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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)?

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closed as unclear what you're asking by Olin Lathrop, Phil Frost, Matt Young, Nick Alexeev, Anindo Ghosh Jul 18 '13 at 7:29

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ 1: Dimensionless values of "load" make no sense. 2: Why do you need a shunt regulator? Why not a linear pass regulator? 3: What is the "dissipating resistance" in this context? -1 for ambiguous and sloppy question. \$\endgroup\$ – Olin Lathrop Jul 16 '13 at 12:57
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    \$\begingroup\$ Load is in Ohm, I need Shunt reg because of very low current. The dissipating resistance is a resistance in serie with the shunt that dissipate power. Why Wouter understood my questions? \$\endgroup\$ – Mark Jul 16 '13 at 13:07
  • \$\begingroup\$ @OlinLathrop can you answer me? \$\endgroup\$ – Mark Jul 16 '13 at 13:22
  • \$\begingroup\$ Yes I can. No I won't. Instead of fixing issues with your question, you decided to get snarky instead. I have a lot of things I can chose to do with my free time, so I don't have to spend it helping those that have shown disrespect. Next time remember who is coming here asking for a favor from whom. \$\endgroup\$ – Olin Lathrop Jul 16 '13 at 13:30
  • \$\begingroup\$ @olinlathrop Sorry but why you offend me writing sloppy question? And why the -1? Is not better to ignore the question instead of offending other users who are looking for help? If another user answered the question so it wasn't so unclear. Or not? I apologize for my reply comment. \$\endgroup\$ – Mark Jul 16 '13 at 20:07
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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.

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