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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

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    \$\begingroup\$ Did you check section eight of the datasheet? The process to calculate R1 and R2 is described there. Where do you get stuck? Also, in sec. 8.2 the full procedure to select all the components is described. You cannot expect us to do the design for you. \$\endgroup\$ Aug 5, 2018 at 13:47

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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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  • \$\begingroup\$ Ive edited the post and added an image of the completed eagle schematic, Will this setup with the same values the way ive done it give 5v and 5A for sure if my input ranges from 11v to 12v \$\endgroup\$ Aug 5, 2018 at 14:52
  • \$\begingroup\$ Well done, your schematic should work. You might want to add a 470k resistor between Vin and EN, as stated in the datasheet page 9, 7.3.1: "Although an internal pullup is provided on the EN pin, it is good practice to pull the input high when this feature is not used, especially in noisy environments." But even without you should be fine. Just make sure the diode D1 and the coil L1 can handle 5A, which is quite a lot of current. \$\endgroup\$
    – MartinF
    Aug 5, 2018 at 16:53
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To get 5-V output, you need to change the the feedback resistors accordingly, or select the fixed-output version of the IC, as advised by MartinF. However, this should be the least of worries for a novice to make the regulator working, and at full 5A output.

To make it working, please follow the manufacturer's suggested layout as close as possible when making the PCB. Forget any "breadboarding". Then use their "component calculator" to select proper inductor and capacitor types.

The best way is to find a demo board for this IC, and follow its BOM to last specification. There is a demo board for a similar LM22677 IC, use it as the guide. Make sure that the design has sufficient thermal dissipation properties, and described in Section 10.3, since at 5 A the design efficiency is under 70% (or need to dissipate 7.5W), and the IC/inductor will be hot and even might need a substantial heat sink. And good luck.

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