing over the range of 0 V to 6 V.
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| Figure 2. | Power loss comparison: linear regulator vs Figure 1’s power supply. |
Figure 2 shows a comparison of power losses for a linear regulator and the circuit of Figure 1. The load current is 1.5 A in both cases although the LT1076 is capable of 1.75 A guaranteed output current in this application and 2 A typical. If more current is required the LT1074 can be substituted for the LT1076. This change accommodates outputs up to 5 A but at the expense of a heftier diode and coil (D1, L1). An MBR735 and Coiltronics CTX50-2-52 are recommended for 5 A service.
