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I am trying to build a step-up and step-down circuit from a DC-DC converter IC.

This is from the datasheet:

enter image description here

I have two questions in relation to this:

  1. The value of the resistor in the red box. It says 0.22. Is this in ohms? 0.22 ohms seems quite peculiar to me.
  2. Regarding the Vout calculation. You can look at the red line in the bottom of the screenshot. It looks like Vout is calculated by selecting values only for the R1 and R2 resistors. Is this correct? Shouldn't Vin be taken into consideration as well?
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  • \$\begingroup\$ Please add the datasheet link \$\endgroup\$
    – sai
    May 26 at 13:17
  • \$\begingroup\$ What is peculiar about 0.22 ohm? It is a current sensing resistor and has to be a small value. \$\endgroup\$
    – sai
    May 26 at 13:22
  • \$\begingroup\$ 1. Lazy of them to leave out the unit when they have it elsewhere. Could theoretically also be that the Omega-sign was not interpreted correctly and the PDF ended up with a blank character instead. \$\endgroup\$
    – winny
    May 26 at 13:41

1 Answer 1

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\$R_{SC}=0.22\Omega\$. It has to be small, to stay cool, and to avoid becoming a bottleneck for significant inductor current. Such small values are very common in current sensing applications.

The regulator does not need to know what the input voltage is, in order to know if its own output is at the correct potential. To determine if the output is too high or too low, some fraction of it, a fraction defined by R1 and R2, is compared against a fixed internal 1.25V reference. No other information is needed, the formula is correct.

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