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LM49450: Onboard amplifier audio cuts out

John Bae
Prodigy 30 points
Part Number: LM49450
Other Parts Discussed in Thread: LM4950

Tool/software:

Is it normal for the LM49450 amplifier audio to cut out and revert the audio output to the auxiliary headphone jack? Plugging in headphones and unplugging them will make the amplifier output again to a small speaker. It is something that happens sometimes on our boards, but not on all boards. I'm not sure if it's a defect in these chips design or manufacturing problems. Again not all our boards suffer from this, just some. One thing I noticed in the datasheet is that it does not mention or recommend a pull down resistor on the auxiliary headphone jack for headphone detection. Is this something that should be done for these?

Is there a recommended replacement for this chip that has simliar specs (stereo, class-d, i2c control etc)?

  • 0
    Prodigy 30 points

    Another to note, if the speaker output pins are too close (surrounded) to a ground plane on either side (top or bottom of the boards), could that trigger the overload protection and force the audio to go out of the aux headphone output instead? 

  • 0 in reply to John Bae
    TI__Guru** 102045 points

    Hi John,

    Have you inspected the voltage on HPS pin? Is it close to the threshold rather than being strongly pulled up or down?

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    Ya, it is measuring 0.02V. With a headphone plugged in then it goes up to 2.49V as expected. When the amplifier cuts out audio to the speakers on it's own, the HPS pin still stays at 0.02V until headphones are plugged in.

  • 0 in reply to John Bae
    Prodigy 30 points

    I can replicate the problem 100% everytime if I ground a speaker output pin, which is why it's leading me to believe that since they are surrounded by ground planes that it could be triggering the short circuit protection in the amplifier. We are considering adjusting the ground planes so they are not immediately next/close to the speaker pins and give it a bit of breathing room.

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    Another question. We have all the recommended caps close to the pins as described in the documentation. When powering the system direct with 1 cable, either from a 12v source which drops it down to 5v, or 5v directly, the cut out happens as described. However when adding an additional cable between the power cable and to the board with the lm49450, then the audio never cuts out from the speakers. 

  • 0 in reply to John Bae
    TI__Guru** 102045 points

    Hi John,

    Can you share a schematic to better understand how the power is connected?
    Perhaps you have a ferrite or choke with limited current spec before reaching the amplifier, and it helps when you add a direct connection to the supply bypassing those components.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to John Bae
    TI__Guru** 102045 points

    Hi John,

    Can you specify where in the schematic the cables you mentioned are connected to?

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    On the 5V. There are some pads to connect a cable at DTR / RX / TX / 5V / GND / GND

    I am thinking of adding 100nf caps next to the three 5v pins on the lm49450 (which is located on the far left in the image). I think something may be triggering the circuit protection.

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    Ivan,

    A few changes were made, 100nf caps were added close to the three 5V pins on the lm4950 (pin 16 / 18 / 24) and removed the copper pour that is close to the speaker pins.

    Do you think the C8 capacitor for pin 10 and 11 (DVDD and IODVDD) should be changed to two 1.1uf caps as per the datasheet or the one 2.2uf is ok? We have seen setups similar to this and they work with no issue. THe same with the BYPASS, 

  • 0 in reply to John Bae
    TI__Guru** 102045 points

    Hi John,

    I'll review this and get back to you by the end of the day tomorrow.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    I believe you are correct about the choke effect of the cabling. Using longer cabling makes the system run longer before cutting out, which we have been able to test repeatedly. The extra decoupling caps should do the trick. Will test it out soon.

  • 0 in reply to John Bae
    TI__Guru** 102045 points

    Hi John,

    On a quick glance I noticed that the input barrel jack connector does not seem to have any decoupling caps, you may want to add some bulky capacitors there as well.

    Thanks for the updates let us know how the new test goes.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 30 points

    Thanks, will do.