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Regulator wiring, in-line or not — Parallax Forums

Regulator wiring, in-line or not

Bobb FwedBobb Fwed Posts: 1,119
edited 2009-06-13 14:51 in Propeller 1
So I saw on one of the Parallax-fab propeller board (don't remember with one -- one of the big proto boards) that they had the 3.3V regulator supplied by the 5V regulator (Vin -> 5V -> 3.3V). I've always wired up my boards so the 3.3V and 5V regulators are supplied directly by the input voltage (Vin -> 5V, Vin -> 3.3).
What is the benefit of supplying feeding the 3.3V via the 5V.

In a project I am working on, the 5V circuit uses over twice as much as the 3.3V, so the regulator is already nice and toasty, putting the 3.3V load on there as well will only make things worse. I understand that the 3.3V regulator will need to dissipate less heat.

Is it just a matter of preference or is there some other thing I am overlooking?

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-12 16:42
    Feeding the 3.3V regulator from the 5V regulated supply has the advantage of reducing heating of the 3.3V regulator. It's particularly useful when there's not a lot of use of the 5V supply. The disadvantage of this scheme is that it reduces the amount of 5V supply current that can be drawn before over-current shutdown or overheating will occur in the 5V regulator.
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-06-12 16:49
    So it is a matter of the specific application. Right? I have more current draw on my 5V...so probably I should keep them separate.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    April, 2008: when I discovered the answers to all my micro-computational-botherations!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-06-12 17:03
    Feeding the 3.3V regualtor from 5V also gives you a wider selection of regulators to choose from. Some 3.3V regulators I've seen have a top-end input voltage of 6.5 or 7V. An additional advantage of reducing the step-down voltage and resulting power dissipation is that you can use a 3.3V regulator in a smaller package, thus reducing board space and, usually, cost.

    -Phil
  • mctriviamctrivia Posts: 3,772
    edited 2009-06-13 14:51
    another advantage is you can make a cheap cool supply by making the 5v a switching regulator. You can then get inpu voltages of 7-40v with little heat

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