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Click/Pop LM4950

Anand Sethi
Prodigy 115 points

How does it work. The spec sheet says when it stabilizes then outputs are connected. Would this not cause a pop?

I am referencing this from sheet 19 of datasheet.

The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches VDD/2. As soon as
the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are
reconnected to their respective output pins. Although the BYPASS pin current cannot be modified, changing the
size of CBYPASS alters the device's turn-on time.

  • 0
    TI__Intellectual 1240 points

    Hello Anand-san,

    I'll explain about pg19 on datasheet that you are referencing.

    The most of LMxxxx Boomer series amplifier is taking following archtechture. Until the voltage on Bypass pin reach Vdd/2,the amplifier is configurered unity gain amplifier which means internal analog SW close between Output and -Vin pin. This makes unity gai amplifier. After the voltage on Bypass pin reached Vdd/2, then the analog SW open then the amplifier gain is set by external resister ratio. From this operation, during the analog SW close, output pin would not output the signal. After the analog SW was opened, then the output pin will have output signal. This will help to suppress pop and click sound in BTL connection but in SE connection wouldn't help. In SE connection, Pop will causes due to how fast the output DC bias rising to Vdd/2 whic means Bypass pin voltage how fast rising to Vdd/2. So in SE connection, to minimize the pop sound, you need to take large Cbypass cap on Bypass pin to get slower rising.

    I hope this helps for your question.

    Best regards

    T NAOKAWA 

  • 0 in reply to Toyojiro Naokawa
    Prodigy 115 points

    Thanks you Mr. Naokawa for your response. I have a follow-up question to your response. As soon as the switch is opened the gain is set by external resistor components. What is the state of the capacitor at this point. If there is voltage differential between the amplifier output and coupling capacitor would that still not cause a pop? Thank you in advance.

  • 0 in reply to Anand Sethi
    TI__Intellectual 1240 points

    Hello Anand-san,

    Regarding Pop & Click sound, there are couple of reason you will have pop & click sound. If you take BTL connection on load, Pop will cause the voltage differences between Vouta-Voutb at internal SW opened. If -Vin voltage is not followed to Vbypass, +input and -input will have voltage difference. Then the Amp-A try to compensate the voltage different and as the result Vouta goes high to charge Cina via Rina. So it will be depending on input time constant ie Cina*Rina and Vbypass voltage rising time. Cina*Rina<<Vbypass rising time can minimise pop & click. If you take SE connection on load, there is additional pop reson. In SE connection, you will have relativelly large cap on output as DC cut to the load. But Amplifier have to charge this large cap to Vdd/2. In this case the charge current flow into the load. this charge current will cause the pop sound. To minimize this charge current, you need to use large cap on Bypass pin to have slower rising time. I'll recommend to take BTL connection to minimize the pop sound.

    Best regards

    T NAOKAWA

  • 0 in reply to Toyojiro Naokawa
    Prodigy 115 points

    Hello Mr. Naokawa,

                                           Thank you for your reply and time. I think I am understanding this a little bit better now. As soon as I get the board I will scope out the relevant points and then observe how the system reacts.

    Thanks,

    Anand