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LM7480-Q1: Max rating

JLA
Intellectual 2565 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM7480

Dear team, 

A customer is asking this question:

We are going to use LM7480 in common drain configuration with maximum supply voltage of +36V. Maximum negative transient at input is -40V during ISO pulses. This means that we would momentarily have 76V difference from C to A during transient.

 

LM7480 datasheet indicates that max rating of C to GND is limited to 70 +Va during negative input transient. In our system this would mean that max rating from C to GND would be +30V and we would have +36V in worst case. However, it is said on datasheet also that C to A max rating is +85V which would mean that this would not be a problem.

 

 

I noticed that in LM74700 datasheet this is informed differently. There is no separate C to GND limitation during negative transient.  Both chips seem quite similar out regarding ideal diode functionality so I wonder why other chip would have separate C to A limitation during negative transient while other one does not.

 

So do we have a problem with LM7480 if C to A is 76V during negative transient or is it safe just to ensure that C to A is kept <85V?

Best regards

Jorgen

  • 0
    TI__Guru 57737 points

    Hi Jorgen,

    I understand your question. Let me look into this and get back to you by tomorrow.

  • 0
    TI__Guru 57737 points

    Hi Jorgen,

    The ISO pulse of -40V is only for a small duration of Time. We can clamp the -ve voltage at the input using a TVS diode as shown below.

    You can clamp the voltage on the -ve side to a suitable voltage to achieve (70 + V(A)) > 36V.

    Is there a possibility of input reverse polarity in your system ? i.e. Can -36V be applied at input ?

  • 0
    TI__Intellectual 2565 points

    Hi Praveen, 

    I accidentally talked about ISO pulse but in this case we have to tackle IEC 61000-4-5 1kV negative surge pulse. We use following circuit to limit input to -40V during surge pulse. We have -26V reverse polarity requirement so we can’t use any lower voltage TVS at input. We have tested several TVS components and we haven’t find any component which would limit lower than -40V without compromising -26V reverse polarity requirement.

     

     

    So if we have +36V input voltage before surge, we’ll have +76V from C to A during surge(50us).

     

    Do you think this is a problem? It just feels strange to me that it is said on datasheet that C to A can be +85V but then at the other line it is said that in can only be +70V.

  • 0 in reply to JLA
    TI__Intellectual 2565 points

    The picture missing above:

  • 0 in reply to JLA
    TI__Guru 57737 points

    Hi Jorgen,

    Thanks for the detailed explanation. 

    I am discussing this with our internal team to clarify the about the Rating of the device and to come up with a solution for your requirement. Please give me  a couple of days before I can get back to you. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Jorgen,

    I am working on your query with internal team and is taking more time than expected. Will give you an update again within 2 days. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Jorgen,

    Apologies for the delay in response. It has taken some time to verify with our design team. 

    As long as we don’t violate the “VCAP to A” 85V Abs Max, we should be okay. If we can discharge VCAP-VS very quickly, then the problem is solved.

    Using a PNP and some passives, we can discharge the VCAP when Vin goes from 36V to -40V. Please see circuit and simulation results below.