LM4950: Bypass pin

Part Number: LM4950
Other Parts Discussed in Thread: TS5A22364


When BYPASS is high, VOUT is only at 12V and there is no sound output.
When the BYPASS is shorted to GND, sound is output instead. Please help to confirm what is the problem?

When the Bypass pin is pulled low, there is sound output, but the sound is abnormal and noisy

  1. Pin 1:0.2V AC, 2V DC inputBypass pin
  2. Pin 2, 8:5.6V DC
  3. Pin 9:0V AC, 6V DC input

  • Hello, 

    Our team is looking into this thread and will respond as soon as possible. 

    Best Regards, 

    Justin Beigel

  • Hi

    Are you forcing voltage/GND at bypass pin 8? This pin should be connected only to the correspondent decoupling capacitor.

    Bias for the device is set internally, and decoupled through pin 8. -VINB should not be connected to any external signal, and only use -VINA. Make sure to place series AC coupling capacitor and resistor at the input of -VINA, as shown in the data sheet. Please confirm you're using BTL connection at the output, similar to Figure 66 from data sheet.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators

  • Hi lvan

    The attachment is the customer's final schematics as follows
    LM4950 switches through TS5A22364
    Will ARIN R577 R575 10K, R519 R527 24.9K cause abnormal sound?

    New USB Audio (002) (2).pdf

  • Hi Gareth,

    The bypass seems to be pulled to some external voltage. Is there a chance customer can try to connect bypass pin only to decoupling capacitor? Perhaps removing the series resistors.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators