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LM4890: The problem of supply voltage and start-up time

Shaw Wong
Intellectual 590 points
Part Number: LM4890

Hi,

Our customer use LM4890 to drive a loudspeaker as following schematic, VDD is 3.3V from LDO which can provide 250mA current. But they encounter some problems.

1. Does LM4890 work when SHUTDOWN pin is high or low? In datasheet, it says the chip works when SHUTDOWN pin is high. However, out customer tell us the chip works when SHUTDOWN pin is low.

2. When a 3.3V level PWM signal about 8 KHz input -IN pin, the loudspeaker can make a sound, but the VDD voltage is pulled down from 3.3V to 2.2V. The loudspeaker is 400mW level, 8 ohm impedance and the drive current is about 100mA. In datasheet, we can find that when VDD is 3.3V, the output power can reach 400mW.

3. When input capacitance is added on the left of R14, and power on, the chip works after about 200ms. When input capacitance is removed, the chip works after about 100ms. And the VDD voltage are both pulled down in the two condition.

Please help to analyze these problems, thanks!

  • 0
    TI__Mastermind 20085 points
    Hello Shaw,

    1) This is so weird. The shutdown mode should work how is explained in the datasheet.
    Sorry, but I can't understand which are the questions in the following points. Could you be more specific?

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer
  • 0 in reply to José Luis
    Intellectual 590 points
    Hi José,

    Thanks for your reply. I think it is weird too, but our customer said that when SHUTDOWN pin is low, the chip can work only, that is opposite to datasheet. And the following question is that when the chip drives a loudspeaker, the supply voltage VDD is pulled down from 3.3V to 2.2V, which should be abnormal, and we think that the chip can not provide enough output power, so the supply voltage is pulled down. So we want to know the reason why the supply voltage is pulled down. Thanks!
  • 0 in reply to Shaw Wong
    TI__Mastermind 20085 points
    Shaw,

    If the device can't supply enough output power, so you should verify if the source supplies enough current to the system in order to avoid this kind of problem. You can prove with another power supply in order to verify it.
    Therefore, Are you using an LC output filter? If so, an inductor wrong election could produce this problem too.

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer