ADJUSTABLE_0_TO_5_V_SUPPLY

Published:2009/6/24 2:50:00 Author:Jessie | From:SeekIC

Although linear-regulator ICs are frequently used In variable p。wer-supply applications，they might not always be the best choice，At low output voltages，power losses in these regulators cancause 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 dc-dc converter that functionally replaces a linear regulator in the just clescribed application. The converter not only-eliminates the problem of power loss，but it can be adjusted for output voltages (as low as 25 mV)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 of the LT1076 regulator IC，making it possible to adjust the output from0 V to approximately 6 V. This range encompasses the 3.3-V and 5-V logic supplyvoltages for portable and desktop equipment，as well as battery-pack combinations of one to four cells.As R4is driven from0 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 2.37V． This forces the converter’s out-put to swlng over the range of0 to 6 V．The LT1076 is capable of 1.75-A guaranteed output current In this application，and 2 A is typical．If more current is required，the LT1074 can be substituted for the LT1076.



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