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I am designing a board that will need 3.3 V, 2.8 V, 1.5 V, and ideally 1.2 V rails. The board will be powered by a 12 V power supply so for stepping from 12 V to 3.3 V I am planning on using a buck converter for efficiency, but since stepping from 3.3 V to 2.8 V is much smaller, I was thinking about just using a linear voltage regulator, and then chaining regulators to get to 1.2 V.

Whatever power distribution I go with, I need it to be compact, cheap, and efficient. Is it smarter to use multiple buck converters, multiple linear regulators, or a mix of both?

Ideally, I would find some part that has all 4 rails in a convenient SMD package, but I haven't found that part yet.

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    \$\begingroup\$ I wouldn't get your hopes up about finding a single part that can give you all four (or three, if you're using a buck for the 3.3, which I conditionally recommend) voltages. I'm looking at some that can do 2.8, 1.8, and 1.5 together, but I think you'll have a much easier time just getting some single- or dual-output LDOs and running them from your 3.3V rail. Of course, any more specific advice would depend on your requirements for current, ripple, transient response, power dissipation, etc etc. \$\endgroup\$
    – vir
    Jan 6, 2022 at 0:19
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    \$\begingroup\$ This sounds like exactly the type of application SIMO are designed for. (Single Inductor Multiple Output buck switching regulators) See for example this paper (note: I am an applications engineer at Maxim - now Analog Devices) maximintegrated.com/content/dam/files/design/… \$\endgroup\$
    – MarkU
    Jan 6, 2022 at 1:03
  • \$\begingroup\$ @MarkU These look really cool. Thanks \$\endgroup\$ Jan 6, 2022 at 1:38
  • \$\begingroup\$ @MarkU - that's neat. But 2.8V might be a problem if it uses up too much of the on-time duty cycle. \$\endgroup\$ Jan 6, 2022 at 1:43
  • \$\begingroup\$ You could just use a diode in series from the 3.3v to generate your 2.8v rail. Likely it will be 2.7v but most chips can handle a 0.1V delta. \$\endgroup\$ Jan 7, 2022 at 5:31

3 Answers 3

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If you chain all the lower voltage through the 3.3V converter, the power dissipation of the first LDO (3.3V -> 2.8V) could be rather higher, as all current has to pass through it.

Instead, you could derive them all directly from the 3.3V with individual converters. If current demand for a particular rail is low, you can also consider switched capacitor converters. They are quite simple/small, have high efficiency, but bring quite a bit of output ripple, so check carefully.

It all depends on the current ratings of the individual rails.

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If you can mount a BGA chip, this one can supply your 4 different voltages by setting each with a cap and resistor. They are a bit spendy but you didn't give a budget.

https://www.digikey.com/en/products/detail/analog-devices-inc/LTM8001IY-PBF/3934651

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I was just looking for a similar solution and ran across the MAX679X series, several of which are dual-output.

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