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Help with USB specs
April 2, 2008
A reader e-mailed with a question about inrush current in USB 2.0 devices. Can you help? Post a comment below.
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I'm reading "USB 2.0: Compliance and beyond", which uses Agilent Application Note 1400 - "USB 2.0 Compliance Testing with Agilent Infiniium" as a reference.
This application note says "The USB 2.0 specification outlines a total inrush surge current limit of 50 μC." The USB spec itself does not mention microcoulombs, instead specifying a load of no more than 10 μF in parallel with 44 Ω. So, when converting this to an "area under the curve" measurement, I would expect the 50 μC requirement to apply only to the area that is under the curve but above the 100 mA line. Is this correct? It appears this way from the image in Figure 1, with "inrush threshold" at 100 mA.
Figure 1.
In other words, in my attached image (Figure 2), do I measure the area of the blue part, or the blue part plus the red part? (Of course, this particular wave fails no matter how you measure it...)
Figure 2.
Posted by Martin Rowe on April 2, 2008 | Comments (0)
