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LM4670: Audio Power Amplifier Layout clarifications

HARI T O1
Expert 1385 points
Part Number: LM4670
Dear Team,

I am using LM4670SD/NOPB in my design.The IC is working at 5V Supply.

This IC requires an 8 Ohm speaker.

My first question is,

  1. In that case do the lines Vo1 and Vo2 needs a differential impedance of 8 Ohm(Each line 4 Ohm Single ended).

May I know my assuption is correct or not.

My second question about the current carrying capacity of Vo1 and Vo2 lines.In the datasheet

page no 4 you can see the power specification as given below.

Considering the maximum power as 3W from the 5V rail,I calculated the current carrying capacity of Vo1 and Vo2 as 0.6A.

May I know is this correct?

Do we need to provide guarding for IN+ and IN-

Regards

HARI

  • 0
    TI__Guru** 102355 points

    Hi HARI,

    No need to route with specific impedance for the output traces. Regarding the current capacity, 0.6A should be good enough for this amplifier output power.
    By guarding do you mean adding diodes like D113 and D127? These should not be needed at the input as long as there's no external contact to these nets.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Expert 1385 points

    Hi Ivan,

    Thank you very much.

    By guarding I mean adding guard traces around IN+ and IN-

    Regards

    HARI

  • 0 in reply to HARI T O1
    TI__Guru** 102355 points

    Hi HARI,

    Thanks for the additional information. Being a differential signal, the guarding may not be required. Are these long traces and crossing over many digital signals? As long as you have a good GND across the analog signal path you should be OK.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Expert 1385 points

    Hi Ivan,

    Thank you very much.

    Do we need to take care anything about differential impedance.

    Audio signals are not high speed.

    May I know just length matching is sufficient or not or Is there any differential impedance requirement (like 100 Ohms) that needs to be followed in the PCB layout while routing the audio inputs to LM4670

    Regards

    HARI

  • 0 in reply to HARI T O1
    TI__Guru** 102355 points

    Hi HARI,

    No need to care for differential impedance. Length matching is enough for audio signal traces.
    As you mentioned, audio is not high speed so most of the PCB trace constraints are not as critical.

    Best regards,
    -Ivan Salazar
    Applications Engineer