Electrical – Altering switching power supply schematic to generate 5v instead of 3.3v at 5A

buckpowerswitch-mode-power-supply

Im fairly new to this level of more complex circuit design so forgive me if I sound like a newb.

I'm creating a DC/DC Step down buck regulator using the LM22678 switcher from texas instruments: http://www.ti.com/product/LM22678

The documentation depicts an application of the switcher for an output of 3.3v at 5A. enter image description here

The documentation is rather complicated for someone who is not an expert. What values of the capacitors, resistance and inductance would I have to modify in order to generate a 5-6v output at 5A?

Below is just a start point of my design on eagle:enter image description here

EDIT:
enter image description here

Best Answer

There are two variants of the regulator you are using. The schematic you posted from the datasheet depicts the LM22678-ADJ. This one is best used for voltages from 1.285V upto 5V. If you want 5V or higher, you'd best use the LM22678-5.0 variant. This is the one you used in the Eagle schematic you posted.

The datasheet says on page 14 - 8.1.1: "For output voltages of 5 V, the -5.0 option should be used. In this case no divider is needed and the FB pin is connected to the output." That means:

  • Make sure you buy the LM22678-5.0
  • You omit Rfbt
  • You omit Rfbb
  • You connect the FB pin straight to Vout
  • The rest of the parts you can leave as they are depicted in the datasheet.

Now whether this will work depends on the INPUT voltage you are planning to feed to the regulator (which you didn't share about), and the amount of current you plan on drawing from it. 35.6uF worth of capacitors on the input of the regulator is not a lot. That means your INPUT voltage cannot have a large ripple. For instance if you are rectifying AC voltage, you might need some more (e.g. bridge rectifier) or 'bigger' (e.g. a bigger C2 capacitor) parts on the INPUT.

Also keep in mind the minimum INPUT voltage you need, to make sure the OUTPUT can reach 5V. You can read about that at page 13, including a formula that you can use to calculate the minimum Vin. As you can see there, the minimum Vin depends also on the current you plan to draw from the regulator.

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