Voltage regulator calculation

Published:2011/8/17 1:51:00 Author:Amy From:SeekIC

Victor Himpe

Before you con design on adjustable voltage regulator into your circuit, or do a redesign, you need to calculate the values for two resistors. This is not diffi­cult in itself, but actually finding the right resistors may pose prob­lems. Fortunately a trick is avail­able to make it all much easier. With most adjustable voltage reg­ulators like the LM317 and LM337, the input voltage has to be 1.2 to 1.25 volts above the desired output voltage. This is because the voltage at the ADJ (adjust) input is internally com­pared to a reference voltage with that value.

The reference voltage always exists across Rl. Together with preset R2 it determines the current Rowing through the ADJ pin, as follows:

Vout = VREF [1+(R2/R1)]+IADJ R2
If for the sake of convenience we ignore Iadj. enter the reference voltage (1.2 V) and for Rl select a value of one thousand times that voltage (i.e., 1.2 kfi) then the equation is simplified to:

R2 =1000(Vout-1.2)

In practice, simply determine the voltage drop across R2 (output voltage minus reference voltage) and you get your resistance value directly in kilo-ohms. For exam­ple, for 5 V R2 becomes 5-1.2 = 3.8 kΩ which is easiest made by connecting 3.3 kΩ and 470Ω resistors in series.

In the case of relatively low volt­ages, smaller resistor values are recommended. This is because sufficient current needs to flow to enable the voltage regulator to do its job. A simple solution is to choose, say, 120Ω for Rl. R2 then becomes:

R2=100(Vout-1.2)

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