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Muting problems on TPA3121D2

Vic Elias
Prodigy 110 points

Other Parts Discussed in Thread: TPA3121D2

I am seeing a strange problem with the Mute signal on a TPA3121D2.

We are using it in an application for a kenwood radio to mute beeps from radio control heads.

It seems like it doesn't mute the output immediately.   It's hard to tell when it kicks in, because I only have short 50 ms beeps that are coming through it, but in any case it does not mute the first 50 mS beep after the mute line goes high.   I am driving the mute pin from a gpio pin on a PIC microcontroller running at 3.3 volts, so I think I am within VOH of the mute signal.  The incoming audio beep comes in 7.6 mS after the mute goes high.  Note this application is only mono and uses the LOUT output only.

Is there some delay necessary?  What is meant by when muting,  the "outputs run at a 50 percent duty cycle"?  shouldn't they just be grounded when the mute goes high?  Is there a problem since we are only using the LOUT output instead of both ROUT & LOUT?

Should we be using the -SD shutdown input instead of mute?

Thanks,

Vic Elias

  • 0
    Prodigy 110 points

    Ok, now I see the problem, buried under the AC characteristics there is a 'mute delay'.  Why wouldn't this important fact be put in the Mute Pin description in the data sheet?  Why is there a mute delay?  why does it need 'typical 120 mS'  to shut off the output??  I guess you wouldn't notice it on a "DLP TV" as referred to in the applications on the datasheet because nobody cares about 120 mS in a TV application and nobody buys DLP TV's anymore anyway (TI & nascar failure).

    This is great after someone designed this part in to our product and we have production boards coming in.

    If I use the -SD shutdown pin instead will it provide fast muting?  Will I hear pops or clicks?   Is there a "shut down delay" or "power up delay" buried somewhere in the data sheet that I have missed? 

     

     

  • 0 in reply to Vic Elias
    TI__Intellectual 1050 points

    Hello Vic,

    You are right that the mute has a delay. Shutdown will have a longer delay. If you do a quick Ctrl+F for "shutdown", under the section "SHUTDOWN OPERATION" you will see that it both mutes and brings the amplifier to a low-current state. A search for "shutdown" would show that in the section just preceding, "VBYP Capacitor Selection", that the value of C_BYP relates to time taken for amplifier to start. The EVM value for C_BYP is chosen for best pop performance. The purpose of SD is to allow a low-power state.

    When mute brings the outputs to 50% duty cycle, that is normal under the AD mode "Class-D Modulation Scheme" (can also be found with Ctrl+F for "duty cycle"). 50% duty cycle is the default for AD-mode PWM outputs, and carries no audio signal. After filtering, the low current results in average of 0 V under differential / bridge-tied load (BTL) config. (At switching frequency, the instantaneous amount will be miniscule and out of audio band.)  Since you are using single-ended (SE) (LOUT and ground), the average output current and voltage after the output filter will not be 0 V. That is the reason for the "Single-Ended Output Capacitor". Thus if you choose to do single-ended you will need this cap and trade-off lower cost / simpler config for less noise rejection and attenuate your lower frequency content. These are all explained in the data sheet in these two sections.
     
    Best regards,
    Arnold Zhang