RT9706 datasheet(13/14 Pages) RICHTEK

13 / 14 page

RT9706

DS9706-02 March 2007

www.richtek.com

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 RT9706 as close as possible to the output

port to limit switching noise.

ESD

Because USB is a hot insertion and removal system, USB

components (especially the connector pins) are subject

to electrostatic discharge (ESD) and should be qualified

to IEC801.2. The RT9706 is designed to withstand a 8kV

human body mode, as defined in MIL-STD-883C. The

requirements in IEC801.2 are much more stringent and

require additional capacitors for the RT9706 to withstand

the higher ESD energy.

Low-ESR 1

μF ceramic bypass capacitors and output

capacitors should be placed as closely as possible to the

VIN and VOUT pins to increase the ESD immunity. The

RT9706 may pass the requirements of IEC 1000-4-2

(EN50082-1) level-4 for 15kV air discharge and 8kV contact

discharge tests when these capacitors are added.

GND

FLG

GND_BUS

USB

Controller

Board Layout

OUT

BUS

RSWITCH : Resistance of power switch

(80m

Ω typical for RT9706)

VPCB : PCB voltage drop

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 RT9706, with its maximum

100m

Ω on-resistance over temperature, easily meets this

requirement.

Locate the ceramic bypass capacitors as close as

possible to the VIN pins of the RT9706.

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