LM4951: Audio Amp output pin shorted and click pop sound

[Bball4life]

Hi All,

In my design, we use the LM4951 amp and noticed it has some problems.
https://www.ti.com/product/LM4951

After some customer feedback, the LM4951 under unknown condition was shorted. After this, there was a "click-pop" sound during the turn on.

During the debug and repair phase, it was noted that Pin 6 was shorted to GND and Pin 10 was at half the impedance.
Input to the chip was verified and it is correct. The output to the speaker was checked and there were no shorts detected.

Below is the schematic for reference.

Has anyone encountered Pin 10 or Pin 6 being shorted under any conditions?
Any advice will be greatly appreciated.

Thanks,

 

[Easy peasy]

sounds like some one has played with it and buggered it up - is it protected for speaker shorts and shorts to gnd or Vcc ?

 

[KlausST]

Hi,


I've checked your circuit against the datasheet.
You added the capacitor at the feedback ... and the circuit at the output.

Here I'm especially worried about the 1nF. Did you do some stability check? ...just to be sure it does not oscillate.

With stability check I really mean a phase margin check.
It may be that in your lab the circuit is working fine ... but starts to oscillate with different conditions like:
* temperature, different speakers, or even no speaker connected.

Klaus

Added:
Btw: for better readability next time keep on standard signal flow direction of schematics: from left to right

 

[Bball4life]

Hi Klaus,

During the design and development time, various conditions as what you mentioned, different temperatures, different speaker impedance was used to test if there was any potential problems. And there was none discovered.

I want to check the phase margin for the audio amp. Is there some way that I can check it outside of simulation, like maybe using a vna or spectrum analyzer? Please share some links as I couldn't find a good link online.

I also read the datasheet (page 12 || 'bridge configuration explanation' section) once again, and it said one sentence which is, "under rare condition the audio amp can oscillate at low frequency". Any thoughts on what rare conditions are?
**Attached a screenshot**


Lastly, after googling around, I saw a similar topic in Texas forum mentioning Audio Amp shorted.

LM4950 blew up when both outputs shorted to Ground. - Audio forum - Audio - TI E2E support forums

Other Parts Discussed in Thread: LM4950 We using TI LM4950 in our production and we fail average 1 a month in production area due to the output cable shorted to

e2e.ti.com

The issue is that, I am confident that the output of the audio amp is not shorted as the output of the audio amp is not accessible by any person after the product is assembled into its chassis.

Any advice is greatly appreciated.

Thanks,

--- Updated Mar 9, 2025 ---

sounds like some one has played with it and buggered it up - is it protected for speaker shorts and shorts to gnd or Vcc ?

Hi Easy peasy,

This chip does not have short circuit protection.
The output of the audio amp is not accessible to anyone once the product is assembled into the chassis.
Some kind of internal or rare condition cause by the chip itself is causing the audio amps to fail.

Also, it is not all audio amps that fail, it is 5 in nearly 1000.

I am chasing the root cause for it.

Thanks

--- Updated Mar 9, 2025 ---

sounds like some one has played with it and buggered it up - is it protected for speaker shorts and shorts to gnd or Vcc ?

Hi Easy peasy,

This chip does not have short circuit protection.
The output of the audio amp is not accessible to anyone once the product is assembled into the chassis.
Some kind of internal or rare condition cause by the chip itself is causing the audio amps to fail.

Also, it is not all audio amps that fail, it is 5 in nearly 1000.

I am chasing the root cause for it.

Thanks

 

[D.A.(Tony)Stewart]

Is it possible full amplitude oscillations can short the output stage or both at high Tj temps?

I agree with all of @KlausST 's concerns. In addition, any time you have a differential output, one of the inputs in non-inverting and stray capacitance must be less on the positive feedback side than the negative feedback time. I think that may be why the 1nF's and RC gain reduction were added. Perhaps high ESR in the Bypass Cap or V+ caps may contribute to low f oscillation while the 1nF and gain margin opportunities in layout to reduce stray +ve feedback contributes to the high f phase margin issues.

Power on tests and and step voltage input load response may be the tell with some functional test added to screen out these boards to cull them out,

 

[Bball4life]

How does one measure the phase and gain margin from the schematic that I attached before?
After reading several online articles, I understand that I have to add a smaller resistor value in the feedback path. Then probe using an oscilloscope.also inject a square wave into the input of the audio amp.
And then some how capture the bode plots.

I wondering if that is the correct process or if there is any other way or easier way?

Thanks

 

[D.A.(Tony)Stewart]

On this IC configuration, I don't think it is possible with cascaded inverted and diff. output.

But a step response you can do with various amplitudes and width pulses.

 

[KlausST]

Hi,

ESD during handling and assembling of the IC may also cause long term failure.

Klaus