RT9711 datasheet(13/15 Pages) RICHTEK

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RT9711

Preliminary

DS9711-04 March 2007

www.richtek.com

The USB specification defines the maximum resistance

per contact (RCONN) of the USB connector to be 30m

and the drop across the PCB and switch to be 100mV.

This basically leaves two variables in the equation : the

resistance of the switch and the resistance of the cable.

If the hub consumes the maximum current (II) of 500mA,

the maximum resistance of the cable is 90m

Ω.

The resistance of the switch is defined as follows :

RSWITCH = { 4.75V

− 4.4V − [ 0.5A x ( 4 x 30mΩ + 2 x

90m

Ω) ]

− VPCB } ÷ ( 0.1A x NPORTS )

= (200mV

− VPCB ) ÷ ( 0.1A x NPORTS )

If the voltage drop across the PCB is limited to 100mV,

the maximum resistance for the switch is 250m

Ω for four

ports ganged together. The RT9711, with its maximum

100m

Ω on-resistance over temperature, easily meets this

requirement.

PCB Layout

In order to meet the voltage drop, droop, and EMI

requirements, careful PCB layout is necessary. The

following guidelines must be considered :

Keep all VBUS traces as short as possible and use at

least 50-mil, 2 ounce copper for all VBUS traces.

Avoid vias as much as possible. If vias are necessary,

make them as large as feasible.

Place a ground plane under all circuitry to lower both

resistance and inductance and improve DC and transient

performance (Use a separate ground and power plans if

possible).

Place cuts in the ground plane between ports to help

reduce the coupling of transients between ports.

Locate the output capacitor and ferrite beads as close

to the USB connectors as possible to lower impedance

(mainly inductance) between the port and the capacitor

and improve transient load performance.

Locate the RT9711 as close as possible to the output

port to limit switching noise.

Locate the ceramic bypass capacitors as close as

possible to the VIN pins of the RT9711.

Board Layout

GND

GND_BUS

OUT

BUS

FLG

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