SCHEMATIC NOTES
The AN2720SC includes two onboard regulators. The USB suspend current specification
only permits a device to draw 500 uA in the suspend mode. The pullup resistor on D+
draws 200 uA by itself, without any assistance from the AN2720SC. The second
regulator allows each side to have its own 500 uA suspend budget.
The transistor and diode configuration on the BDPLUS pullup is used for power
management. While only one side is connected, it is important not draw power from the
host side even though it is physically connected. To simplify power management, we
have assumed power is expected to always come from the A side of the chip. So when
the A side is not connected and thus not receiving power, the B side must be
disconnected from the host power source since the peripheral is not in use. The pullup
on the B side is switchd by the AVBUS line via the transistor, electrically disconnecting
the B side when AVBUS is removed.
The “MNT” of the “MNT5” and “MNT6” in the JP1s USB-B represent mounting tabs on
the USB B connector. They are on the schematic because they must be attached to the
ground plane.
The schematic proposed will work for any number configurations, including the use of 2
B connectors or a complete cable with 2 A side connectors on each side of the chip.
The 24LC00 device is used for the VID and PID of the EZ-Link device. While we have
included the most space efficient one in the schematic, any version will suffice.
The SN75240 is a USB surge suppressor from Texas Instruments. This surge suppressor
is optional, but it does provide up to 8 KV of ESD protection. Since the USB interface
has wires exposed to the outside world, some level of ESD immunity higher than the
2KV level inherent to the AN2720SC is recommended.
Should there be difficulties in passing the difficult EMI standards, a white paper at the
Intel website should help guide the developer in resolving EMI problems. The white
paper explores the variety of different methods of EMI reduction. For reference, the
URL is http://developer.intel.com/design/USB/papers/EMI_APPS.HTM.
AN2720 Data Sheet Version 1.0
04/27/01
Page 15
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