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LMV761: Absolute Maximum Ratings Check

Julian Chen
Expert 2550 points
Part Number: LMV761

Dear E2E member,

I want to know the differential input which is IN+ - IN-,  the maximum rating is  5.5V, right?


Is it OK for this design as below?

  • 0
    Guru 140200 points
    Hi Julian,

    looks good so far. But why have you mounted C36 and C66 to the output? The LMV761 toggles very fast at the output and toggling into a cap will generate a considerable load current spike. This can produce lots of noise on the supply line and signal ground.

    Kai
  • 0 in reply to kai klaas69
    Expert 2550 points

    Dear Kai,

    Please refer the question as below,

    Customer means the CMVR just only -0.3V~3.8V@VCC=5V

    But the pin 1  of LMV761 is 3.9V, Pin 3 is 0V when 3.3V not ready.

    So Pin1 - Pin3 = 3.9V when   3.3V not ready.

    It's a risk for this design?  Does IC will abnormal at a moment when 3.3V not ready?

  • 0 in reply to Julian Chen
    Guru 140200 points
    Hi Julian,

    is this the schematic you originally posted?

    Well, of course, the input voltages must not violate the common mode input voltage range! Change the voltage dividers so that the input voltages are allways within -0.3V...2.1V.

    And I would remove C36 and C66.

    Kai
  • 0 in reply to kai klaas69
    Expert 2550 points

    Dear Kai,

    Thanks for you reply.

    I still want to confirm some questions.

    1. Now 3.9V sequece always prior than 3.3V. So What can we do in this design? Do we need to change others part?

    2. If over CMVR, what the issue does IC happened?

    3. What the CMVR SPEC at VCC=3.3V of LMV761?

    Thanks!

  • 0 in reply to Julian Chen
    TI__Intellectual 1740 points
    Hi Julian,

    Below are the answers to your questions:

    1. I would recommend that you change the power up sequence so that the device is powered up with 3.3V before signals are applied to the inputs. The ESD diodes will always be conducting otherwise and this is not expected use for ESD diodes of the device.

    2. Going over the CMVR of the device will lead to unexpected behavior and can also damage device. As I saw in the first screenshot you posted, you are seeing a propagation delay of 270ns which is double what it should be.

    3. The common mode voltage range of the device is from (Vee-0.3) to (VCC-1.2V). So in the case of 3.3V this would be from -0.3V to 2.1V.

    Regards,
    Jas
  • 0 in reply to Jaskaran Atwal
    Prodigy 30 points

    Hi Jas,

    Thanks for your reply.

    What will happen when signals are applied to the input before IC VCC is ready?

    In the current application, it's a bit hard to change power up sequence to meet your requirement.

     

    Gary

  • 0 in reply to Gary kung
    TI__Intellectual 1740 points
    The input ESD diodes will be conducting every time the device powers up which is not their expected application. If there is no other option do you know what the input current is at the inputs when the signals are applied before VCC is ready? It should be kept to less than 1mA.

    Regards,
    Jaskaran
  • 0 in reply to Jaskaran Atwal
    Prodigy 30 points

    Dear Supporter,

    We'd like to add 3 capacitors to reduce input noise.

    Please help to review this circuit or provide your idea to reduce input noise.

    C34,C72 and C73:1000pF

    Gary

     

     

     

     

     

  • 0 in reply to Gary kung
    Guru 140200 points
    Hi Gary,

    looks much better now. :-)

    I would omit C72. C34 and C73 is enough filtering of inputs. And, as mentioned earlier, I would omit C36 and C66. The LMV761 toggles very fast at the output and toggling into a cap will generate considerable load current spikes. These can produce lots of noise on the supply line and signal ground. Be very careful with caps at the output of a fast comparator...

    Kai
  • 0 in reply to kai klaas69
    Prodigy 30 points

    Hi Kai,

    Got it.

    Thanks

    Gary