[STUSB4500] Vbus_EN_SNK behaviour in single power path application

[aleruggeri87]

Hi,

I’m using the STUSB4500. I want to have a device that can be supplied at standard 5V or at higher voltages (9V and 12V) on a single power path, so I followed the schematic of EVAL-SCS001V1 reference design. The power MOS is controller by the Vbus_EN_SNK pin which is configured as default. In NVM I also set PDO1 5V@3A, PDO2 9V@1.75A and PDO3 12V@1.25A. However I’m encountering an issue when enabling the power path at high voltages, due to the large capacitive load I have. This is what I see:

- if a connect to a 5V-only capable supply, Vbus_EN_SNK goes low together with the ATTACH pin, even before PD contract is established. The soft-start on the MOS is doing his work and everything starts-up correctly.

- if a connect to a >5V capable supply, Vbus_EN_SNK goes low together with the ATTACH pin, even before PD contract is established. The MOS is switched on, powering the circuit to 5V. Soft-start limits inrush current. Then, when the PD successfully contract a higher Vbus voltage, the source start increasing the Vbus voltage beyond 5V, but since the power MOS is already on, a high inrush current makes the source issue a hard reset of the link. Actually the soft-start circuit is not operative during the bus voltage change, due to the fact that Vbus_EN_SNK in always low even during contracting.

- if I connect to a >5V capable supply but change the behaviour of Vbus_EN_SNK to “active only on Vbus>5V�?, it remains high-z till after the contract at >5V is in place, then goes low, the soft-start makes his work and everything it’s ok. Of course in this case I cannot use it in conjunction with sources that provide 5V only, which is not good for my application.

So it appears that I can only have a system that work at 5V OR at higher then 5V, but not at any voltage as stated by EVAL-SCS001V1 documentation. The point is that I would have expected Vbus_EN_SNK to be low only AFTER a contract is established and not also DURING a negotiation. Also, I do not understand the difference between this pin and the ATTACH pin, since with default configuration goes low at the same time, as soon as 5V is present on the bus.

Is there any solution to this problem?

Thanks.

[Didier HERROUIN]

​Hello,

It is normal that a classic negotation starts with a 5V, before rising up eventually to a higher voltage.

Nevertheless, it is true that for some applications, inrush current during transition phase could be detected as too high by the source.

The solution would be to limit the current during transition phase:

• By embedded current limitation, but most of the time, SINK chipset does not include any current control (like STUSB4500)
• By controlling the current from the SINK application (by the Host) during negotiation phase.

Could you provide a bloc diagram of your application ? To understand what is connected behind the MOS, and if there is a way to implement a control of STUSB4500 through I2C link.

What is the size of your large capacitive load ? Have you tried to add a coil after the MOS to limit inrush current ?

ATTACH  is not driven exactly as EN_SNK: it is a "logical signal" whereas EN_SNK is purely analog.

Most of the time, both are at the same level, but it could happen some exception where EN_SNK is disabled whereas ATTACH is still active.

If you want to communicate directly, please send a mail to smart-grid-emea@st.com

[aleruggeri87]

Hello Didier,

the schematic is exactly the same as in your EVAL-SCS001V1 reference design; capacitive load after the MOS is about 300uF.

Ok, it's a bit high, but I don't understand how EVAL-SCS001V1 can work... unless the load is purely resistive, the inrush current is unavoidable when the source changes the bus voltage and the load switch is already active.

Ideally, I would like to have a pin that signals when ANY of the 3 contract is in place. And activate the MOS afterwards (and its soft-start will limit the current)

Right now, I can use POWER2 to signal a contract with PDO2, POWER3 to signal a contract with PDO3. I’m also using GPIO to signal if 5V@3A is available, but actually I found no way to signal that PDO1 contract is in place.

That is the reason why I assumed that VBUS_EN_SNK was an OR between the 3 possible PDOs, and not a Vbus >=5V indicator.

Is it possible to configure a pin with such behavior, maybe using I2C?

VBUS_EN_SNK will act differently if the STUSB4500 is configured as passive device (USB_COMM_CAPABLE=0)?

Can I use I2C to query the STUSB4500 about the PDO status? Unfortunately I did not find any clear documentation about the available I2C registers.

Regards,

Alessandro.

[Didier HERROUIN]

​Hello Alessandro,

Some user guide to read STUSB4500 registers should be put on st.com soon.

But of course, it requires a µcontroller to read the registers and act consequently.

Is your application able to reduce the current during a while, if it is aware that we are in a negotiation phase ?

By the way, I have tested your use case with a 330µF as a load, and with STUSB4710 (EVAL ISC004V1) on SOURCE side, but I was not able to reproduce your issue.

I will try with other values, but unfortunately I will not be able to perform any test next week.

The GPIO feature can not be programmed, as it is a full-HW chipset.

Regards,

Didier

[aleruggeri87]

Hello Didier,

I'm using a commercial PC-board for the source; employed chip is TPS25741a.

Slew rate during the voltage change is approx 30 mV/µs, that is about the maximum allowable by the USB-PD standard.

This slew rate, combined with my load capacitor gives a calculated inrush current of 9 A. I cannot find a specification on the maximum allowable current during this phase, thus I conclude that is equal to the contracted current.

I guess your source chip is applying a softer voltage change and thus you don't have any current limit protection issue.

My application is not able to reduce the load; DC/DC converters that follow the capacitive load are already OFF during the negotiation phase.

The only load seen during this phase is the 330 µF cap.

I have a microcontroller connected to the STUSB4500; it would be nice to have a user guide that clearly explain registers content.

Regards,

Alessandro.