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, 10-12-2024, 11:14 AM

Hello everyone,

I am creating a mixed signal PCB which has two separate supplies: A buck converter for the digital section and an LDO Regulator for the analog one.

Each regulator steps down 5V from a USBc supply to 3.3V.
The digital section draws up to 3A, while the analog section draws up to 500mA.

For the digital side, I am using a TPSM82864AA0SRDJR Buck Converter and for the analog section, an AMS1117-3.3V, both powered by the VBUS supply line (USBc, see schematic on the top left).

Following Phil Salmony's advice on the Mixed Signal Hardware Design course, I want to place a filter to suppress the high frequency noise from the (usually very noisy) USBc supply rail before reaching my voltage regulators.

This filter should be able to:
1) filter out the high frequency noise from the USB rail
2) while also being able to handle the 3A needed by the analog section
3) while also not dropping more than 1V across itself (keeping the needed dropout voltage of the LDO regulator in mind).

In his course, Phil used a Pi-CLC network filter to suppress the USBc noise, but sadly I cannot employ the same tactic here because my current needs are way higher than the ones from his project. The DC Resistance of the inductor of the Pi filter would drop a massive voltage at 3A.

Thank you very much in advance.
Greetings,
Leandro

Gaskoin , 10-12-2024, 11:24 AM

Are you using PD? You know that standard usb 2.0 allows only for 500 mA consumption?

, 10-12-2024, 11:25 AM

yes, i am using PD

Mini , 10-12-2024, 01:36 PM

Which inductor do you mean? 3A sounds too much. Can you give exact parameters of PI filter?

, 10-12-2024, 01:38 PM

The inductor was on Phil's tutorial, but like i mentioned above, his project consumed way less current, so he could use a Pi filter network with L=10nH and 0.1 Ohm DC resistance, C=1uF. But for my application an inductor will not be practical (i think) due to the high current. Thats why i was asking for any alternatives 🙂

QDrives , 10-12-2024, 07:34 PM

https://www.ti.com/lit/wp/slyy109b/slyy109b.pdf
What are you space constraints?
If you are going to filter, then filter each input separate. Otherwise you SMPS will cause noise in you LDO.

, 10-13-2024, 06:48 AM

I hav eenough space to play with. What filter arrangement would you recommend?

QDrives , 10-13-2024, 08:17 PM

The cheapest filter arrangement is a pi filter. And pi for filtering incoming noise (what you mentioned) and more importantly, outgoing noise (EMC requirement).
With 10uH and your 22uF tantalums you get about 10kHz bandwidth.
10uH inductors can be as low as 20mOhm for acceptable sizes.