Low Dropout Regulator 5V 1A

I am not an electrical engineer and so I could use some help here.

We are a bit concerned about the speed controls pumping too much voltage into our APM2.

It appears our ESC's are outputting nearly 6 volts.  Is this too much?

We started looking at some options.  A little voltage regulator LM7805C came to mind, but it appears most suck up quite a bit of voltage (drop out voltage) ~ as much as 2 volts ~ so then the board would only get nearly 4 volts.  And it sounds like that too could be a problem....yes?

So we found what is called a Low Dropout Regulator (see link).  It appears this guy has a drop out voltage of only 0.15 volts at 100mA (did I read that right?) (How many amps do our APM2's need?)  see specs at link below


So it sounds like if the ESC's produce anything more then 5.1 volts we could be assured of a safe 5 volts.  Of course if the voltage from the ESC's gets down to 5 volts, our board would only get 4.9 volts etc. 

I think I would like this little bit of insurance....am I on track or way off the wall ?

Thanks for your input.


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  • I got with Jordi and the 3DR team and we researched and clarified the power requirements for the APM2.0 and there is new, more detailed information on the power requirements that are approved by the 3DR engineering team. This information is based off the manufacturers data sheets and an understanding of how the board is designed and what the original design targets were. Here is the link: http://code.google.com/p/arducopter/wiki/APM2board

    This is now the "official" recommendation and specifications  for the APM2.0





  • I have ordered my APM2 just today and plan to push it to an undervoltage situation to find out. I suspect it will either operate erratically or perhaps simply halt machine cycles and maybe freeze. It is unknown...at least to me.

    My application is the quadrotor. The 5VDC is sourced from the ESC and piped to the PDB (power distribution board). From there it would feed the APM2.

    What is very confusing/funny is what the APM2 documentation states:

    Difference between APM 1 and APM 2

    The main functional differences between APM 1 and APM 2 is in the sensors. (MAJOR TEXT EDIT - STARWALT)

    No on-board power regulator. You must power the board via USB (on the bench) or your regular ESCs (via the ArduCopter power distribution board) in the aircraft, with power coming in through the APM 2 Output pins.


    If the schematic is correct, this statement must be incorrect. Then again I would need to look over the APM1 to determine what they really mean  by 'No on-board power regulator'.

    There is also a section discussing alternate methods of powering the APM2. HERE

    Do you mean if I came up with a protected board you would be happy to try it? My initial ideas would be to modify the PDB to incorporate fusing and power on indications or make a new PDB that has some of the features we are looking for.

    Let me get my APM2 integrated into the Quadrotor project and make some measurements. I am using the 3DR ESC sold for the Arducopter so maybe their output really is just 5V and not what you started this discussion about.

    Up to this point, I have just typed myself smart. Soon enough I can provide some data.

  • Found my notes.  The regulator I was looking at is a Micrel MIC2937A.

    -300mV dropout at 500mA.

    -Line regulation 0.03%

    -Load regulation 0.04%

    -Voltage Accuracy 2% max

    -Noise 260-400uV RMS

    Also the MIC29150:

    Dropout: 80mV @100mA, 220mV @750mA

    Line Regulation: 0.06%

    Load Regulation: 0.2%

    Noise is similar to above

  • Moderator

    Here's an off-the-shelf item that does what you want.  I use it to power a 3.3V camera.


    I'm totally ignorant on most things power, but I've been running APM 1 and APM 2 off of the 3A BEC of a 30A ESC.

  • Linear regs unreliable? Twas not I my lord!

    I side with Andrew in that they are one of the most reliable components in any market.

    They are probably also the most misapplied and abused in any market.

    I mentioned witnessing an oscillation behavior when the regulator is used at or just above maximum load current. This oscillation was really a on-off-on-off-on... event. This was the device doing what it was designed to do, protect itself. As the regulator heats up (bleeding off the 'extra' energy above the drop out level) the specifications of the device change - they de-rate.

    That is why there is so much literature on thermal management of regulators. When they heat up, the performance shifts. The same thing happens in bearings and mechanical things. Bridges change length when they heat up. So do roads, engine blocks, etc. (That is why you took the thermodynamics class in college).

    What is a BEC afterall? A form of regulator circuit, that is all. 'BEC' is a hobby name. We EE types call them by function, a voltage regulator circuit.

    It takes a power source, a battery in our case, and reduces the input value to a designed output value that will operate up to certain limits. The limits are input voltage, output current, and heat management in a simple case. Component designers then try to think up myriad ways to make the regulator work in extreme conditions. Those are more of the charts and graphs in the component literature.

    Looping this back around to the APM... any form of well designed regulation that keeps the APM running within its operational specification is a good thing.

    Having worked in electronics for over 35 years (and getting the B.S. later in life), now is a wonderful time to learn electronics. It has never been cheaper to experiment simply because we are buried in so much surplus/salvage/junk that has been tossed out.

    You should see my 'stock pile'. I will never live long enough to employ all the parts I have. Resonable offers to come get some will be accepted. ;)


    Rick asks another good question to clarify my statements about regulator non-function. I defer to one of the manufacturers, Texas Instruments... TI Regulator Link

    For the short course version, see the graph taken from Section 8.2 below. I think a C.E. major can understand most of the paper but it does go into depth on thermal management of components. The bold box below says it all, Dont't go below the minimum input voltage. Note also the quiescent voltage difference in the two lines. That is what Rick is referring to when the input drops off. Switching regulators (actually a switching regulator *circuit*) usually have an output that goes to zero when the input falls below the magic point.


  • It sounds like Rick needs a product, not a part, unless he wants to build a product from parts.

    The drop out specification is to let the user know when the regulator stops working (ideally).

    For a 5 VDC regulator with a 1 VDC drop out spec, 6.x VDC input is the minimum input level where the regulator will still output a voltage/current. If the source, for instance a battery, drops below 6.x VDC the regulator should stop functioning. This means that whatever is connected to the regulator is shut off (at least when the magic point happens).

    A higher drop out specification means that the input source must be greater. The regulator will stop functioning just like the above situation but at a higher input voltage level. In either case, you do not want the regulator to drop out while in flight.

    The 1A value of your initial post title refers to the maximum current the device can provide to the load. If you exceed that current (not the case probably with an APM), the regulator may also shut down - again no output (bad news part 2).

    I have seen situations where 780x series regulators oscillate when close to the maximum loads. That drives a system load bananas. An flying aircraft is not a good place for this to happen. The newer devices usually protect against this behavior.

    The system/product designer has to pick a suitable input voltage that also does not waste energy by dissapating the unregulated input differential as heat.

    Specifications have tolerances and each regulator will be slightly different from another of a different batch/lot, thus the plus/minus caveat on a value.

    In the long run this discussion is basically one of cost. If the aircraft is valuable and $$ resources are comfortable, rather than spend time developing another product; I would run a separate battery -> BEC -> APM and be done with it.

    This way you have a known product, support of that product, and a basis for use.

    If you want to 'roll your own', you should bench test all the parts to failure (not necessarily smoke failure) while measuring the important parameters to know where your critical points are.


  • I'll just be another voice in the 7805 chorus, with the caveat that I'm a capacitor snob (from my old audiophile days) and hyper about stable supply rails. 

    Most application notes suggest something tiny at the output, but I suggest a 4.7 uF tantalum cap vs. a cheap electrolytic.  If you choose the latter, go 47 uF.  A similar or smaller cap at the input is recommended to protect against input noise.  A 47 pF ceramic in parallel with both seals the deal for noise suppression.

  • Clever way to get 3 power sources, I would not of thought to only hook up the ground and the signal wire from my ESC and leave out the RED 5V wire from the connection.  TYVM, for redundancy in aircraft is a beautiful thing.

  • Hold on a moment.

    When you say the ESC produces 6 volts, are you measuring this open with a voltmeter?  The ESC regulation circuit may be a switching type, and you can't rely on the voltage output being sane on them when you're not drawing a load through it.  They float horribly when they aren't under load.

    Hook up a 50 ohm resistor across the output of the BEC, then measure the voltage across that.  Keep in mind that the resistor will dissipate half a watt, so pick an appropriate sized resistor.

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