Little help with a LM2596 inverter...


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  1. #1
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    Default Little help with a LM2596 inverter...

    Hello all,

    Been away for a while, hope everyone is well. I've got a question in regards to the LM2596 INVERTING switcher. I'm brain frying over a "shutdown", or rather a "start up" circuit for this rig. Can you fine folks check out these datasheets real quick and let me know if I'm over-thinking things (which would not be abnormal for me ).
    I would like to turn ON the switcher with +12vdc...
    It appears to me that these two datasheets conflict each other?
    http://www.ti.com/lit/ds/symlink/lm2596.pdf FIGURE 40
    http://www.onsemi.com/pub_link/Collateral/LM2596-D.PDF FIGURE 24
    Thanks a ton guys,
    Chris

  2. #2
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    Default Re: Little help with a LM2596 inverter...

    It appears to me that these two datasheets conflict each other?
    in what way can you expect then to be equivalent. ?

    you are not comparing identical devices one is a fixed output voltage version the other an adjustable output voltage version of the switcher. it's little wonder their different

  3. #3
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    Default Re: Little help with a LM2596 inverter...

    Umm, Ok,
    Maybe I'm missing something but, negative, is negative. It is a function of the voltage divider on the output. This chip can be set to fixed or variable. This clearly was not my question. If you look at the logic states expressed in the datasheets aforementioned, you will see that ON-Semi specs that a HIGH specifies an "OFF" as TI specs a HIGH as output "ON". The schematics are effectively identical.
    Thanks for your input,
    Chris

  4. #4
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    Default Re: Little help with a LM2596 inverter...

    read it again
    This chip can be set to fixed or variable
    the way I read it ,it comes in fixed or variable flavours for output voltage not either /or

    reguards shut down both are active low ie look at the circuits .....
    onsemi
    It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply
    current to approximately 80 A. The threshold voltage is typically 1.6 V. Applying a voltage above this value (up to
    +Vin) shuts the regulator off. If the voltage applied to this pin is lower than 1.6 V or if this pin is left open, the regulator
    will be in the “on” condition.

    ti
    ON/OFF CONTROL
    VIH Threshold Voltage Low (Regulator ON) 0.6 V(max)
    VIL High (Regulator OFF) 2.0 V(min)

    you say
    TI specs a HIGH as output
    I say where ???
    Last edited by richard; - 25th April 2015 at 08:18.

  5. #5
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    Default Re: Little help with a LM2596 inverter...

    Pulling the ON/OFF pin on the LM2596 enables the regulator, both datasheets agrees on that. What low is depends on if you're using it in the inverting configuration or non inverting configuration.

    What's different between the two datasheets is the notation of the input signal driving the LED in the optocoupler in the circuit examples mentioned. If I'm not mistaken they've seem to have got it backwards in the ON Semi datasheet because forward biasing the LED will make the transistor start conducting pulling the ON/OFF pin low which turns ON the regulator, yet the "timing diagram" next to the pin for the LED signal says HIGH=OFF.

    /Henrik.

  6. #6
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    Default Re: Little help with a LM2596 inverter...

    well picked henrik
    it would have much easier to say ti sheet fig 40 vs onsemi fig 24 .

  7. #7
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    Default Re: Little help with a LM2596 inverter...

    Didn't he say exactly that in the first post?
    It appears to me that these two datasheets conflict each other?
    http://www.ti.com/lit/ds/symlink/lm2596.pdf FIGURE 40
    http://www.onsemi.com/pub_link/Collateral/LM2596-D.PDF FIGURE 24

  8. #8
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    Default Re: Little help with a LM2596 inverter...

    I must be going blind I did not see that at all

    jeese it's even in bold type
    Last edited by richard; - 25th April 2015 at 11:31. Reason: jeese

  9. #9
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    Default Re: Little help with a LM2596 inverter...

    Thank you, thank you, thank you guys . My thoughts were that ON Semi were backwards, but I needed a second opinion. I think I've looked at those two schematics 50 times each trying to figure out what I was missing. I may still be nuts but, maybe not so much in regards to this question.

    Thank you so much again,
    Chris

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