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LM4951: Use case of Single ended

Masayoshi Takahashi
Intellectual 1530 points
Part Number: LM4951


Hi team,

my customer would like to use LM4951. Then I got some question.

Could you give your advice?

1. The customer would like to use single-ended. To use for single-ended, the customer should add coupling capacitor. Is this correct?

2. If yes, could you tell me the recommended capacitor value and schematic?

3. The customer plan to use valuable resistor on input as below picture. Would it be ok to use? If there is any issue for gain of amp, please let me know.

Regards,

Yoshi

  • 0
    TI__Guru* 84915 points

    Takahashi-san,

    Please take a look at my responses below and let me know if you have additional questions or comments.

    1. The customer would like to use single-ended. To use for single-ended, the customer should add coupling capacitor. Is this correct?

    Yes, this is correct. The coupling capacitor is required since there's an internal bias.

    2. If yes, could you tell me the recommended capacitor value and schematic?

    The recommended capacitor value is 0.39uF as shown in figure 1 of datasheet:

    https://www.ti.com/lit/ds/symlink/lm4951.pdf#page=2

    3. The customer plan to use valuable resistor on input as below picture. Would it be ok to use? If there is any issue for gain of amp, please let me know.

    This resistor value should be considered also as part of the Ri resistors array, so the gain may be modified. Take into account the equivalent resistor in order to calculate the gain.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    TI__Intellectual 1530 points

    Hi Luis,

    Thanks for your answer. I'd like to ask more question about below.

    Usual single-end is like below.

    1. Does LM4951 need output capacitor as 220uF same as above? I'm confusing figure 1 is BTL mode. Do you have schematic example of single-end?

    2. Regarding gain, I understand the valuable resistor change the gain. Could you tell me how to calculate the gain in use case of single end? Is this same as AVD＝2(Rf/Rj)?

    Regards,

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Intellectual 1530 points

    Hi Luis,

    Can I get any reply?

    Can't LM4951 use for single-end output?

    Regards,

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Guru* 84915 points

    Hi, Yoshi,

    1.- When a BTL configuration is used in single-ended mode, a big capacitor is required. The 220uF capacitor will be needed in this design as you correctly showed in your picture above.

    2.- The single-ended gain will be the half of the value that you showed Avd = Rf / Ri. As the datasheet states, a BTL output doubles the voltage swing across the load. The output power is then four times compared to a single-ended amplifier with the same conditions.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    TI__Intellectual 1530 points

    Hi Luis,

    Thanks for your answer. I almost understand your suggestion.

    Regarding 1, The customer would like to know the example of schematic. Could you show the example? 

    I also would like to know the recommended output capacitor.

    Regarding 2, I understand the gain will be half when using single-ended. If they are using valuable resistor in input as I already showed, the equation is Avd = Rf / (Ri + Rv) / 2. Is this correct?

    Regards.

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Guru* 84915 points

    Hi, Yoshi,

    Please take a look at the suggested connections for a single-ended configuration.

    Regarding the gain formula for single-ended config, I think it should be Avd = Rf / (Ri + Rv). The gain for a BTL configuration is AVD = 2(Rf / Ri) as mentioned in the datasheet.

    http://www.ti.com/lit/ds/snas453c/snas453c.pdf#page=12

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    TI__Intellectual 1530 points

    Hi Luis,

    Thanks for sending the schematic example.

    I'd like to confirm the recommended capacitor as C3 and C4.

    In you previous answer, recommended capacitor value is 0.39uF. I thinks this is for C4. Is this correct?

    Also, 22uF is recommended value for output capacitor?

    Regards,

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Intellectual 1530 points

    Hi Luis,

    I confused in your schematic example because NC is connected. So could you check whether below schematic is correct or not?

    The customer plan to use gain = 20. So they need to change the resistor as Ri = 1k ohm and Rf = 20k ohm . Is this correct?

    I also would like you to answer the previous question about recommended capacitor.

    Regards,

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Guru* 84915 points

    Hi, Yoshi,

    Sorry for my mistake on the schematic, the NC pin must be floated as you correctly showed in your last diagram. The connection should be made to the CCHG pin instead of NC pin.

    For a gain = 20, you are correct, you will need of Rf = 20k and Ri = 1k.

    The rest of you schematic looks correct.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    TI__Intellectual 1530 points

    Hi Luis,

    Thanks for checking the schematic. 

    BTW, I'd like to ask about shutdown signal behavior more. Could you check below question below?

    1. How long does the customer need to set the duration of shutdown time? Is the device work when the customer apply shutdown signal for 1ms?

    2. After shutdown signal, the device will wake up after 35ms. The device will not output the signal even though input the signal. Is this correct?

    Regards,

    Yoshi

  • 0 in reply to Masayoshi Takahashi
    TI__Guru* 84915 points

    Hi, Yoshi,

    1.- Due to the Cb capacitor, the LM4951 needs to wait for 10ms in order to ensure that the shutdown process has been finished correctly. During this time, the device could be still working.

    2.- Typically, the device requires of 25ms to complete the wake-up time. The device will not output a signal before this time.

    Best regards,
    Luis Fernando Rodríguez S.