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LM4951A: Amplifier switching off when driving 4-ohm speaker

Dzenan Omanovic
Prodigy 30 points
Part Number: LM4951A

Hi, I'm using LM4951A with a 4-ohm speaker as advised by TI support.

However, I note that at high volume setting, the amplifier is shutting down for hundreds of miliseconds. 

This seems to be a current limiting rather than a thermal limiting factor as happens immediately.

I can't see in the datasheet if there is a current limit and to what value it's set to. 

Perhaps the simple answer is that the part is not suitable for 4-ohm despite previous info.

Graph below shows the following signals.

Ch1: Vout- (pin6)

Ch2: Bypass (pin1)

Ch3: VDD (pin9)

it's noticeable that Vout- is turned off by the amplifier.

Many thx for any replies.

  • 0
    Prodigy 80 points
    Hi,

    The datasheet says that the minimum load is 8 ohm, so it could be possible that your speaker pulls to much current.

    What VDD voltage do you use? Depending on the voltage are you able to calculate the max current according to the max power output. Looking at your scoop I assume it is 7.5 volts. With that VDD the amp is able to source 1.8 Watt typically.

    The thing that caught my eye is that CH3 (that you say is VDD) increases when the signal (CH1) has a high amplitude. I would expect that the signal clips a couple of mV below your rail voltage/VDD
  • 0 in reply to Tom Salm
    Prodigy 30 points
    Hi Tom,
    I used around 7.5V. The application uses 6-8.4V.
    I think Ch3 increase is probably down to noise picked up by probes due to grounding connection made with long wires.
    Is there an over current protection in this device and if there is, do you know what it's set to internally?
    thx again
  • 0 in reply to Dzenan Omanovic
    TI__Mastermind 20085 points

    Hi Dzenan,

    It seems that the device is exceeded the power dissipation. You can refer the Power Dissipation section in the amplifier's datasheet to calculate the maximum power dissipation that the device can endure.
    In fact, the device has short-circuit protection. You can refer the Thermal Shutdown and Short Circuit Protection section in the amplifier's datasheet to find this information.

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer

  • 0 in reply to José Luis
    Prodigy 30 points

    Hi Jose Luis and thx for your feedback,

    Having re-run the test at lower voltage, I agree with you that this is most likely to be down to thermals rather than short circuit. 

    The datasheet states 73C/W thermal impedance from junction to ambient with 1in2 copper. It would help if there is data for thermal impedance of junction to case.

    Also it would help if there is data on the lowest thermal impedance that could be achieved with this package (for example with a 2-3x the ground copper area to the one stated in the datasheet). Let me know please.

    Thx again,

    Dzenan

  • 0 in reply to Dzenan Omanovic
    TI__Mastermind 20085 points
    Hi Dzenan,

    Unfortunately, this information is not available and it is quite difficult to get it as this device doesn't have EVM in order to make tests.
    It is always recommended to use a copper plane of at least 1in2. You can use the formulas in order to avoid exceeding the maximum power dissipation.

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer