Most efficient/simple voltage reduction

Discussion in 'General Electronics Chat' started by Col, Oct 17, 2014.

  1. Col

    Thread Starter Member

    Apr 19, 2012
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    Hi,
    I want to generate 1.8V (1.6-2.2 is ok) for a very low power application (active RFID, needs about 150uA). Batteries are unfortunately not available in this range (correct me if I am wrong). I want to getenrate a voltage suitable for my RFID, and it does not need to be well regualted. An LDO is out as it will just burn power, similar for a simple R divider. I'd rather not go down the roade of switching regulators if I can avoid it.

    So im thinking along the lines of diode. Could I just put a 1.2V Vf diode between the battery and load? I thought about a zener with breakdown of 1.8V but wont this just short out higher voltages and quickly drain the battery

    Tnx
     
  2. riko

    New Member

    Sep 7, 2014
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  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What kind of RFID? Right now I am playing with 14443 at 13.56Mhz, and I am using LTC1771 step down converter.
    I am still trying to finish the data transmission, and after that I will again try to get it to work from just the RF field energy.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you rule out linear regulators, switching regulators, and voltage dividers, I'm not sure what's left. Unless you are dealing with a relatively high voltage source, like 6 volts, a switcher is way too complex for the job. A series regulator will be more efficient than a shunt regulator like a zener diode or LED. But no matter how you do it or how small the load is, regulation requires headroom, and headroom costs watts. Without some operational parameters, there can be no "best" answer.
    What is the battery voltage and capacity?
    What is the expected battery lifetime?

    ak
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    On the other hand, a shunt regulator used on the primary side of a step down converter might have some benefits for this particular purpose, as it will keep the load on the LC tank fairly constant.
    This will keep your tuning frequency and bandwidth constant as well, which should help with reliable data transmission.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Sounds like you are using a 3 volt cell. So my thinking is just a couple of silicon diodes (1N914) in series with the 3 volts. Follow this with a 10 Meg resistor to ground and a small filter cap. Probably around 65% efficient. Not sure you are going to do much better.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Not necessarily. You could set the zener current to, say, a max of 400µA. For instance: 3V cell + 3KΩ resistor + 1.8V zener.

    When the load comes on line, it will take 150µA and the zener will take 250µA. I believe a zener would have no trouble holding the voltage within the range you gave while passing these low currents.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Maybe, but the LDO mentioned above has a static current of only 125 uA when on, and 0.3 nA in shutdown mode.

    ak
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Hmm, yea. does he have a way to turn it on and off?
     
  10. nigelwright7557

    Senior Member

    May 10, 2008
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    What ever you use to drop volts will use power.
    The only way around that is a DC-DC converter.
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    A single cell lead acid battery puts out about 2.1 volts open circuit. ;)
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Reference
     
  13. Col

    Thread Starter Member

    Apr 19, 2012
    44
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    Thanks all for the comments.

    Its UHF RFID, 868MHz, aiming for 6-8m distance, NFC band could more easily take power from the reader

    Won't the LDO be much less than 50% efficient factoring both the voltage 'burn off' and quiescent current. I don't have a way to shut it off
    wirelessly (infact the reason i need to add power is to drive an RFID digital output wirelessly) but its for a demo so the enable of the LDO is handy, but equally could just break to circuit at the diode

    Rony: series diodes sounds the simplest and 65% efficiency I can handle, probably I'll go with this solution

    Analog Kid: Thats just what I was thinking too, Battery will be a 3V button cell like CR1220 (40mAhr), Battery life should be as long a possible.

    Lead acid battery it is then, I guess they can integrate them on silicon now. :)

    Thanks for the reference Mike, but am i missing something here? 1.55V is too low, I need 1.8V (or an absolute min of 1.65V, not recommended)
     
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