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TAS5760MD short circuit

toshi-e2e
Genius 4705 points
Other Parts Discussed in Thread: TAS5760MD

Hi, my customer has questions in case of connecting SPK_FAULT pin and SPK_SD pin of TAS5760MD as below.

Case 1.  When the below cases happened,   SPK_SD was toggled automatically.
      Is it a nomal operation? Why is SPK_FAULT not latched?

     - SPK_OUTB- and SPK_OUTB+ are shorted.
     - SPK_OUTB+ and SPK_OUTA+ are shorted.
     - SPK_OUTB- or  SPK_OUTB+ is shorted to GND.



Case 2.  When the below cases happened, SPK_SD was not set to low, and TAS5760MD was operating
     continuously uner the condition that an input frequency and level of both channels were the same.  
    - SPK_OUTB+ and SPK_OUTA- are shorted.
    - SPK_OUTA+ and SPK_OUTB- are shorted.
  
     But when an input frequency or input level between channels was different, Case 1 was occurred.
     Why does not TAS5760MD detect short condition?

There is an additional question about case1.

　  When SPK_OUTB- and SPK_OUTB+ are shorted  and SPK_SD is returned from low to high, 
 　 PVDD/AVDD current flows for a moment.
 　 But when SPK_OUT is shorted to GND or PVDD/AVDD, PVDD/AVDD current does not flow.
　 Why such differences occur?

Regards,
Toshi

    

  • 0
    TI__Mastermind 34665 points

    Hello Toshi-san,

    Case 1 is normal, since by connecting /SPK_FAULT to /SPK_SD when a fault occurred as in case 1 the /SPK_SD is pulled low by /SPK_FAULT and once the /TSPK_FAULT has passed the /SPK_FAULT goes high again pulling /SPK_SD high until another fault occurs. It repeats until the fault condition is removed. Connecting /SPK_FAULT to /SPK_SD creates an automatic retry circuit. See page 33, table 1 of the TAS5760MD datasheet Over-Current Error and page 32 "Speaker Amplifier Protection Suite."

    In case 2 as in case 1, TAS5760MD does't have short ciruit detection. It has Over-Current protection circuit that can detect current that are potentially damaging to the output FETs and shutdown the Class D to protect the output FETs. Above certain power levels shorts can draw sufficient current to trigger the OCE. If the current drawn by the short isn't high enough to trigger the OCE as in case 2, the TAS5760MD isn't going to shutdown due OCE as seen in case 1.

    The best way to understand the difference between case 1 and case 2 is to monitor the current between the output pin and the inductor. It should show the difference in the current levels. In case 2 if you increase the input level (output power), it will reach to a level where the current is high enoug to trip the OCE.

    The follow up question on case 1 PVDD/AVDD supply current, I'm not sure I fully understand your question. Can you provide scope captures. I will try to answer it and you can correct me if I interpreted the question incorrectly. In case of OUTB+ to OUTB- the device probably starts switching for a some time before the OCE occurs, where as with OUT to GND OCE occurs immediately upon /SPK_SD going high. This may explain the behavior of PVDD/AVDD current observed.

    With OUT shorted to GND the FET sees ~1/2 impedance than OUT+ shorted to OUT-.

    OUT to GND impedance: FET Rdson + R inductor

    OUT+ to OUT- impedance: 2x FET Rdson + 2x R inductor