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LM324: How to respect Maximum Current constraint?

Sean McBride
Prodigy 80 points
Part Number: LM324
Other Parts Discussed in Thread: LM2902, , LM358, NE5532, SA5532

I am aware that these general purpose opamps (the LM324, LM358, LM2902, etc.) expect their load to be returned to ground, and provide a "Short-Circuit Output Current" specification in the datasheet.  (Where "ground" may be a low-impedance node at a voltage between the supply rails, as in a split-supply application, or the low supply rail itself as in single-supply usage)

But many of them say shorting them to the high rail may cause overheating and destruction of the device.

So am I wrong to interpret that as meaning there is some maximum output current that we could cause to occur and must take care to avoid.  And also that it is a maximum sink current, since the ability to short to the low supply rail surely means it's current-limited when sourcing current, right?

But there's nothing in the "Max" column under the row for "Output Current" on the datasheets!

NE5532 / SA5532
  • 0
    TI__Guru*** 145996 points

    Sean,

    The sourcing current in the amplifier is internally limited (by a circuit to reduce drive) and only thermal overheating from a sustained short is a problem.  The sinking current is not internally limited by any circuitry for that purpose. The PNP sink driver has finite current capability and that sets the maximum current.  Both of these are obvious from the device schematic in the data sheet. 

    The data sheet does have a 60mA maximum for supply set to 10V that covers sourcing and sinking. For sourcing there is a 60mA maximum for 15V supply. The sinking current for 15V supply has no maximum. With supply voltage over 15V, it is advisable to avoid output shorts of any kind.   

    Do you have a supply voltage in mind? 

  • 0 in reply to Ron Michallick
    Prodigy 80 points
    Thanks for your response.
    What I want to know most is how to interpret the TI datasheet correctly.  Where does it say there's a 60 mA maximum that covers both sourcing and sinking?  What page?

    But to answer the question, I'm expecting to use them in the 5-15V range.

  • 0 in reply to Sean McBride
    TI__Guru*** 145996 points

    Sean,

    On page 5, see green rectangle. 

  • 0 in reply to Ron Michallick
    Prodigy 80 points

    Thank you.  Yes I saw those, but the top one is for source not sink, and the bottom one's test condition is with the output at halfway between the supply rails, not any higher.

    Should I, in general assume this? (at least with TI datasheets) :

    If there is no maximum current on a pin, and if there is a current-limiter which will kick in at one voltage but not another,

    That when the pin is in that other voltage range the current-limiter doesn't work at, I'm supposed to ensure the current doesn't exceed some maximum value

    And that maximum value is the same number of milliamps/etc. in this voltage range as the current limiter would have limited it to in the original voltage range the limiter works at?

  • 0 in reply to Sean McBride
    TI__Guru*** 145996 points

    Sean,

    The sourcing current limiter works the same at any supply voltage. See figures 2 and 3 for typical behaviour (The data sheet says VCC=15V, but it is the same for all VCC). The power dissipated is IOUT * (VCC - VOUT).  The device limits IOUT, the device has no control over VCC or a forced VOUT.

  • 0 in reply to Ron Michallick
    Prodigy 80 points

    I know, I meant a different voltage range of the inputs/output not the supply rails (perhaps I should've said current polarity, but either way let's pretend I said "a different context than the current-limiter will operate in".

    Ultimately what I want to know is, on the next datasheet I have an issue like this with, how can I know that the maximum sinking current I'm expected to make sure the device is only ever exposed to, is the same as the current the sourcing circuit-limiter caps it at when sourcing?

  • 0 in reply to Sean McBride
    TI__Guru*** 145996 points

    Sean,

    The "Absolute Maximum Ratings" table in the data sheets plus all the table notes is the first place to look. Look at the load resistors and currents in the "Electrical Characteristics" tables (test conditions) to know the loads and output currents that work best. Sometimes the "Recommended Operating Conditions" may give some advice, but usually not.

  • 0 in reply to Ron Michallick
    Prodigy 80 points

    Okay, thank you very much.