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What resistors do I need to get 5V on this chip? Sorry, I don't understand the reference to the Iadj in the formula. The section on the Iadj in Adjustable Voltage Design seems circular. http://www.onsemi.com/pub_link/Collateral/NCP59302-D.PDF

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  • \$\begingroup\$ It doesn't seem circular to me. Adjust Pin Bias Current (Iadj) is given on p.3 in the datasheet. \$\endgroup\$ May 21 '13 at 17:15
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The ADJ pin is an input pin that compares the the output voltage through a potential divider made up from R1 and R2 to an internal 1.24V reference (a typical bandgap ref. value). There is a (very) small current taken by this input (Iadj). On page 3 of the specs it gives the value of this current as about 200 to 350 nA. Unless you are going to use very high value resistors or extremely accurate resistor values for setting the output voltage you can safely ignore its effect.

The 'extra' voltage drop is shown as Iadj x R1 because the input current flows from the output into the ADJ input through R1. For practical purposes just use the first term of V(out) = 1.24 * (1 + R1/R2).

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It's not really circular. The "Adjust Pin Bias Current" is specified in the table labeled "ENABLE INPUT" (sic), typically 100nA.

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