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tas5631 current limit?

geert de vries
Intellectual 580 points
Other Parts Discussed in Thread: TAS5631, TAS5630, TAS5162

Hi,

 

Currently we are evaluating a new design incorporating the TAS5631.

We use the DKD version, 2 x BTL at 48V. AD modulation scheme

At 3.5 Ohms load, some 2dB before clipping, spikes apear on the output signal.

This starts at the negative side first. It seems the output current limit kicking in, altough the

current is below the specified 18-19A. We made shure the output filters (MPP core) are

not into saturation and the inductance value is larger than 10uH at this current.

Maybe it is possible to tweak the oc trip resistor, however there is no documentation about this.

Anybody any suggestions or identical experience?

 

kind regards

Geert

  • 0
    TI__Guru**** 189399 points

    Geert,

    Can you attach a screen shot from your oscilloscope?

    -d2

  • 0 in reply to Don Dapkus
    Intellectual 580 points

    No problem, however  this will take a few days since new boards are populated at this moment.

    The proto type boards are at the EMC lab at for pre-compliance test this week. Concearning the current limit spikes, we noticed

    that we can get rid of these by reducing the OC resistor slightly, to 20K or 18K.

    Can it be that ground noise coupled into this oc reference circuit causes this effect?

    The load current was measured with a current clamp and showed no strange things .

    The new boards have an extra layer with groundplane, so I hope that it will cure the problem. I will

    post the scope shots when I have these.

  • 0 in reply to geert de vries
    Intellectual 580 points

    and.... this is what it looks like.

     

    meanwhile I found out that reducing modulation limit works also, however the gain is slightly reduced as well. How doues this work?

    In the TAS5630 topics I found something about grounding the heat-pad. Is this nescessary? now we use a silicon pad which transfers the heat ok but isolates as well.....

     

     

  • 0 in reply to geert de vries
    TI__Mastermind 18510 points

    Geert, grounding the heatsink is vital.  The heat slug conducts substrate currents which must be returned to ground.  The bottom surface of our heatsink is machined after anodizing to provide a good electrical as well as thermal contact, and we use thermal compound rather than a silicon pad to complete this contact.  Lack of good heatsink grounding may be the cause of the problem.

    In general, it is very important to follow the rules about PCB layout shown in the data sheet and the EVM PCB layout.  If your grounding is not as good as that this could cause the problems you are seeing.  My advice is simply to copy the EVM PCB layout as closely as possible.  This also could be the difference between the EVM and your layout .

    If this does not solve the problem I think we will need to see copies of your schematic and PCB layout, in PDF form.  But please do not spend your time on that until you have been able to check the first 2 items I mentioned above.

    Regards,

    Steve.

  • 0 in reply to SteveCrump
    TI__Mastermind 18510 points

    Geert, please note one other thing.  TAS5631 data sheet SLES221C, page 4, Recommended Operating Conditions, specifies minimum load impedance of 3.5ohms for BTL operation.  This is not a load rating, it is true impedance.  Generally, a 3.5ohm speaker load will have regions in which its impedance falls below 3.5ohms.  These would violate the minimum impedance specification and could activate TAS5631 overcurrent protection.

    Regards,

    Steve.

  • 0 in reply to SteveCrump
    Intellectual 580 points

    Hi Steve,

     

    Thanks for the reply.

    From the datasheet it was not very clear to me that the slug should be grounded. In our case, the heatsink is attached to an other part of the enclosure, offering no direct HF path back to the board. However we can use o.1mm grounded copper foil between heatsink and pad. I tried it to see if the current limit trip point would shift up, this was not the case. We will re-evaluate THD at higher levels  with grounded pad as well, probably this will show better results.

    Concearning the minimum impedance, you are right. We only looked at the max current the op fets could deliver, which leads to a theoretical lower impedance.

    This figure is of course derated by the hysteresis switching current and if drain-source diodes start to conduct near clipping, this current has to be taken as well by the

    opposite fets. We noticed that lowering the modulation index a bit improves the situation at low impedances which could be an indication of this..

    So at the end we are not doing too bad if we can drive 3.3 Ohms from 48V telecom supply.

    I must say the the previous design, bases on the TAS5162 in parallel was not as critical as this chip, and delivered breath taking specs right from the start. Due to the higher

    idle power dissipation we had to abandon this path...

     

    Thans for the forum,it is a great help

     

    Geert

  • 0 in reply to geert de vries
    TI__Mastermind 18510 points

    Geert, I hope you meant to type 3.5ohms, not 3.3ohms.  The second figure is outside our spec.  Please be careful about this.

    Also, I think when you say "" you mean current limit.  But current limit is not precise, and it must be at least a little above the maximum audio currrent.  So it is best to use the output power and impedance numbers from the data sheet.

    Regards,

    Steve.