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I have attempted to design a 12V to 3.3V switching regulator for my PCB that will draw 0.5A Max. The problem is that after assembling the components, the output voltage is the same as the input voltage, meaning something is wrong with my design, and I just cannot figure out what it is.

I am using THIS regulator

The data sheet calls for this layout on page 13, using a recommended list of components:

enter image description here

And here is my layout. In addition, I am using 2 seperate power planes. The dashed orange line on the bottom represents the 12V power plane (and everything below the line), and the dashed line on the top is the 3.3V power plane.

So the large vias at the top are connected to the 3.3V power plane, and the large vias at the bottom are connected to the 12V power plane.

enter image description here

Here is the schematic (This version is fixed, now that the feedback pin is connected): enter image description here

There is a list of recommended components in the datasheet on page 13 as well, and I am using components that are substitutes for these, since they are now outdated.

  1. CBYPASS (C21) - C3216JB1C106K160AA
  2. COUT (C19,C20) - C2012X5R1A226M125AB‎
  3. LOUT (L2) - NRS5020T4R7MMGJ
  4. CBST (C22) - CL05B223KA5NNNC

Is there something that I am completely missing from this design? It appears that it should work, but does not.

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  • \$\begingroup\$ If the regulator isn't working at all, it's as likely to be a problem in the schematic as in the layout. Could you edit your question to include the schematic, please? \$\endgroup\$
    – TimWescott
    Jan 24, 2019 at 21:19
  • \$\begingroup\$ We're also missing something, a schematic! We want to see the schematic that you used so not a link and we're also not going to reverse engineer your layout. So include the schematic from which you made the layout. \$\endgroup\$
    – Bimpelrekkie
    Jan 24, 2019 at 21:19
  • \$\begingroup\$ I have not read the datasheet but, at first sight, many components from the first design are missing in the second design. And as others have told, a schematic would certainly help a lot. \$\endgroup\$
    – user2233709
    Jan 24, 2019 at 21:21
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    \$\begingroup\$ Please correct your title, I think you mean Not working, rather than Now working. (unless a little green wire to pin 5 solves your problems). \$\endgroup\$
    – TimWescott
    Jan 24, 2019 at 21:26
  • \$\begingroup\$ Title corrected, schematic attached. The problem has been fixed. The missing components connected to the feedback pin as noted by @user2233709 are not required due to the fixed voltage of this regulator. However, a connection from the feedback pin was required, which I did not realize. Thank you for the help. \$\endgroup\$
    – Daniel Frenkel
    Jan 24, 2019 at 22:08

2 Answers 2

Reset to default
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You have nothing connected to the feedback pin. If it drifts to 0V when unconnected, that's what's going on.

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  • \$\begingroup\$ Yep, that'll do it. \$\endgroup\$
    – Hearth
    Jan 24, 2019 at 21:29
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    \$\begingroup\$ That appears to be it. According to the datasheet, the feedback resistors are not required since this regulator is fixed, so I did not attach anything to the pin. But you are correct, I just need to connect the feedback pin to Vout and it will work. Thank you. \$\endgroup\$
    – Daniel Frenkel
    Jan 24, 2019 at 21:58
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Also

The ESR of your Caps @ 1MHz is 2mohms ( very good ) but that is the lower IC Spec limit in the spec and you have two in parallel. This won't affect DC error but pulse load tests may lag response due to the ability of the current source (dI/dt=V(RdsOn+DCR)/L) to recharge the cap. in time and then overshoot i.e. It will affect phase margin and overshoot. ESR is a tradeoff between efficiency, load regulation and overshoot. Low is great. Too high or too low is worse. enter image description here

Vcc is the input and the output is Vout. You called the output Vcc ! Perhaps Vin & Vdd might have been better for output...

Try this test after you add FB jumper to your Vout using AWG30 magnet wire.

enter image description here

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