ISOUSB211DPEVM: downstream VBUS output power

Hi Koteshwar

Thank you for the quick reply.

Best regards

Ueli

Good afternoon, I am "the customer" in question. A quick glance showed me that this high current consumption of the ISOUSB device is actually expected in High Speed mode. We only intend to use the device in Full speed mode. Am I correct to estimate that the downstream USB would then support some 390mA to 450mA? I got these figures by adding to the 200...260mA thresholds you mentioned above the 95mA saving on each IVCCx or IV1P8Vx supply input (so 2 x 95mA in total).

I am not sure, though about the efficiency of the SN6505A based power supply, I don't think it will provide a full 500mA at its output when supplied from a 500mA capable upstream USB port.

Best regards,

Cristian

Hi Cristian,

Thanks for reaching out and for sharing more details about your application requirements.

The current range that I provided earlier is only a rough estimate considering typical laboratory test conditions under which case the current consumption by device is not expected to be at its max. I did factor in roughly about 85% efficiency for the SN6505A based power supply in giving out 200-260mA. To elaborate on this,

1. I considered 120mA for the upstream side of ISOUSB, although current consumption under typical test conditions will be lower.
2. Input current available for SN6505A is ~380mA.
3. Assuming an efficiency of 85%, the output current available is ~320mA.
4. Considering 120mA for downstream side of ISOUSB, there is about 200mA that can be sourced to external loads.
5. Since laboratory test conditions will only lead to typical current consumption by device, I expect the actual current available to external loads will be >200mA and I roughly estimated that to 260mA without any detailed calculations.

Please note that these are only theoretical estimates and the EVM is not characterized for its max load conditions. Hence, the actual test data could slightly differ from the calculations.

Yes, you are mostly right to consider adding 190mA (=2*95mA) to the current range I had provided to get a rough estimate of available current in Full Speed mode at the downstream for external loads.

Is there any particular load current that you would like at the downstream? Like I mentioned above, I will only be able to provide a rough theoretical estimate and not the true worst-case values as we do not characterize EVM for worst-case test conditions.

Let me know if you have any questions, thanks.

Regards,
Koteshwar Rao

Thank you very much for your detailed explanations, they do make sense to me. The maximum current we would need downstream is still a "fluid" figure, but the ballpark would be some 300...350mA. I believe we have a fair chance with that, considering Full-speed operating mode only, and I am eager to give it a try. Thanks again!

Regards,

Cristian