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OPA341: if shut down for an extended period, does not return.

Ken Henderson
Intellectual 670 points
Part Number: OPA341
Other Parts Discussed in Thread: OPA363

Hi,

I'm using an OPA341UA under the following conditions:

Power: V+ = 5V, V- = 0V. Decoupled with 100nF

Output driving a NPN base through 100R.

SD pin: 0V or 3.3V (data sheet says thresholds are 0.8V and 2V) from microcontroller.

SD signal is on 250us every 25ms, or 1% duty.

But at times the SD os off for an extended period, e.g. several minutes. When this occurs the opamp does not come back to life. It just stays in shutdown and I have to cycle the 5V power to get it to come back.

Any ideas what may be causing the opamp to stop responding?

I've tried all sorts of decoupling on various pins...

Is it possible 3.3V is not enough for the SD pin (even though the data sheet shows 2V threshold)?

I'm stumped! Any help appreciated.

PS; for now I have made a work-around by powering the opamp off for 5ms every 25ms during a time when it's not needed, but I don't really like this solution!

Thanks, Ken

  • 0
    TI__Genius 15410 points

    Hi Ken,

    You are correct in that above 2V ((V-)+2V) the device should turn on.

    Has the device been damaged in any way? ESD? Applying an input to the amplifier while power is off? Etc.?

    Can you please provide us with a schematic?

    Can you please provide oscilloscope shots of the input, output, and SD pin when the device is working as expected (SD off for a short period)?

    Can you please provide oscilloscope shots of the input, output, and SD pin when the device is NOT working as expected (SD off for a long period)?

    Providing this information will help us troubleshoot your issue.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Intellectual 670 points

    Hi Tim,

    Thanks for your reply. I don't think there has been any ESD damage as I followed the usual precautions. Also there are 6 opamps in the circuit and it happens to all of them. None of the inputs are driven while the power is off because the micro is powered from the same 5V supply.

    Here's the circuit. The SD input comes from a micro output (3.3V signal). I also tried 10nF in parallel with R2 to supress any spikes. The REF signal is a DAC output from the micro divided by 2 (1k/1k divider, and decoupled with 100nF). The output OUT1 drives the primary of a transformer connected from DCIN (19V) to OUT1. D1 is the free-wheeling diode. There is also a 1uF/10R snubber across D1 (not shown). The requirement is for the opamp to make a current sink with T1 & R3 so that the current in the transformer primary is controlled by REF and ON for a short pulse from IN1.

    Here are the opamp signals while it is working. REF = 0.7V so approx 1.4A in prmary. (all 1V/div):

    Then if the SD pin is not activated for a few minutes, the next time it starts to be activated the opamp out remains OFF, even though +IN > -IN.

    I hope this helps to see what is happening.

    Thanks, Ken

  • 0 in reply to Ken Henderson
    TI__Genius 15410 points

    Hi Ken,

    I have ordered samples of the OPA341 to try to recreate the issue you are seeing in our lab. The samples should arrive tomorrow or Monday.

    While I am waiting for samples to arrive can you please verify that the supply voltage to the device is still there after waiting a few minutes for the SD pin to turn on or that nothing is collapsing the supply?

    If the supply seems to be ok, can you remove the OPA341 that is currently on the your board and replace it with a new OPA341? If by chance the device was damaged, changing out the device should fix the problem.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Intellectual 670 points

    Hi Tim,

    Thanks for ordering the samples to try this. It will be interesting to see what you find.

    I did check the 5V supply when I found this problem and it is rock stable. In fact I have to remove the supply to get the OPA341 to come back to life!

    So that is the work-around I am using now. The micro shuts the 5V supply to the OPA341 down for 5ms every 25ms during a time it is not needed. I'm hoping this will not stress the OPA341.

    Unfortunately I cannot change the OPA341 in this prototype as it has to go out next week for field testing, but I will also get some more samples next week and try them.

    Have a good weekend!

    Thanks, Ken

  • 0 in reply to Ken Henderson
    TI__Guru*** 140186 points
    Ken,

    We have confirmed your test results. My lab testing has found that the turn on time (propagation delay) is correlated to the duration of shutdown time that proceeds the turn on. Resetting the power supply resets 'memory' of shutdown time.

    There is no harm in cycling the power supply. Power up and power down is a normal and non stressful event for the op amp.
    We can provide alternate part numbers if you want to change the device for a longer term solution.
  • 0 in reply to Ron Michallick
    TI__Guru*** 140186 points
    Ken,

    OPA363 is a good replacement for OPA341 in most applications.
  • 0 in reply to Ron Michallick
    Intellectual 670 points

    Hi Ron,

    Thanks for testing this and very interesting that you get the same result. Do you have any correlation between off time and turn-on delay?

    I will test the alternative you have suggested.

    Thanks, Ken

  • 0 in reply to Ken Henderson
    TI__Guru*** 140186 points

    Ken,

    I tested units from two different lots. Here is the result.

    Turn on propagation delay (seconds) vs. Shutdown duration (minutes, not milli-minutes).