ADM3251E Latchup

Hi Brian,

The ADM3251E has ESD protection built-in only for HBM (human body model) ESD. I presume your testing is to IEC 61000-4-2, which is a much harsher system level test. I do note the external protection/filter circuit (D4, FB3, FB4, C18, C19). The exact components chosen here will affect whether ADM3251E is successfully protected from disruption.

Another consideration is the effect of discharge routed to GNDISO vs. the other side GND. ADM3251E is rated to minimum common-mode transient immunity of 25 kV/us, but it is possible the ESD discharge could exceed this.

One option sometimes used for ESD protection on isolated designs is to place high voltage capacitors from GNDISO to GND. C17 might be helping here, but it might be possible to connect D4 to GND rather than GNDISO via a capacitor. Or high voltage capacitors could be used for C18 and C19, to connect to GND vs. GNDISO.

One fundamental consideration too is where the equipment is earthed - for example is GND earth referenced, or is GNDiso earth referenced (GND floating/isolated). This is important in terms of immunity offered by any cable shield as well.

In general the system design will need to confer the ESD immunity if testing to IEC 61000-4-2. The built-in HBM ESD protection is really just for handling in a controlled environment for PCB assembly/component stocking/handling.

Best regards,

Conal

Hi Brian,

    As Conal mentions, your exact protection circuit component selections will affect the success of this circuit function.  This is why I'd suggest you may want to select a different TVS diode to protect the TxD line.

   The ADM3251's Tout pin's ABSMAX is +/-15V.  The TVS diode in the schematic (STS232120B301) has a 19-25V clamping voltage depending on the peak transient current (19V @ 1A, 25V @ 12A).  I suspect that this TVS's stretch allows the Tout pin to go high enough to trigger the latch up.

Eric

Hi Conal, Eric,

Thanks for the suggestions.  We were able to find a solution by populating both R2 & R3 as well as using some capacitors instead of GDTs.