5. Design Considerations

1. USB Host Controller (UHP)

External Parts.  A few external parts are required for the proper operation of the UHP:

  • With Panel-Card 35, pull-down resistors on each line of approximately 15 kΩ. These should be installed even if the UHP is not to be used at all in order to keep the signals from floating. Panel-Card 57 and Panel-Card 70 don't need them.

  • With Panel-Card 35, series resistors of 27 Ω (5%) on each line. Panel-Card 57 and Panel-Card 70 don't need them.

  • Small capacitors (e.g. 15pF) to ground on each line (optional).

  • ESD protection devices are recommended for applications which are subject to external contact. The restrictions with regard to capacitive loading have to be applied when selecting a protection device.

  • A circuit to generate the 5V VBUS supply voltage.

VBUS considerations for USB Host.  A USB host port has to provide a supply voltage VBUS of 5V +- 5% which has to be able to source a maximum of 500mA, or 100mA in case of battery operation. Please refer to the appropriate rules in the USB specification. A low ESR capacitor of at least 120µF has to be provided on VBUS in order to avoid excessive voltage drops during current spikes.

VBUS has to have an over-current protection. The over-current drawn temporarily on VBUS must not exceed 5A. Polymeric PTCs or solid state switches are recommended by the specification. Suitable PPTCs are "MultiFuse" (Bourns), "PolyFuse" (Wickmann/Littelfuse), "PolySwitch" (Raychem/Tyco).

It is required that the over-current condition can be detected by software, so that VBUS can be switched off or be reduced in power in such a case.

Layout considerations.  If external resistors are needed, they should be placed in the vicinity of the module's connector. The two traces of any of the differential pairs (USB-Host A+ and USB-Host A- , as well as USB-Host B+ and USB-Host B-) should not encircle large areas on the base board, in order to reduce signal distortion and noise. The are preferably routed closely in parallel to the USB connector.

USB High-Speed.  If designing USB High-Speed a wave impedance of 90 Ω on the traces should be respected. The traces shoud be routed as short as possible and in parallel with as low parallel capacitance as possible.