3
\$\begingroup\$

The USB spec calls for a bulk capacitor of no less than 120uF on each downstream port. I have a total of 7 ports on my USB Hub, 6 of them are connectors where you can attach USB devices, however there is one internal device: FTDI's FT232R VCOM IC. This device is always attached and I'm wondering if I still need the bulk capacitance.

Based on what I've read of USB design guidelines the bulk capacitance is there for when you attach a device you don't cause too much droop on other ports.

Any advice?

Schematic:

Distribution Switch:

Power Switch for VBUS

VBUS:

VBUS5 Circuit to power FT232RL and RS232 Transceiver.

FT232RL and RS232 Transceiver:

FT232RL Circuit

\$\endgroup\$
9
  • 1
    \$\begingroup\$ You may not need it. But I would just keep it in there. My thinking is, when you insert other devices in the external ports, other devices may pull the rail down and cause YOUR device to malfunction. \$\endgroup\$
    – user57037
    Commented Nov 18, 2015 at 3:22
  • \$\begingroup\$ IIRC it's not there for the other ports, it's to keep the inrush from pulling down that port's rail. If you have a permanently connected device, you need to provide the proper capacitance for those circuits as defined by the IC datasheet. Other ports would still need the bulk capacitance though. \$\endgroup\$
    – Daniel
    Commented Nov 18, 2015 at 5:03
  • \$\begingroup\$ @Daniel, I am sure that is the intent. But there is really only one rail, right? To the OP: are there any components between the capacitors? \$\endgroup\$
    – user57037
    Commented Nov 18, 2015 at 6:33
  • \$\begingroup\$ @mkeith Well, sort of. You can have thermal fuses, EMI suppression, or active current limit devices between the ports which would add extra impedance. \$\endgroup\$
    – Daniel
    Commented Nov 18, 2015 at 8:13
  • \$\begingroup\$ @Daniel, If the OP has such things, then I agree with you, no giant cap needed for on-board device. If not, then might as well put it in the layout. You can still no-load it. \$\endgroup\$
    – user57037
    Commented Nov 18, 2015 at 9:10

1 Answer 1

Reset to default
1
\$\begingroup\$

If it's a guideline or recommendation, then read the datasheet and figure out what the reasoning is behind the guideline. It may not even be applicable to your design.

Since the hub is an internal hub and the devices were not going to change (not plugged in or plugged out by the user), then you probably don't need a big capacitor because you won't have voltage spikes from hot swapping, only during power up. I'd go with a standard bypass value like 1uF.

If, however, you were designing for a consumer device and you don't know what the load is going to be (could be a 2A phone, which will be a heavy load) they you could have a problem, you may want to go beyond the 120uf.

My point is they are recommendations not requirements (or strong recommendations), the manufacture recommended them because they probably had customers that forgot to put them in and they complained. Or they experienced dropouts during testing, so they put the recommendations in the sheet. Since your application is different you can ignore that recommendation.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.