Other Parts Discussed in Thread: DRV8353, DRV8323
Hi, I had a functional design based on the DRV8323RS and have redesigned it around the DRV8353F to work around shortages (I know common story). The problem I am having is unusual noise on the current sense lines. In this design, the schematic and layout are pretty much the same between the two versions, aside from an external regulator instead of the built-in that was used on the DRV8323RS version. In the DRV8353F design though, each of the current sense channels has a board specific and channel specific noise that is both large, and hard to describe.
For some reason, channel C always appears to be relatively noise free, but channels A and B sometimes, but not for every board, have varying amounts of noise measured on the current sense outputs. The driver is configured in 3x mode, with the CSA gain configured to 20. Current sense ADC sampling is conducted at the center of the off time. For the good channel C (and the previous board), with a 50% duty cycle on all channels, I measure a standard deviation of around 1.3 mV (around 1-2LSB on the 12 bit ADC of the microcontroller). On the bad channels of the DRV8353F board there is often a background noise of around 4 mV, with aperiodic spikes up to 80 mV.
I believe the external regulator in the new design is relatively well laid out. There are separate ground planes for the power and logic/sense regions connected under the regulator (and at the AGND pin of the DRV8353). Neither the current sense inputs nor outputs run near anything that is switching during the sampling time and I don't see any significant power supply related noise with the oscilloscope, and in any case, one channel works just fine, which would seem to rule out power supply noise.
Under a microscope, all the solder joints look fine on all components.
Are there notable differences between the current sense amplifiers on the two chips that I should be aware of?
My layout for the FETs and current sense resistors looks like:
And the current sense outputs run to the microcontroller like so:
I don't have the best probing setup, so attaching the scope probes does couple some additional noise into the system. Specifically, when I probe the debug line that signals ADC sample, it shows up as extra noise on the current sense output that is not actually a problem. So first, a scope trace of a "good" channel C. In this plot, yellow is the "SOC" line, AC coupled, teal is a debug line indicating the end of the ADC sample (which starts just a microsecond or so before that line goes high), and purple is the INHA line (all 3 channels are the same).
Followed by a closeup of the sampling window, and a second close-up with the debug probe removed, as it was coupling a moderate amount of noise into the scope reading:
That sag in the line is clearly related to the sampling, and may just be a ground level issue in my probing as the microcontroller draws more current when running the ADCs? This being the "good" channel, I don't see that level of noise in the readings for sure.
Now, for one of the "bad" channels in two variants. Sometimes it has terrible noise for the entire time the gate drive is low, sometimes only partway through.
Now, a close-up of that:
It is almost as if the sense amplifier is becoming unstable for some reason.
However, even for channels without that noise which is obviously bad on the scope, the "sag" associated with sampling is more variable, larger, and results in increased measured noise.
Finally for reference, scope traces taken in the same way from the DRV8323 based design, which for all channels resembles the channel C output from the DRV8353F