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LM4674: Random output channel is damaged

Germán Rosendo
Prodigy 10 points
Part Number: LM4674

Hi everyone,


I am going to expose my problem.

We have had some issues about LM4674 Audio Power Amplifier.

The schematic circuit implemented is shown below. This circuit is used for separating both, L and R audio channels on a 32 ohms impedance headphone.

The transformer is used because headphone has a common pin pulled to GND.

Occasionally, in some devices, it has been observed that the output audio line L (OUTLA, OUTLB) or R (OUTRA, OUTRB) from LM4674 has been damaged and it stops working permanently. We don’t know what is the reason that it deals damage to the amplifier.

 Please, anyone could help?


 

Thanks in advance.

Regards

  • 0
    TI__Guru** 100965 points

    Hi German,

    Since LM4674 is a Class-D, I would not recommend to place capacitors directly at the output of the device. Ideally the first thing the driver should see is a resistive/inductance component. Capacitor at the output of a Class-D can cause high overshoot at the fast switching edges of the PWM signal.
    In addition to that, operation with transformer is not thoroughly tested/considered for this device operation, as use case is BTL load.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi Ivan,

    Thanks for your answer.

    We have performed a test, the test have consisted on adding an extra capacitor on C19, C35 and C21 in order to increase each capacitance and the result has been another channel output damaged relatively fast.

    It seems to be responsible of the damage bercause of current peaks.

    We have removed these capacitors in other device and until now all works fine and none other channel has been damaged.

    Thanks again.
    Regards.

  • 0 in reply to Germán Rosendo
    TI__Guru** 100965 points

    Hi German,

    Than you for your feedback.
    I think that's expected, as mentioned, it is recommended to remove the capacitance connected directly at the output of the Class-D, this would prevent overshoot and thus current peaks.

    Best regards,
    -Ivan Salazar
    Applications Engineer