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LTC4421 ESD protection

NiceJ
on Dec 17, 2020

Hi AD experts,

My board looks very sensitive to ESD (Works one day perfectly and don't work the day after), and I trying to find out.

Don't work means that :

  1. inputs still valid --> OV<500mV; UVF&UVR>500mV (at multimeter)
  2. GATE are not triggered (0V) --> MOSFET are OFF
  3. CH2 FET is closed, even when the LTC4421 power is remooved
  4. INTVCC  is ok : 3.8V measured

Does it make sense to use Bidirectional TVS as ESD protection for source input ? Since input range absolute Maximum Rating of LTC4421 is -10V to 60V (not symetrical).

TVS used in my design is :

Subber is present.

Additional question : if the current limit feature isn't needed. Can we just tie SENSE pin to GND ?

Thanks



detailed description
[edited by: NiceJ at 7:53 AM (GMT -5) on 17 Dec 2020]
  • 0
    •  Analog Employees 
    on Dec 23, 2020 2:50 AM 2 months ago

    Hi NiceJ,
    I spoke to the designer about the bidirectional TVS, and to summarize his response:
    The TVS is for the scenario where both supplies are ON and the output cap is charged, and one supply is shorted to ground.
    Current will flow from VOUT to the shorted supply (see picture). The parasitic inductance stores energy as current flow through it.

    Gate 1 shorts to ground to stop the reverse current flow.
    The parasitic inductance resists the dI/dt by making the red node more and more negative.
    Eventually, the node becomes negative enough to reach the threshold voltage of the FETs.
    The FETs turn on, and the charge from the cap provides current to the parasitic inductance.
    The negative voltage on the red node stops building up.

    So, the TVS is bidirectional because a zener would clamp at 0.7V and get in the way of the mechanism described above.
    For ESD purposes, you could split up the TVS into two zeners: One could clamp at whatever positive voltage you want, and the second one will clamp the rail so it doesn’t go lower than -10V.

    To disable current limit, tie sense pins to output.

    -Aaron