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Audio HP Amp TI LM4811 - am I getting crazy?

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Saimoun:
Hi all!
So glad to join the community here - I have been following Dave's videos for a while now, really really great stuff!!  :-+

My first post - an issue of course! :D

I am trying this IC from TI called LM4811 - datasheet here: https://www.ti.com/lit/ds/symlink/lm4811.pdf

On the paper it's all good, it seems very simple, here is the "typical application" from the datasheet.




So I made a PCB and ordered it, soldered it, no shorts and everything seems perfectly ok! But when it comes to testing, it seems that the Bias voltage (internally created by the IC) is *heavily* influenced by the CLOCK pin! yes - the CLOCK pin!

Here are my schematics:




To test I tried measuring the voltage on the Bias pin directly or on the Vin pin - since this should be at the same average voltage (they are identical since for now I am sending a 1.65V DC from the DAC). Here are measurements when the CLOCK pin is at 0 (GND) and when the CLOCK pin was at 1 (5V) while starting up. The yellow line is the Vin pin and the blue line is the CLOCK pin.


CLOCK at GND:



CLOCK at 5V:



Sometimes the bias voltage changes by itself, oscillating between 0 and 1V when the CLOCK is at GND, and between 5V and 3V when the CLOCK is at 5V.

I double-triple checked the PCB layout and the soldering. Nothing complicated, just the IC and a few through hole components around. Is there something I missed in the datasheet maybe?

Thank you in advance!  ;D

Cheers
Simon

Saimoun:
Ooh and I forgot to say that the CLOCK pin seems like it pulled down somehow - if I put a 1K resistor between 5V and the CLOCK pin it reads at around 0.5V. If I put 100Ohm resistor it reads at around 1V. To put it at 5V I need to connect it directly to 5V.

magic:
Are you sure the chip is genuine?
Are you sure you didn't connect shutdown wrong?
What the clock pin is for and why are you connecting it to some fixed voltage? I'm lazy and didn't download the datasheet :P

Saimoun:
It's all good - we're all lazy :D

Chip is genuine, bought from Digikey. I already tried two of them and both have the same behavior.

The CLOCK and UP/DN pin work to change the volume. As I understood they should be at GND when "resting". Here is all the datasheet is saying about it:



The Shutdown pin is connected to GND and just to check again I am looking at the datasheet that the device is active when Shutdown is low... And actually this is very confusing, I am not sure anymore :/

How do you guys understand it, should it be LOW or HIGH to put the device active?


--- Quote ---In order to reduce power consumption while not is use, the LM4811 features amplifier bias circuitry shutdown.
This shutdown function is activated by applying a logic high to the SHUTDOWN pin. The trigger point is 1.4V
minimum for a logic high level, and 0.4V maximum for a logic low level. It is best to switch between ground and
VDD to ensure optimal shutdown operation. By switching the SHUTDOWN pin to VDD, the LM4811 supply current
draw will be minimized in idle mode. Whereas the device will be disabled with shutdown voltages less than VDD,
the idle current may be greater than the typical value of 0.3µA. In either case, the SHUTDOWN pin should be
tied to a fixed voltage to avoid unwanted state changes.
--- End quote ---

magic:

--- Quote from: simonlasnier on July 29, 2020, 06:26:32 pm ---Ooh and I forgot to say that the CLOCK pin seems like it pulled down somehow - if I put a 1K resistor between 5V and the CLOCK pin it reads at around 0.5V. If I put 100Ohm resistor it reads at around 1V. To put it at 5V I need to connect it directly to 5V.

--- End quote ---
It looks like there is a diode from CLOCK to GND inside the device. With 100Ω to 5V it's consuming 40mA of current if the pin is at 1V :wtf:
And the datasheet says that it should be pulled to 1.4V to register as high? Makes no sense.

If it isn't fake, are you sure you didn't mount it backwards? Does it work at all, by the way? Do the VIN pins really track the voltage at BYPASS or was that just your speculation? Can you get any sound from it?

Shutdown seems correct. "Shutdown active high" means that high will make it shut down.

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