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TAS5630 Amplifier Runs Very Hot and Shuts Down

Ray Ward
Prodigy 120 points
Other Parts Discussed in Thread: TAS5630

We are using the TAS5630 Amplifier in a BTL configuration to drive one 100 Ohm load at 25 VRMS with a 42 Vdc power supply.  The other BTL output is not connected to a load.

According the Class D LP Filter Design formulas, the output inductors should be in the range of 233uH for a 100 Ohm load.  The filter caps are currently 0.033uF metal film 100 Vdc parts.

The amplifier seems to drive the load with these components installed with very little distortion at 25 VRMS as desired, but the TAS5630 Chip runs very hot and will eventually hit the 125 degree shut down point.  With the original inductors and caps, (15uH and 0.68uF) the TAS5630 IC driving the the same load at the same 25VRMS voltage, the temp rise is almost all the result of the normal operating power in the TAS5630 IC.  (IC temp settles around 57 degrees C.)  When the 100 ohm load driven at 25 VRMS is attached with the original components, the IC temp only increases another 10 degrees. (67 degrees C).  All temperatures are measured on the top thermal pad of the TAS5630 IC with no heat sink. 

The Coilcraft MSS1260 250uH inductors have a rated current of 1.0 Adc and a saturation current of 1.5 Adc. 

What is causing the IC to heat up so much when I change the inductor values from 15uH to 220uH and the caps from 0.68uF to 0.033uF??

FYI,  the installed filter components are to big to fit in our current design space.  We only need to drive 25VRMS into an 8 Watt load.

Thank You

 

 

 

 

 

  • 0
    Expert 1240 points

    The nominal oscillator frequency of the TAS5630 is about 4 MHz, and it looks like the Coilcraft MSS1260 220uH has a self resonant frequency of 5.5 MHz.  This would indicate that the inductor is no longer behaving like an inductor at the switching frequencies of the chip.  Have you tried configuring the device for a lower oscillator frequency (from datasheet: System Design Considerations -> Oscillator), and checking to see if this has any impact.  A general rule of thumb asks for inductors with self resonant frequencies at least 2 times greater than the greatest frequency content which is to be filtered, and the larger the margin the better.

    Sparkchaser

  • 0 in reply to sparkchaser
    TI__Guru**** 189399 points

    Sparkchaser,

    I think you're on the right track, I suspect a dip in the frequency response of his filter at the switching frequency, which is 400 kHz nominal (see page 4 of the d/s). The 4 MHz is the internal clock which is divided by 10 to drive the output stage.

    Ray, if you SPICE the response of your filter, I suspect you'll find there is a dip in the freq around 400 kHz which is causing your problems as Sparkchaser correctly identified.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 120 points

    Thank You,

    I dropped the PWM frequency from 400 Khz to 150Khz.  Doing this dropped the device current and the operating temperature without a heat sink to 60 degrees C at the thermal tab on top of the IC.  Loaded to 40 Ohms at 25 VRMS the temperature of the device only increases 4 Degrees C to 64 Degrees C.

    What is the acceptable PWM operating frequency range for the TAS5630 IC?

    I have two different TI Design guides showing me how to calculate the LC filter values.  Both come up with the same values,  L load = 233uH and C load 0.073uF for a 40 ohm resistive load.  Are these correct?

    Thank You

     

     

     

     

     

  • 0 in reply to Ray Ward
    TI__Guru**** 189399 points

    Hi, Ray,

    In another thread, it was mentioned that the part was not designed to operate at 150 kHz switching frequency.

    As for your LC filter, those values sound correct, but we have never tested such a high impedance load.

    -d2