USB hosts hubs transceivers Forum Discussions

[TeMa_2997106]

There is a description of the Cin (Input pin capacitance) parameter in the datasheet.

Is the meaning of this parameter the capacitance that the USB HUB's input pin itself has? Or is it the value that limits the capacitance of the signal coming into the input pins of the USB HUB?

Also, Cin of HX2LP is 10pF. And according to the USB2.0 specification document (Section 7.1.6.2), the capacitance is to allow no more than 5.0 pF for the Die and no more than an additional 5 pF for the Package.

The sum is 10pF, but is it correct to understand that this satisfies Cin's 10pF?

Thanks,
Tetsuo

[MallikaK_22]

Hi,

The input pin capacitance mentioned in the datasheet is the capacitance as seen by a signal when it is  in highspeed output mode. The guideline in the USB2.0 spec measured at the package pin is 10pF. It includes capacitance due to pads and package.

The realistic way to measure this is as stated earlier, i.e. the capacitance seen by the signal.

Regards,

Mallika

[TeMa_2997106]

Mallika-san,

Thank you for your quick response.

In the USB2.0 spec, I thought it was the capacity (red circle) in the following figure, but is it correct that it is the limit of the input signal capacity (blue line) ?

Thanks,
Tetsuo

[MallikaK_22]

Hi,

No, the 10pF limit as per spec is the capacitance on each line as shown in the diagram. The capacitance value as seen by the output signal is a way of measuring these capacitances.

Regards,

Mallika