Lm2595 Output capacitor ESR

Discussion in 'General Electronics Chat' started by mendiola_loyola, Nov 30, 2010.

  1. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    Dear All:

    The Lm2595 datasheet states that the output capacitor with a very low ESR can cause instability
    on the regulator.

    Tha datasheet specifies the output capacitor capacitance range up to 330uF but it doesn't specify
    the lowest ESR value for that capacitor.

    I want to know the lower limit of the ESR for the output capacitor.

    Could you help me with this topic please?

    Datasheet:
    http://www.national.com/ds/LM/LM2595.pdf

    Regards.
    Alfredo Mendiola Loyola
    Lima, Perú
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    On page 8 of the datasheet they show a capacitor manufacturer's name and the value, voltage and series of the capacitor.
    On page 9 they recommend a second capacitor's manufacturer's name and the value, voltage and series.
    On page 11 the datasheet shows 4 capacitor manufacturers.
    On many other pages they talk about the capacitors.
    In figure 14 they show a graph of typical ESR ratings for capacitors.
    In figure 15 they show the effects of temperature on the ESR of an electrolytic capacitor.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    The datasheet pretty much covers it, but in different spots. It seems ESR should be ~0.1 Ohms, preferably > 0.05 Ohm. They recommend directly the Nichicon PL series. The size you need can be found on the table once you know the requirements for current draw of the load.
     
    Last edited: Nov 30, 2010
  4. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    Yes I have read all the datasheet, but it doesn't include nothing about the lowest ESR value that an output capacitor can have.

    For example, Can I use a 330uF/50V as an output capacitor if the output voltage is 5.0V?

    The table in the datasheet indicates that I must use a 220uF/25V, but I have bought a 330uF/25 and I want to know if I can use it.

    I'm using the adjustable version.

    Regards
    Alfredo Mendiola Loyola
    Lima, Perú
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    If you didn't buy the manufacturer's capacitor that is recommended then you must research the spec's of what you have and compare it to the spec's of the ones that are recommended.
     
  6. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    Dear All:

    The Lm2595 inductor Selection table specifies a Maximum and Minimun voltage per Inductance region.

    What would happen if for example I have the inductor L30 (68uH) and my input voltage drops to 9.5 Volts?

    Will the regulator work bad if I don't respect the minimum voltage per Inductance region?

    I will use 1A of current.

    I hope you can help me?

    Regards
    Alfredo Mendiola Loyola
    Lima, Perú
     
    Last edited: Feb 9, 2011
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Look at page 11 of the datasheet.

    For a 15V input, and 5V @ 1A output, a 68uH inductor (with windings large enough to carry 1A, not the little inductors that look like a resistor), and a 180uF-220uF cap (depending on the Manufacturer) are suggested.

    The output cap most likely can vary a bit, as long as it is in the ESR rating required. From the datasheet, the capacitor should have a very low ESR at 100kHz. There usually isn't a "lower limit" for ESR, only "too much". It's a parasitic aspect of lower grade caps that impedes the capacitors function.
     
  8. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    I use the ADJ version.

    My input is: 23.4, output 13.8

    But sometimes my input voltage drops to 18 volts.

    Regards
    Alfredo Mendiola loyola
    Lima, Perú
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Those aren't in the chart, have you run through the equations on pages 11 and 12 to get the values needed?
     
  10. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    Yes: (Input 23.4; Output 13.8)

    E*T =35.8020378

    L29 (100uH)

    But the minimum voltage for that inductance region is 19.9V (25 E*T ) for an output of 13.8V.

    The problem I have is that sometimes my power supply voltage (Input voltage) drops from 23.4 to 18 or 17V at 1A load.

    Is there any problem if my input voltage drops below 19.9V (25 E*T) ?

    Regards.
    Alfredo Mendiola
    Lima, Perú
     
  11. mendiola_loyola

    Thread Starter New Member

    Sep 18, 2010
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    Picture of Inductors Table ADJ
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The purpose of the regulator is to hold the output steady even if the input voltage varies a good deal. The major item that changes is efficiency. The wider the gap between input and output, the lower the efficiency (Bottom right graph, page 5).

    Design for the highest input voltage. The datasheet shows 3.25v headroom, so add 3.25v to the output voltage to know the minimum input voltage (though that may not be optimal). The datasheet has basically all of the answers.
     
    Last edited: Feb 10, 2011
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