March 2006

9

M9999-031306

MIC2551A-2.5

Micrel, Inc.

PCB Layout Recommendations

Although the USB standard and applications are not based in

an impedance-controlled environment, a properly designed

PCB layout is recommended for optimal transceiver perfor-

mance. The suggested PCB layout hints are as follows:

• Match signal line traces (VP/VM, D+, D–) to

40ps, approximately one-third inch if possible.

FR-4 PCB material propagation is about 150ps/

inch, so to minimize skew try to keep VP/VM,

D+/D– traces as short as possible.

• For every signal line trace width (w), separate

the signal lines by 1.5 – 2 widths. Place all other

traces at >2 widths from all signal line traces.

• Maintain the same number of vias on each dif-

ferential trace, keeping traces approximately at

same separation distance along the line.

• Control signal line impedances to ±10%.

D+ and D–.

Configuration Mode

VBUS/VTRM

VIF

Notes

Normal

Connected

Connected

Normal supply configuration and operation.

Disconnect (D+/D-

sharing)

Open

Connected

VP/VM are HIGH outputs, RCV is LOW.

With OE# = 0 and SUS = 1, data lines may be driven with external

devices up to 3.6V.

Disconnect

Ground

Connected

VP/VM are HIGH outputs, RCV is LOW.

Disable Mode

Connected

Open

Logic controlled input pins are Hi-Z. No communication is possible

until interface voltage is restored.

Unpowered

Connected

Ground

Inoperative

Table 1. Power Supply Configuration