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reading Power Dissipation graphs

Discussion in 'Electronic Basics' started by [email protected], Mar 24, 2008.

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  1. Guest

    I'm reviewing the datasheet for the LM1875.

    Page 4, upper-left, has a Power Dissipation vs. Power Output graph for
    a 4-ohm load.

    For +/-30V, to get 30W out, power dissipation is given as 47W.

    What is meant by "power dissipation"?

    Is this total heat generated, for heatsink load calcs?

    Is this the total power consumed ( = the power delivered by the power
    supply)? If so, then the chip is 30W / 47W = 64% efficient, right?

  2. Phil Allison

    Phil Allison Guest

    ** Yep.

    ** Nope.

    ** No.

    The LM1875 is not usable at 4 ohms with more than a +/- 20 volt supply -
    as it has in internal current limit of 4 amps.

    30 watts into 4 ohms = 11 volts rms = 15.5 volts peak.

    So with a +/- 30 volt supply, the IC is only able to use half the available
    supply voltage with a 4 ohm load.

    Makes it very INEFFICIENT !!

    It will go into thermal shutdown and/or gross distortion with a 4 ohm
    speaker load.

    ........ Phil
  3. In <>,
    That is true.
    No, efficiency in this case is 30W / (30W + 47W) or 39%.

    - Don Klipstein ()
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