Question about RP2040-PICO30

[chros]

Dear Olimex,
I have a short technical question about the design of the RP2040-PICO30 device. It is a great device, just one point, which is not fully clear for me.

In the paragraph below, I reference to the RP2040-PICO30 Schematic.

To provide the +3.3V for the RP2040, the SY8089AAA Buck-Converter is used. The Buck-Converter has in input capacitor (C17) of 22uF - which is also recommended by the SY8089AAA Datasheet (see page 7). The Buck-Converter can directly be supplied via USB and the Schottky diode D1.

Anyhow, according to the USB Spec., the maximum input capacitance for USB-powered device should not exceed 10uF due to the inrush current when hot-plugging the device and charging the input capacitor of the Buck-Converter. As reference for my question I am using the journal article by TI "Powering electronics from the USB port".

My questions are:

• Did you specifically consider this point during the design?
• Do/Did you face any issues with inrush current when powering the device via USB?
• Did you implement any inrush current limiting measures, I did not see in the schematic?
• Am I completely wrong about this topic?

Thank you in advance!

[LubOlimex]

In short - you are looking too much into it. It is fine.

It is often an asked question online, for example here:

https://electronics.stackexchange.com/questions/38361/usb-powered-device-with-multiple-decoupling-capacitors

I believe original RPi has even bigger capacitor 47uF (more than double).

The diode on our the input has internal resistance which acts as inrush current limiter.

[LubOlimex]

As more exact answer my colleague found that 10uF doesn't apply for USB-C in this document:

https://www.ti.com/lit/ds/symlink/tps25740a.pdf?ts=1690456306659

Page 19:

8.1.1 VBUS Capacitance
The USB Type-C specification requires that the capacitance on the VBUS pin of an empty receptacle be below
10 μF. This is to protect legacy USB sources that are not designed to handle the larger inrush capacitance and
which may be connected via an A-to-C cable. For applications with USB Type-C receptacles and large bulk
capacitance, this means back-to-back blocking FETs are required as shown in Figure 19. However, for
applications with a USB Type-C plug (that is, a captive cable) this requirement does not apply since an adaptor
cable with a USB Type-C receptacle and a Type-A plug is not defined or allowed by the USB I/F. Figure 20 is a
schematic for such applications.

[chros]

Hi LubOlimex,

thank you for the reply.
I guess I am see this topic too strict and you are right, that the Schottky diode has an effect on limiting the inrush current up to some point.

Did you choose the 1N5822 for a specific reason or is it a common component you use? It seems a bit overpowered for its use in the RP2040-PICO30.

Thank you.

[LubOlimex]

> Did you choose the 1N5822 for a specific reason or is it a common component you use?

Both. Using it is a bonus feature and an extra, tho. USB-C specification allows for more current draw. So if somebody needs more current from the USB he can draw it.

[chros]

Thank you!

[Julian758]

Absolutely, it's a bonus feature in addition to its primary function. The USB-C specification enables higher current draw, offering users the option to pull more power if required.