ADUM2401 VCC ringing issue

Hi Patrick,

Looks like ringing on the power rail caused by coupling due the surge input to the PSU unit.
From your waveforms, it looks like the ringing can go low enough so that it's outside the valid supply range. This might create data integrity issues as the isolator can just turn off. There's also a potential risk of exceeding the abs max rating on the VCC pin if the voltage at the pin undershoots or overshoots beyond the limits given (−0.5 V to +7.0 V).

Solution:
Change the layout to avoid these issues to minimize parasitics. Why is it coupling in the first place?
Secondly, use adequate bypassing. (Caps between 0.1uF to 0.01uF with low ESR are highly recommended, smaller ceramic (bypass) caps = higher esr so go for 0.01uF in parallel with a bulk tantalum capacitor. You want this capacitor to have a good storage capacity. if there aren't enough electrons available locally the supply voltage sags down and this turns off the isolator. We don't want it turning off. We want the supply voltage steady as possible.
The bypass caps should be placed very close to the isolator's power and gnd pins as placing them far away means you're adding more resistance in the path that you don't want hence the low ESR caps.
You could add TVS diode safety diodes to clamp the voltage to acceptable levels as you don't want the overshoot/undershoot beyond the abs max rating either.

Regards,

Naveen

smaller caps = lower esr* correction