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TDA4791LP - no output - SOLVED

Question asked by andrew on Jun 12, 2016
Latest reply on Apr 28, 2017 by Rowe.Martin
I've got a TDA7491LP design that I cannot seem to make function. I've followed the "
Applications Circuit for class D amplifier" example circuit (Figure 26 on the datasheet). I'm powering the amplifier with a 10V switchmode supply.

Some measurements from the board, all referenced to the circuit common (where SGND and PGND are connected):
  • MUTE (pin 21) at 3.5V
  • STANDBY (pin 20) at 3.5V
  • VDDPW (pin 19) and VDDS (pin 26) are both reading 3.2V.
  • PVCC is supplied with 10V
  • SVCC (pin 35) at ~10V
  • VSS (pin 36) reads ~6.5V
  • VREF (pin 34) at ~1.8V
  • SYNCLK output (pin 25) reading ~4V square wave of ~630kHz
I've got the inputs configured in single-ended mode; INNA and INNB are each conected through individual 470nF capacitors to my circuit common. I'm supplying a 1Vp-p 1kHz sine wave on INNA.  I should be seeing a PWM waveform at OUTPA/OUTPN but both these pin sets are reading about 200mV; there is no PWM activity at all.

Now I'm a little confused; SVCC is called "signal power supply" and VSS is called "3.3V regulator output referred to power supply" -- Should I be referring MUTE and STBY to VSS's voltage? 

Updated November 7, 2016

I wanted to update the thread and let anyone searching in the future that I have resolved the issue with ST's help (your FAEs are the best!) My issue was actually resolved months ago but I'd forgotten to come back here to update the thread.

I compared my board against the TDA7491LP eval board provided by ST and everything checked out; I then replaced my output filter section with the section from the eval board and as soon as I did this the fault condition went away. I guess my choice of LC filter was very poor and causing the output to appear as a short, which caused the TDA7491LP to protect itself.

I'm currently running the output section the same as Figure 34 in the datasheet; that is, I'm using a small ferrite (BLM31PG601) instead of an output inductor, using 1nF as the output filter capacitor, and using 2.2nF capacitors from each leg of the output to ground.