LM4917: LM4917 - System power brown-out when headphones inserted

Colin R.

Part Number: LM4917
Other Parts Discussed in Thread: LP5907

I am using the LM4917 Ground-Referenced Headphone amp on a custom board. Occasionally, when plugging in the 1/8'' (3.5mm) headphones to the jack mounted on the PCB, a brownout of all the ICs in the project occurs, USB resets, etc. By brown-out, I mean a momentary loss of 5V power, like it's briefly shorted. This is caused because the tip and the sleeve can momentarily short when plugging in headphones.

Is this a known problem with "capless" headphone amps, that I'm not aware of? I made sure not to connect the exposed pad as suggested in the datasheet and everything else seems correct. Any suggestions would be very helpful. I attached schematics of the amp and the power supply that feeds it.

Looking forward to getting some help with this problem. Thanks!

over 5 years ago

0 Luis Fernando Rodriguez S. over 5 years ago

TI__Guru* 84915 points

Hi, Colin,

Our team is checking this issue. We will answer as soon as possible to provide some suggestions or if we need additional information.

Best regards,
Luis Fernando Rodríguez S.

0 Luis Fernando Rodriguez S. over 5 years ago in reply to Luis Fernando Rodriguez S.

TI__Guru* 84915 points

Hi, Colin,

Unfortunately, this momentary short-circuit between the tip and the sleeve is normal since there's a contact between them during the plugging in.

However, there shouldn't be any issue with the rest of your system even if there exists a short in the headphone output. So, we strongly recommend to try removing the feedback capacitors (C53 & 54). If your system still has this issue, you would need to isolate this section or the headphone ground from the rest of your circuitry.

Best regards,
Luis Fernando Rodríguez S.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

After some more testing, I believe the problem only occurs when audio is playing back while inserting or pulling out the headphones. If audio is not playing back I cannot seem to get the board to fail. Unfortunately, simply not playing audio is not a solution, as this amp is used to monitor a microphone.

0 Luis Fernando Rodriguez S. over 5 years ago in reply to Colin R.

TI__Guru* 84915 points

Hi, Colin,

Could you try applying a shutdown on both left and right channels with the low active pins SD_LC an SD_RC?

Best regards,
Luis Fernando Rodríguez S.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

When the LM4917 is in shutdown, the problem does not occur.

0 Luis Fernando Rodriguez S. over 5 years ago in reply to Colin R.

TI__Guru* 84915 points

Colin,

Yes, even if there's an audio signal at the inputs, if the system is shutdown (SD pins in low state) this could help to avoid the issue that you get. Have you tried this?

Best regards,
Luis Fernando Rodríguez S.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

When the LM4917 is in shutdown, the problem does not occur. But there is no switched jack or anything to detect headphone input, so while this could potentially work, it doesn't help me with these boards.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

Removing the feedback capacitors (C53 & 54) does not solve the problem.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

What method do you suggest I use to isolate the grounds for this chip?

This is on a 4 layer board where much care has been taken to route analog and digital signals correctly. Noise performance is very good and everything sounds great when headphones are plugged in.

On an older board I used this chip and it does not exhibit this problem. The only difference I can tell is that on the old board, I accidentally connected the exposed pad to ground. There was a noise problem, on this older board where over time, noise would appear in the headphones, but I was unsure if this was related to heat (the part was in close proximity to a chip that got very hot) or the connection of the exposed pad. That's why this new design leaves the pad floating as suggested by the datasheet.

0 Colin R. over 5 years ago in reply to Luis Fernando Rodriguez S.

Intellectual 475 points

I changed the current limit on the TPS2113ADRBR that supplies power to the boards from 500mA to 1A and this at least solves the problem for these boards. Not sure if this is a good long term solution, however. The datasheet for the LM4917 should really mention this issue - I've never encountered it on headphones amps that have DC blocking capacitors.

0 kai klaas69 over 5 years ago in reply to Colin R.

Guru 140200 points

Hi Colin,

eventually, a low ohmic resistor in each output line and/or somewhere in the supply line of LM4917 can help. At the input of LP5907 seems to be a good place for a current limiting resistor. I would start with a 4R7 resistor, in series to FB1. Maybe this helps.

Kai

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

Hi Colin,

have you tested the current limiting resistors?

Kai

0 Colin R. over 5 years ago in reply to kai klaas69

Intellectual 475 points

Hi Kai,

I don’t have that stuff option on these PCBs, but I do agree that it should help and I will add it on the next rev. I changed the current limit on the system load switch I mentioned above to accommodate the momentary current draw and it’s working ok for now.

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LM4917: LM4917 - System power brown-out when headphones inserted