RP2350 alternate 1.1V

[phil from seattle]

There has been a fair amount of discussion about the RP2350's built in switching regulator to produce DVDD (1.1V). In particular, the datasheet and Hardware design guide both go on at length about having the proper orientation of the inductor and note that most small inductors don't have an orientation mark.  RP recommends an Abracon part that does not appear to have been released (it has a TBD part number). There are several threads here and elsewhere with much teeth gnashing over this. I've seen several suggestions to use an LDO regulator to supply DVDD but no details. So, some details...

I was able to find one of the launch partners (Bus Pirate) that uses a 1.1V LDO and discussed what they did - https://forum.buspirate.com/t/rp2350-bus-pirate-5xl-and-6/529/12.  The short form is they connect VREG_AVDD and VREG_VIN through an LPF (10 ohms/2.2 uF) to 3.3V and leave VREG_LX, VREG_FB and VREG_PGND unconnected. The output of the LDO is connected to the 3 DVDD pins, with bypass caps of course.  The LDO circuit takes up less than half the space of the components for the switcher and is cheaper than using an Abracon inductor (well, a similar sized one).

This sort of thing really needs to be at least in the "Hardware design with RP2350" document. How to do it may be obvious to many but I prefer to hear these kind of details from the manufacturer.

[SpacedCowboy]

Looking at their circuit, it wants a 0-2R ESR capacitor on the output. I'm not sure how to qualify a capacitor against that requirement...

• ESR varies by frequency (though data sheets usually quote it at 120Hz AFAICT).
• It's not clear what frequency is relevant for the output of the LDO - in fact I can't find much about frequency in the LDO data sheet at all, what I can find is mainly frequency-dependent parameters...
• Looking at Murata's capacitor tool to see their ESR values, they vary from ~10R at 100Hz to a low of 6 milli-ohms at 1MHz

So if I were to choose to do it this way, how do I go about checking that the capacitors I choose will be suitable in terms of ESR (ok, I get X7R or X5R, the LDO datasheet says 1uF but they've used 4.7uF here, no problems there) ? Any clues ? Clearly if the frequency to care about is > 1kHz, there's no problem, but how do I tell what the requirement needs ?

I guess it could be the clock frequency of the part (12MHz) or the internal frequency of the part (whatever it's overclocked to, like anyone's running this thing at 150MHz...) Or is it more to do with the LDO and I'm just missing it...
 

[phil from seattle]

Yes, the 74311 data sheet calls out X5R or X7R ceramic caps so I was planning on going with that and not terribly worried about the exact ESR of a ceramic. TI also says 0.1 to 1 uF rather than 4.7 so, like you mentioned, that may be a factor.

[Geoff-AU]

LDO output capacitor (bypassing) matters just for the LDO's stability, it doesn't matter what device is downstream.

The 74311 datasheet simply says use a X5R/X7R 1uF ceramic (or higher).  If they required anything more specific (like aligning a bespoke inductor with the current position of the moon) then they would say so in the datasheet, and we could make suitably derisive comments forthwith.

[iMo]

..The short form is they connect VREG_AVDD and VREG_VIN through an LPF (10 ohms/2.2 uF) to 3.3V and leave VREG_LX, VREG_FB and VREG_PGND unconnected. The output of the LDO is connected to the 3 DVDD pins, with bypass caps of course..

That is a big question whether to connect the VREG_VIN to the LPF or to the Vcc or to let it floating..
Has to be elaborated, imho..
PS: they have replaced the 1.1V switcher because it is cheaper to make it with the LDO, we have discussed that change in the "2350" thread here mostly because of the EMI/noise.. (and therefore you have to replace the 3.3V switcher as well)..