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The output of one voltage regulator feeding the input of the next? — Parallax Forums

The output of one voltage regulator feeding the input of the next?

MarkSMarkS Posts: 342
edited 2007-09-21 01:46 in General Discussion
I'm working on something that will use an ARM9 microcontroller, which uses 1.8v, but will also use a Prop at 3.3v and various other components at 5v. The input voltage will be between 5v and 9v, unregulated from a battery. I don't want to hook up each voltage regulator directly to the battery. Feeding 9v to the 1.8v regulator means that 7.2v (80%) is being wasted as heat. This is totally unacceptable and I can imagine that the battery will not last long on such a set up.

What I want to do is shown in the attached image. Is this acceptable?

Post Edited (MarkS) : 9/21/2007 12:55:34 AM GMT
Regulators.jpg
813 x 385 - 30K

Comments

  • pwillardpwillard Posts: 321
    edited 2007-09-21 01:09
    It would work as expected, but make sure to use LDO (Low Drop Out) types of regulators.

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    There's nothing like a new idea and a warm soldering iron.
  • RDL2004RDL2004 Posts: 2,554
    edited 2007-09-21 01:35
    "Drop Out" is also known as "headroom".

    The typical 3-terminal regulator needs a voltage at its input that is over and above what it must output by a certain amount or it will fail to regulate.

    LDO (low drop out) regulators need less "headroom".

    Your basic 78xx or LM340 type may need anywhere from 1.5 to 3 volts overhead (depending on manufacturer and other things).

    An LDO type of regulator can work with very low headroom 0.5 to 1 volt is typical - this helps also with power dissipation

    Check the data sheets of the regulators you plan to use to be sure they will work as intended.

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    - Rick
  • JSWrightOCJSWrightOC Posts: 49
    edited 2007-09-21 01:46
    May I mention that the total power consumption will be approximately the same regardless if you connect the regulators in series or independently, for any linear type voltage regulator. Any variation between the two configurations would be dependent on the curve of the quiescent current draw vs. input voltage for each regulator. No matter how you slice it, if you are consuming 100mA from the 1.8V regulator, you're still going to have to lose 720mW somewhere, be it divided up among 3 regulators or all in one regulator. The only way to achieve higher efficiency would be to use a lower voltage power source (6V instead of 9V perhaps) or switch-mode buck regulators.
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