rt9711pb Richtek Technology Corporation, rt9711pb Datasheet - Page 13

• Manufacturer Part Number: rt9711pb
• Description: 1.5a High-side Power Switches With Flag
• Manufacturer: Richtek Technology Corporation

The USB specification defines the maximum resistance
per contact (R
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 (I
the maximum resistance of the cable is 90mΩ.
The resistance of the switch is defined as follows :
R
If the voltage drop across the PCB is limited to 100mV,
the maximum resistance for the switch is 250mΩ for four
ports ganged together. The RT9711, with its maximum
100mΩ 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 :
DS9711-04 March 2007
SWITCH
Keep all V
least 50-mil, 2 ounce copper for all V
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.
= { 4.75V
= (200mV
90mΩ) ]
BUS
CONN
traces as short as possible and use at
−
) of the USB connector to be 30mΩ
−
−
4.4V
V
V
PCB
PCB
−
)
}
[ 0.5A x ( 4 x 30mΩ + 2 x
÷
÷
( 0.1A x N
( 0.1A x N
BUS
PORTS
PORTS
traces.
I
) of 500mA,
)
)
Preliminary
GND_BUS
V
BUS
Board Layout
V
OUT
FLG
GND
RT9711
www.richtek.com
V
IN
EN
13