Strange response from L6981N50DR. (L6981)

[mert_gulsoy]

Hi,

In one of my designs, I used the L6981 5.0v low-noise variant. I followed the datasheet, and this was my first time using this chip. However, I added an option that separates the AGND and PGND pins with a ferrite bead which is not present in the datasheet. As shown in the schematic with a red arrow below:

When this ferrite bead is present on the circuit, the L6981 fails to boot properly. Thus gave 2.5v output with a strange signal shape (below).

When I short the ferrite bead, it seems like the L6981 is working properly.  I wanted to filter 24v supply ground with 5v ground. But as it seems the way that I do is possibly wrong. Why is the ferrite bead causing that effect?

Thank you in advance.

[TDK · Accepted solution]

A ferrite bead acts as an inductor, which allows your grounds to move relative to one another. It actually forces them to move relative to each other since the current draw is constantly changing. This is a bad thing.

Grounds should always be connected directly, even analog and digital grounds. Use different planes and connect them at a single point if noise is an issue.

Measure AGND vs PGND with the ferrite bead in place to see the effect.

[mert_gulsoy]

Thank you for your precious answer. I didn't think this could be that serious. Here are the measurements between AGND and PGND:

This is taken when the L6981 is in 2.5v state. (trying to boot I guess):

And then I shorted the bead and saw that L6981 finished booting. After this L6981 started to work normally. While bead is not shorted anymore. This is taken when the device outputs 5.0v:

I think the fix is to remove the bead.

Best regards.