SparkFun Forums

[kirash4]

I'm working on a design that has an LM2621 DC-DC Step-Up. The typical application circuit is below:

lm2621.png

My question is specifically regarding that R1, 500 Ohm resistor. How critical is that? I want to stick with either a 0402 or 0603 package and Mouser doesn't have any, and Digi-Key has one in the 1/20W range. So I'm wondering if I can substitute it for a slightly different value and if so, which way should I swing, down to 499 Ohm or up to 510 Ohm? Both of those are rather common at Mouser.

The same question for that RF2, 50K Ohm. Another that doesn't seem very popular ...

[142536789]

You should be fine with a 499 ohm. It's only a 0.002% difference from a 500 ohm. (1 - 499/500) * 100 = 0.002% Tolerances on resistors are typically orders of magnitude higher anyway. 0.002% vs 2% As for high value resistors like 10k and above, it matters even less if my math is correct.

[leon_heller]

Or use two 1K or 100K resistors in parallel.

[waltr]

R1 and Rfq are not that critical so a 499 would be fine. Two 100k in series would work for Rfq or a 199k.
The criteria resistors are Rf1 and Rf2. These set the output Voltage by dividing the output Voltage into the Reference or Feed Back pin. Many times when I am designing these circuits I will add another resistor in parallel with Rf2 but not install it until the circit is tested. Then I can twiddle the values to get the desired output Voltage.