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NCV47701 linear regulator formula

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el00

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Hi, I need to use this regulator in a design.
I noticed that the formula in the datasheet is reversed compared to all other regulators working with internal reference:
pag. 10:
https://www.onsemi.com/pub/Collateral/NCV47701-D.PDF
Vout = Vref x (1 + (R1/R2))
while for LM317 it is the opposite
Vout = Vref x (1 + (R2/R1))
where R1 is the upper resistor in the totem.

I wonder if this is a mistake or not. Any idea?
 

Hi,

The given formula is correct.

In my eyes the LM317 is reversed. ;-)
With your regulator the feedback voltage is referenced to GND (1.275V wrt GND)
While with LM317 the feedback voltage is referenced to OUTPUT (-1.25V wrt OUTPUT)

Both clearly explained in the datsheets.

Klaus
 
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    d123

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The reason the LM317 was designed that way is so that it didn't need a ground connection, and thus could be put in a standard 3-terminal power-transistor package, such as the TO-3 and the TO-220.

The disadvantage of this simple configuration, is that the output current through any resistance between the output pin and the top sense resistor connection will appear as an apparent change is the reference voltage, which can then significantly degrade the load regulation.
Thus that top sense resistor should be connected directly to the output pin, and not to the output connection.
 
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