TPS25751: TPS25751 Vbus Discharge Time Inquiry

pierre rouaze

Part Number: TPS25751

Tool/software:

Hello TI Support Team,

I am working with the TPS25751 USB PD controller in a power sink application and have a question regarding the Vbus discharge behavior.

In my design, the total capacitance on the Vbus line is approximately 20.3 µF.

According to the datasheet, the Vbus discharge current is specified to be in the range of 4 mA (min) to 15 mA (max). Based on this current range and the 20.3 µF capacitance, calculations suggest the discharge time from typical PD voltages (e.g., 5V or 9V) down to vSafe0V (approx 0.8V) should be in the range of approximately 5.7 ms to 41.6 ms.

However, in my actual hardware setup, I am observing Vbus discharge times consistently in the range of 1 to 2 seconds after the sink detaches or power is removed. It is quick to reach 2V but after it is quite slow:

Could you please advise if this 1-2 second discharge time is considered normal behavior under certain conditions or configurations, despite the specified discharge current? Or could it indicate a potential issue with my hardware implementation?

Thank you for your support.

Pierre

5 months ago

0 Chris Sterzik 5 months ago

TI__Guru 64311 points

Hello,

In your application is the TPS25751 powered from the VBUS (dead battery mode)?

Regards,

Chris

0 pierre rouaze 5 months ago in reply to Chris Sterzik

Prodigy 50 points

Ah yes, I have a use case where the Controller is in dead battery mode, so it should be a fast discharge in normal mode and slow in dead battery?

I have tested it and it seems to do it with a Vin3v3 supply as well.

Thanks,

Pierre

0 Chris Sterzik 5 months ago in reply to pierre rouaze

TI__Guru 64311 points

Hello Pierre,

I was able to see this with the EVM. The point is that when the device is no longer receiving a valid supply voltage the discharge circuit is not functional.

With no supply to VIN3V3.

With supply to VIN3V3.

Regards,

Chris

0 pierre rouaze 5 months ago in reply to Chris Sterzik

Prodigy 50 points

Ok so it seem this is normal behaviour then, no concern from your side?
when there is no current drawn from the power to the DUT I have seen it take a long time even with 3V3 connected:

Thanks,
Pierre

0 Chris Sterzik 5 months ago in reply to pierre rouaze

TI__Guru 64311 points

Hello,

pierre rouaze said:
Ok so it seem this is normal behaviour then, no concern from your side?

What you are showing is not consistent with what I am seeing with the EVM. There is no active discharge at all in the above waveform. At least in your first waveform you can see the active discharge until the voltage drops below the POR level of the PD controller. Can you confirm where you are measuring VBUS relative to the TPS25751?

Thanks,
Chris

0 pierre rouaze 5 months ago in reply to Chris Sterzik

Prodigy 50 points

yes it is Vbus of the tps, we are configuring the controller with an other MCU sending the config file via I2C, could it be beause the configuration was not finished when the plug was removed?

0 Chris Sterzik 5 months ago in reply to pierre rouaze

TI__Guru 64311 points

Hello,

That is possible. The device would be in the PTCH mode and may not be able to successfully transition to the Unattached type-c state.

Regards,
Chris

0 Chris Sterzik 5 months ago in reply to Chris Sterzik

TI__Guru 64311 points

Regards,

Chris

0 pierre rouaze 5 months ago in reply to Chris Sterzik

Prodigy 50 points

Thanks Chris,

Do you think the controler would remain compliant with the USB PD spec with the delays caused by the dead battery mode? I have looked at the spec and i found that the PD port needs to reach vSafe0V in tSafe0V. but I cant quite tell if it is valid for a sink only device.

thanks,

Pierre

0 Chris Sterzik 5 months ago in reply to pierre rouaze

TI__Guru 64311 points

Hello,

This is not the intended application. The dead battery mode is intended to provide a means to charge the battery. Once the battery is charged the expectation is that the PD will be powered from the battery and support the proper termination (and discharge) in the event of a disconnect.

Another way of looking at it is that the compliance is considered in the recommended operating conditions of the product. When VBUS and consequently VIN_3V3 goes below the recommended operating conditions we can no longer guarantee the performance of the device and you would need to provide additional measures to support compliance in this scenario.

Regards,

Chris

0 pierre rouaze 5 months ago in reply to Chris Sterzik

Prodigy 50 points

I see, but actually it seems that in dead battery mode the discharge is fast enough to reach Vsafe5V in the required 275mS (20mS in our case):
PDcomp
I think this is acceptable. let me know if i am incorect.

Thanks a lot

Pierre

0 Chris Sterzik 5 months ago in reply to pierre rouaze

TI__Guru 64311 points

Hello,

We have discussed a number of scenarios with dead battery and the state of the VIN_3V3, so I am little confused about the statements regarding the specification.

This PD has passed compliance to 3.1 and will soon be updated to reflect the recent certification to USBC-PD 3.2. I hope this is clear.

Regards,
Chris

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TPS25751: TPS25751 Vbus Discharge Time Inquiry