I need a 2.5V regulator (http://www.onsemi.com/pub/Collateral/LM285-D.PDF) with a 3.3V power supply and a variable load (3kOhm up to 60kOhm). How can I calculate the dissipating resistance (see the voltage reference circuit in the datasheet)?
closed as unclear what you're asking by Olin Lathrop, Phil Frost, Matt Young, Nick Alexeev♦, Anindo Ghosh Jul 18 '13 at 7:29
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The limiting case is when your load draws maximum current (at 3k) and the shunt regulator draws 0 current. In that situation the voltage across regulator and load must still be 2.5V
Disregarding any tolerances the series resistor must be 1.8/2.5 * 3k = 2.16k. Choose a lower value (1k8?) to be on the safe side.
If you know the possible variations in your 3V3 you must redo the calculation with the lowest value.