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I need to have a 5v supply with around an amp of current draw max, and a 6v supply with around a 4amps current draw max. I was thinking: instead of having two switching regulator circuits, just having one 5amp 9v switching regulator circuit feeding into an Lm7805 and an lm338t (linear regulators) to generate my 5v, and 6v rails, respectively. My Vin is around 18v. Pololu makes some nice switching regs that fit my needs.

Are there any potential drawbacks to this arrangement? I heard it referred to as a switching pre regulator. When answering please bare in mind I'm a hobbiest.

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    \$\begingroup\$ That's a grand total of 12 + 4 = 16 W of heat. If it is ok for you, go for it. Usually you have linear SMPS cascaded with linear regulators to have a quiet supply, if yours is not the case two SMPS are better. Or one 5A SMPS down to 6 V, and an LDO down to 5 V (1 W of heat only - probably ok even without an heasink). \$\endgroup\$
    – Vladimir Cravero
    Sep 20, 2017 at 7:55
  • \$\begingroup\$ Can you specify what you meant by cascade? \$\endgroup\$
    – Alex
    Sep 20, 2017 at 8:03
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    \$\begingroup\$ With cascading I mean one connected after the other, e.g. in your original plan you would cascade the 7805 to the switching regulator. Please remember that using "in series" in this context is usually wrong, and always inaccurate. \$\endgroup\$
    – Vladimir Cravero
    Sep 20, 2017 at 8:09

2 Answers 2

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No, this sounds reasonable, if it is what you want to do. One problem is that you will drop 9-6=3 volt at 4 amperes, that's 12 watts. You need a hefty heat sink for that.

A better option is perhaps to have a 6V output from a switching regulator, and a 5V low-dropout linear regulator, converting 6V to 5V. The 5 volt regulator would only have to drop one volt, converting 1 watt into heat. This is still a non-trivial amount of heat for one IC, but as peufeu mentions in a comment, is small enough to be dealt with using the copper on the PCB as a heat sink.

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  • \$\begingroup\$ Both you and Vladimir have saved me some board space because for some reason I overlooked the option you outlined. \$\endgroup\$
    – Alex
    Sep 20, 2017 at 7:58
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    \$\begingroup\$ @Alex Too bad Vladimir didn't write his answer in the correct place then. ;) (Yes, I'm on a crusade to get rid of answers posted in the comment section) \$\endgroup\$
    – pipe
    Sep 20, 2017 at 7:59
  • \$\begingroup\$ I did not write an answer because I did not have time. A proper answer, or at least what I'd call a proper answer, would have taken quite a lot more space, dealing with all the pros and cons of the various solutions. But it is just a matter of taste, @pipe \$\endgroup\$
    – Vladimir Cravero
    Sep 20, 2017 at 8:06
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    \$\begingroup\$ If the 6V to 5V LDO dissipates 1W, then a LDO in SOT89 or DPAK using the ground plane as a heat sink will be a good match. \$\endgroup\$
    – bobflux
    Sep 20, 2017 at 11:45
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    \$\begingroup\$ @vladimir It's not a matter of taste. SE comments are intended to only be used for clarifying the question, not a second-class of "partial answers", and can be deleted at any time. The network wide guidelines on this are clear, for various good reasons. \$\endgroup\$
    – mbrig
    Sep 20, 2017 at 15:12
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Using a switching regulator upfront and a linear regulator for final regulation can make sense if you want to reduce power wastage while reducing the amount of switching noise that reaches your final power rails.

I question why you would go as high as 9V though, nowadays you can get voltage regulators with dropout voltages well under a volt, so you should be able to use a 7V output voltage on your switching regulator dramatically reducing power wastage.

If you don't care about noise suppression then I would be looking to a 6V switching regulator in combination with a 5V linear.

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  • \$\begingroup\$ Note that a lot of the "good old" linear regulators have very little noise suppression in the frequencies used for switching, so it's worth shopping around for newer regulators if you want to reduce switching noise. \$\endgroup\$
    – pipe
    Sep 20, 2017 at 15:15

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