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TAS5731M achieving maximum specified O/P powers

Chris Woolrych
Prodigy 160 points
Other Parts Discussed in Thread: TAS5731M, TAS5731

Hi

I’ve been evaluating the TAS5731M class D amp for possible use in a new product and don’t seem to be able to achieve the specified power o/p into 4/8 ohms. 

Please see outline of issues I’m encountering below:

I am currently evaluating the TAS5731M (higher power version of the TAS5731) using the TAS5731 dev board.

In 2.0 BTL mode when checking the maximum power at 1k into various loads I found that I could only achieve 30Wx2 into 8R 10% THD (60W tot) with max PSU voltage of 21V and 20Wx2 into 4R approx 0.4% THD (40W tot) with a max PSU voltage of 20V. Beyond these powers the device shut down immediately. Note that these were pretty much instantaneous tests only switching on for less than 10 seconds to minimize heating effect. 

The graph on p18 of the data sheet shows that should be able to achieve 35W into 8R (x2?) with 24V PSU and 40W into 4R (x2?)with 20V PSU in BTL 2.0 mode with the 2 layer Evaluation PCB. 

I think that over current/over temp flag is being set by the amplifier but cannot check to confirm because over current /over temp flag in the error status x02 register is not shown in the I2C output in the graphical GUI (is there a way to show this?). Also thinking the heat sinking has to be completely optimal to achieve high O/P powers.

Also note the following additional comments on setup/configuration:

All filters set to flat freq response. DRC off.
Have fitted large aluminium block completely flush to bottom of eval board with heat sink compound for max heat dissipation.

Please can you assist with understanding this issue. 

Also would like confirmation that the o/p power graph for 2.0 BTL mode (p18)is for both channels driven as this is not explicitly mentioned. 

Finally could it be possible that the default over current setting trip for the original TAS5731 is still set for the TAS5731M somehow?

The TAS5731M is attractive since it has the integrated DSP and also allows us to independently EQ channels individually ( ie not ganged) for the speaker product that we’re developing.

  • 0
    TI__Guru**** 189399 points

    Hi, Chris,

    Welcome to e2e, and thanks for your interest in our products.

    It sounds like you are suffering from an over-current shutdown issue. Is the impedance of your loads constant across frequency? Do you have excess capacitance in the output somewhere? Do you have a stiff power supply?

    The graph on page 18 is for stereo operation, but this is electrical limitations, not thermal. The EVM will not be able to duplicate that plot due to thermal limitations.

    You can read the register for over-current by going to the I2C tab in the GUI.

    -d2

  • 0 in reply to Don Dapkus
    TI__Mastermind 34665 points

    Hello Chris,

    As Don mentioned the plots are stereo (both outputs driven in phase). Your most likely experiencing thermal shutdown, not over current. The plots on page 18 of d/s are done instantaneously (<2s) for quick bursts, not continously or sustained power. The TAS5731M is rated at 30W continuous power into 8ohms. To distinguish between OC and OT shutdown, you can provide a bursted sine wave with 5-ON 20-OFF to limit the heating effect. If the peaks cause the device to shutdown it's most likely OC. If it doesn't shutdown then it most likely a thermal limitation that peak power.

    Let us know if you have any further questions. Please provide feedback on your findings and we will do our best to support you.

  • 0 in reply to Don Dapkus
    Prodigy 160 points

    Hi Don,

    Thanks for the assistance.   I have managed to read the thermal/over current flag and to determine if over current or over temp carefully lowered the temperature of the device to around 0 deg C with freezer spray then applied signal to momentarily drive above the 20Wx 2 into 4 R causing shutdown as originally seen.  Interrogating the flag then showed that it had tripped and by deduction, since device at around 0 deg C would assume that over current detected.

    Also tried burst mode as per your colleagues suggention 5 cycles on 20 off but still cut out. 

    The PSU is a reasonably stiff  bench supply and I added additonal 2200uF caps on L  and R PSUs  close to IC to prevent sag.  Looking with scope can see some class D switching noise but not any major sag on the PSU.

    I also crudely measured the instantaneous current with 0.1 R series resistor in one channel and found that instantaneous current quite high at around 17A with 10W into 4R.  The average current was much lower at around 300mA on one ch PSU.

    Can you confirm what the internal trip is set at and whether it works on instantaneous or average current.

    Finally I wonder if we are actually asking too much of this part.  The headline figures on the first page show 12V/4 ohm/15W in BTL mode.  It does seem to heat up very quickly with 4 R load and I worry that difficult to get the heat out just via the bottom thermal pad even with 4 layer PCB.

    This part is attractive with the combined DSP but wonder if we would be better choosing alternative part.  The TPA3116 looks like it would be up to the job but has analog i/p so not as ideal for us as would prefer to drive from I2S.

    Regards

    Chris