Hi,
we have an issue with the LM4991. In our design the startup time is very important. We want to switch off the amp via the shutdown-pin between audio signals. 90 ms after we pull the shutdown to GND (to activate audio) the audio signal is applied.
We now have LM4991-ICs from different two suppliers that show a different startup and shutdown behaviour.
One chip ("MJAB LM4991MA") has a startup time of about 84 ms.
Here is the switch-off time of the same chip:
The other chip ("62AC LM4991MA") needs 250 ms to start.
Here is the behaviour of the same chip when shutting down:
At the moment we can only use the first chip in our design, as the second needs too long to start. We are now wondering if one chip is faulty or if the chips are from different fabs and behave differently but as intended?
The measurements were taken in the same circuit, where only the LM4991 was swapped.
Cb = 1 µF
Ri = 20k
Ci = 1 nF
Rf = 16 k
Cf = 220 pF (parallel to Rf)
Vcc = 3.3 V
The input signal was applied all the time for the tests. Only in the final application it will be triggered 90 ms after the shutdown-pin.
Thank you very much in advance.
Best regards,
Joachim Hoppe