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MC33078: MC33078 failure after burn in

yh rdcde
Prodigy 20 points
Part Number: MC33078

Tool/software:

Hello everyone:

If any of the outputs of U12, U13, or U14 fails, the audio circuit will fail (no audio output).
This typically occurs after about four hours of device burn-in.
When this happens, the op amp becomes hot, and the impedance measured between pins 8 and 4 is only 275 ohms. (typically 8.5 megohms)
I don't understand what's wrong with this circuit design. I haven't measured any abnormal voltages or inrush current at the op amp inputs or outputs. Is this MC33078 not suitable for use with the DAC(AK4458)?
Can anyone help me?2451.DAC.pdf

  • 0
    TI__Mastermind 25355 points

    Hello, 

    I can assist in your inquiry. 

    yh rdcde said:
    MC33078 not suitable for use with the DAC(AK4458)

    There should not be any issue using the MC33078. Based on the information I was able to find online, this may just be a component value fix that can resolve what you are observing. 

    I found some information to help debug the issue. From this tech document DAC there is a note on page 12:

    •  Note 14. Regarding Load Resistance, AC load is 2kohm (min) with a DC cut capacitor (Figure 83). DC load is 3.5kohm (min) without a DC cut capacitor (Figure 83). The load resistance value is with respect to ground. Analog characteristics are sensitive to capacitive load that is connected to the output pin. Therefore the capacitive load must be minimized.

    The op amp may be burning up for the possible reasons below:

    1. Common-Mode violation/DC current fight. This can happen if the DAC common-mode voltage applied to the op amps inputs violates the abs max common mode of the op amp. 
    2. Downstream the DAC expects the 3.5kohm or greater DC load when DC coupled, i.e without a DC blocking cap between the output of the DAC and the op amp. If the DC load is too low then the DAC and the op amp may be fighting each other which can cause unintended DC currents to flow heating up the op amp. 
    3. Power Sequencing can also present a problem if the DAC's common mode voltage is outside the op amp's supply rail during power up. The input or output stage of the op amp can be driven outside the abs max conditions and lead to damage. 

    Debug Steps:

    1. Check the common mode voltage at the op amps pins both during power up and normal operation. 
    2. Check the DAC's common mode voltages at the DAC's output during power up and normal operation. 
    3. Try using a DC blocking capacitor (1uF) between the output of the DAC and the op amps to ensure there is no unintended DC currents flowing that can create a scenario where the op amp may be damaged. 
    4. Alternatively, ensure that the equivalent DC load resistance is 3.5kohm or greater if a DC blocking cap cannot be used to ensure that the design adheres to the requirements of the DAC. See page 86 of the DAC's datasheet for their example impedances when DC coupled. 
    5. Verify the power sequencing and voltages

    I hope this information helps. 

    Best Regards, 

    Chris Featherstone