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LM4673 failing

Calvin Diller
Prodigy 20 points
Other Parts Discussed in Thread: LM4673

This design uses an LM4673 with 47uH series inductors on each output pin and 0.1uF to ground on the load side of each inductor for EMI filtering.  PVDD, AVDD and SHDN are wired together to 3.3V and bypassed to ground with 10uF X7R.  Both inputs are AC coupled and the source impedance driving each is about 7.5K.  The output circuit has three possible load configurations: Open, 500 ohm and 4 ohm speaker. 

This circuit is implemented now in two prototypes and so far we have experienced four failures.  The failures are all the same.  Both output lines stop switching and appear to float at around 300mV.  The supply current to the part is constant at about 30mA.

I have verified that there is no overshoot on the 3.3V supply during power-up.  The circuit is operating with enough gain to result in clipping with some signals.

I have not yet established any pattern for when the failures occur.  In some cases the parts failed almost immediately on power-up and in others they operated as expected for weeks.

Does anyone have any insight around why this is occurring?  My best guess is that the output filters are causing the output clamp diodes to carry more current during commutation than with a direct speaker connection, especially when there is no load.  Waiting on more parts to test this theory by installing external schottky clamps.

  • 0
    TI__Mastermind 20085 points
    Hi Calvin

    Welcome to E2E, thanks for your interest in our products.
    You could try to use the device without output filter, that would be a way to check if the problem is the output filter.
    I think, you could have reason regarding to the output filter.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team
  • 0 in reply to José Luis
    Prodigy 20 points

    With some more experimentation I found that the parts are not actually failing.  What appears to be happening is that they try to start but after one switching cycle they give up.  I am now using two stages of 100uH/10uF filtering on these outputs for EMI filtering (this is the audio channel of an EMI receiver with coverage below 300KHz).  Since the LM4673 needs to charge the 20uF of capacitance on each side up to 1.65V during startup, there can be quite a large current spike and this appears to be resulting in shutdown of the part.  With this configuration the parts will start up and run properly less than 10% of the time.  If the outputs are pre-biased with 10K resistors toward the mid-point voltage before power is applied to the LM4673 then the problem goes away.

    I found a solution to this problem by separating the SHDN pin from the supply pins and driving it with an enable signal that is delayed one second from the time 3.3V power is first applied.  With this connection the parts start up reliably every time.  It is not clear to me why this works.  The only difference between working and non-working configurations is the delayed SHDN signal.  But this solution is practical for me so the issue is resolved.

  • 0 in reply to Calvin Diller
    Expert 1930 points
    I'm seeing a similar issue during power up with my design. I have SHDN tied directly to Vdd. Sometimes when I power on the device the amp starts and other times it doesn't. This is extremely frustrating behavior as NO WHERE IN THE DATASHEET is it mentioned that SHDN needs to come up after VDD. That said, this could be some other fault which is causing the amp to turn itself off... none of which (other than short circuit and thermal) are documented in the datasheet.

    Whats the deal with this amp TI? Does the SHDN pin need a filter so it comes up after VDD? Is there some other fault that could be causing these amps to turn off?