Behavior of POWER_OK2 pins and GPIO pins STUSB4500

[MCapu.2]

Here is my requirement for my USB-C powered system:

• I need 5 V @ >= 3 A.

• It doesn't really matter to me if the above is achieved from a USB PD contract in place, or if a non-PD USB-C Source is advertising 5 V @ 3A via a 10k pull-up.

• I don't want to use a micro controller.

If I cannot get 5 V @ 3 A either from a non-PD USB-C or a USB PD contract, I do not want the  STUSB4500 to allow the rest of my system to power up. For example, if someone uses a USB-A legacy charger with a 56 kohm resistor (i.e. default USB power of 1 A) built into the cable: my device shouldn't be allowed power up.

I see that Table 4 in the datasheet states a few configurations. I have a question about configuration 3. In this configuration, "POWER_OK2" asserts when a 3.0 A USB Type-C @ 5 V driver is attached (I believe it does this by detecting a 10k pull-up on one of the two CC pins). Will this pin also be asserted if a suitable power delivery contract (i.e. >= 3 A @ 5 V) is negotiated?

If so, my plan is to use this output pin to enable a load switch to power up my system.

[Benoit FORET]

​Hello,

No, using config 3, the POWER_OK pins only reflects the pull-up resistor information from the SOURCE. It does not allow to distinguish a configuration in which the device is attached to a USB-C (5V; 3A) SOURCE from a USB PD (5V;4A) SOURCE for instance.

However, it should work anyway in your case, as a USB PD capable source offering 5V;4A PDO must present a 10kohm pull-up resistor to be consistent.

If this is acceptable to you, then STUSB4500L could be an alternate solution. At the opposite of STUSB4500, it does not handle USB PD stack, but only USB-C. One benefit in your case is that the "config 3" is available by default with STUSB4500L, therefore not requiring any customisation on your side.

rgds

[MCapu.2]

Thanks very much for your response!

Another quick question, if possible.

My understanding is that unless a contract is in place, a PD device is supposed to current limit to "USB Default Operation" (1A).

As stated, I plan to use POWER_OK2 to enable my downstream circuitry.

As you stated, a PD device capable of supplying more than 3 A should present 10 kohm.

Is there a chance that POWER_OK2 asserts before PD negotiation is finished? My fear is that the STUSB4500 will assert POWER_OK2, thus enabling my downstream circuitry. My downstream circuitry will pull > 1 A before a PD contract is in place, and so the upstream PD source will begin current limiting and cause problems with my circuit.

[Benoit FORET]

Hello,

about your sentence: "My understanding is that unless a contract is in place, a PD device is supposed to current limit to "USB Default Operation" (1A)"

==> not exactly, USB PD contracts between compatible devices supersedes USB-C current. So if either the SOURCE or the SINK is not PD compatible, USB-C current advertised by the SOURCE will apply. Depending on the pull-up resistor applied at the connection, it can be either 3A, 1.5A or USB default (BC1.2 or by default USB 3 current or by default USB 2 current etc...).

So POWER_OK2 (with config 3) will assert ASAP at the attach, before PD negotiation is finished and can be a minimum current value to rely on if a PD contract is negotiated (if so, more current could be available at the end of the nego). If the SRC is not PD compatible or does not support more than 3A in PD mode, this min becomes also a max.

About your STUSB4500L question, the answer is yes: the device is allowed to start pulling 3A as there is no guarantee the SOURCE is PD compatible. If not, the USB-C information is the only one to rely on for setting the max current limit. basically, USB-C (without PD) power nego applies similar mechanism as BC1.2 but on a dedicated channel (CC instead of D+/D-) and supersedes it in case the Default USB power is not showed by the SOURCE.

Hope this makes sense. Anyway, this is good questioning before starting a design.

rgds

[NBALL]

​Hello

Till the device does not recognize the source connected to, device needs to limit itself to 500mA.

As soon as it has discovered TypeC source 1.5A capable, for example, it can increase its power budget to 1.5A, during the PD negociation phase.

When in explicit contract, device should limits its power to the negociated contract; if contract cannot be established, max power is defined by Rp value from source.

Best regards

Nathalie