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I've got an LM22678TJE-ADJ/NOPB regulator providing a 3.3V rail from a (roughly) 15V input. The circuit is shown below:

3.3V supply

According to the datasheet, the choice of output resistors should produce a voltage of 3.341V. The product of the inductor and output capacitor values is 1.034e-9, which is pretty close to the suggested 1.1e-9 in the datasheet.

What I actually get is an average 3.5V, with a 600mV triangular wave imposed on top of it.

Can anyone suggest what's causing this?

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  • \$\begingroup\$ Please give detailed specs of output caps (especially ESR). How many ceramic caps on the output? Also please post layout and scope shot of output. \$\endgroup\$
    – bobflux
    Mar 10, 2017 at 15:27
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    \$\begingroup\$ You do understand that your Schottky is in the wrong place, right? \$\endgroup\$ Mar 10, 2017 at 15:33

1 Answer 1

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Firstly, the diode is in the wrong place, I am surprised the regulator chip did not die, the catch diode needs to connect to the end of the inductor connected to pin 1, note that this is a high di/dt loop.

Secondly you have drawn an electrolytic cap there, that is not going to cut it at the 500KHz switching frequency of that part.

Try again with some MLCCS in parallel with that electrolytic, you will be much happier.

Also, what is your layout like? The layout recommendations for high frequency switchers really, really mean it.

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  • \$\begingroup\$ Thanks for pointing out the silly diode mistake. That's inherited from an older design - it actually works pretty well at 5V output. But you're right, it's not where it should be. \$\endgroup\$
    – Tom
    Mar 13, 2017 at 10:26

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