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LM2903-Q1: Input voltage exceed negative rating

Scarlett Cao
Expert 3410 points
Part Number: LM2903-Q1
Other Parts Discussed in Thread: LM2903, LM339, LM2903B-Q1, LM2903B

Hi team,

My customer is using LM2903-Q1 in their traction inverter system. During the inverter switching there will be noise coupling to the input pin, making the input exceed the negative -0.3V spec. Please see below a screenshot that is happening every cycle (20k frequency), could you help us analyze the risk? Thank you

In below waveform, C7 in pink is the Input at LM2903 pinout. The duration of below -0.3V is around 7ns, the minimum input voltage is -0.86V. 

Thank you

Scarlett

  • 0
    TI__Guru 50016 points

    Hello Scarlett,

    At -0.8V, parasitic junctions are being energized and the response of the comparator is undetermined. Effects could be on that channel, or even on other channels.

    The datasheet is full of warnings about negative voltages - for a good reason.

    Please see section 2.9 of the LM339 family application note:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    They should add Shottky diode clamps if this waveform is part of normal operation.

  • 0 in reply to Paul Grohe
    TI__Expert 3410 points

    Hi Paul,

    Thanks a lot for your quick feedback. As the section 2.9 point out, dedicated ESD protection structures were not included in the LM339 family. LM2903 also do not have ESD protection? What about LM2903B-Q1?

    Customer has this question because also may other TI sockets have a transient spec which says lower negative voltage is okay if the duration is short, also current need to be limited. So they want to know if it's the same case for 2903.

    Thank you

    Scarlett

  • 0 in reply to Scarlett Cao
    TI__Guru 50016 points

    Hello Scarlett,

    The newer LM2903B does have dedicated ESD structures. But they do not clamp until after -600mV - so the inputs are still subjected to negative voltages. If there is enough negative current, it can cause SCR latching.

    We cannot condone exceeding the abs-max ratings. We cannot say that it will not cause damage.

    Repeated excursions can eventually degrade the performance of the device, up until possible device failure.

    If these negative transients are present, thee should be some current limiting resistance in series with the inputs to limit the current to less than 1mA. This resistance can be part of any input divider resistors.