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LM49450: power on/off sequence pairing with audio amp

Roy Chou1
Genius 14255 points
Part Number: LM49450
Other Parts Discussed in Thread: TPA3110D2

Hi Team,

My customer used LM49450+TPA3110D2 in their application. 

We found there was noise output the speaker when system turning on and turning off. The voice like bobobobobo....

I want to know if there is recommended power on/off sequence with codec+amp. Thanks.

Roy

  • 0
    TI__Guru** 102045 points

    Hi Roy,

    Is there any schematic and layout you can share? Have they tried to isolate each stage by disconnecting LM49450 output from TPA3110D2 input?

    Best regards.
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    TI__Genius 14255 points

    Hi Ivan,

    Please see below schematic. Can you let me know how to do below action? Do you mean disconnect between LM49450 and TPA3110D2 first to see the noise source is from TPA3110D2 or LM49450?

    Ivan Salazar said:
    Have they tried to isolate each stage by disconnecting LM49450 output from TPA3110D2 input?

    Roy

  • 0 in reply to Roy Chou1
    TI__Guru** 102045 points

    Hi Roy,

    Yes, disconnect between the two amplifiers to check where is the noise coming from.

    From the schematic, I see Class-D speaker outputs LLS+/LLS- and RLS+/RLS- connected to LINP/LINN and RINP/RINN respectively. This could actually be causing the issue:

    • Class-D outputs from LM49450 are switching signals, not analog signals.
    • The connection from LM49450 outputs to TPA3110D2 inputs would be correct if signals is analog, but in this case a Low-Pass Filter is required.
    • You can refer to this diagram for the LPF connection between the amplifiers:

    Best regards.
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    TI__Genius 14255 points

    Hi Ivan,

    1. Is there any method that our LM49450 can support more than 32-step volume control? 

    2. Is it possible to let LM49450 gain setting = 0dB? Because LM49450 has 2.5W AMP function, and as you know, we place TPA3112D2 as 2nd stage. Much more gain may cause signal defective.

    Regards,

    Roy

  • 0 in reply to Roy Chou1
    TI__Guru** 102045 points

    Hi Roy,

    1. 32-step is how the device is designed, I don't think there is a way to change this on the device itself.
    2. From Table 16 and 17, there is a setting for 0dB both on headphone and loudspeaker. There are also negative gain settings in case the signal needs to be attenuated.

    Best regards,
    -Ivan Salazar
    Applications Engineer