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I am working on an Electronics design which incorporates DC-DC buck converter voltage regulators. This is the first time I've been working with buck converters, so I have no experience and only limited knowledge on their operation. I've already found a design using the SY8113 for the same purpose as what I'm designing (power for a RPi CM3), but when I use the formulas in the Datasheet, the Inductor value I get does not match the "reference" design I've used.

To explain a little bit further; The RPi CM3 pulls a max load of 250mA on the 3.3V (and 1.8V) supplies, which in turn are supplied by the SY8113 circuits. These supplies also power some peripherals, but the total current should not go beyond 500mA. However, using the Inductor value from the reference design (2.2uH), it seems the power supply circuit is designed for a 3A power draw.

So my first question is: What is the consequence of designing such a power supply circuit for 3A, and then only pulling 250 - 500 mA from it?

Secondly, if I calculate the Inductor value from the datasheet formula, I get a minimum inductor value of 11.22 uH (so 15 uH should be a usable value). However, finding a 15 uH inductor with a DCR below 50 mOhm (which is recommended in the datasheet) seems quite hard. So what is the consequence of picking a 15 uH inductor with a DCR of 98-115 mOhm for instance?

I would simply like to know if I should use the 2.2 uH inductor from the reference design, or if I should use the value I get from the datasheet formula. Also, I would like to know about any problems which could occurr from choosing an inductor with a slightly higer DCR than recommended.

Datasheet: https://datasheet.lcsc.com/szlcsc/Silergy-Corp-SY8113BADC_C78989.pdf

Reference design: 3.3V SY8113 based DC-DC Regulator

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What is the consequence of designing such a power supply circuit for 3A, and then only pulling 250 - 500 mA from it?

The data sheet says: -

  • 3A output current capability

This means it is capable of delivering 3 amps. If your load is 250 mA then there is no problem. There is also a typical circuit on the front page that shows efficiency vs load current: -

enter image description here

As you can see, the load current range if from 10 mA to 3 amp.

In addition the regulator is a synchronous type and this means that it will handle a large range of load currents without excessive ripple voltage on the output i.e. it won't go into burst mode unexpectedly.

Secondly, if I calculate the Inductor value from the datasheet formula, I get a minimum inductor value of 11.22 uH

You don't need to - stick with the 2.2 uH design and you'll be OK.

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  • \$\begingroup\$ Thank you for a very clear an elaborate answer :-) I guess using an inductor rated for less than 3A would be OK then also, as I'm never pulling the full load capability of the regulator? \$\endgroup\$ – Johnny Egeland Jul 16 '18 at 9:42
  • \$\begingroup\$ Higher inductance means more losses so I would stick with the design they recommend. \$\endgroup\$ – Andy aka Jul 16 '18 at 9:43
  • \$\begingroup\$ I meant using a 2.2 uH inductance, but with "Imax/Isat" lower than 3A would be OK (as I will never be pulling 3A). This is simply to save a little space on the board. \$\endgroup\$ – Johnny Egeland Jul 16 '18 at 10:36
  • \$\begingroup\$ You should be OK but when you have decided on one, provide a link in a comment and I'll check it out. \$\endgroup\$ – Andy aka Jul 16 '18 at 10:39
  • \$\begingroup\$ Likely slightly overkill, but seller provides datasheet and I think they will fit the board. aliexpress.com/item/… \$\endgroup\$ – Johnny Egeland Jul 16 '18 at 12:41

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