RT9711A/B/C/D
13
DS9711A/B/C/D-03 April 2011 www.richtek.com
V
PCB
= PCB voltage drop
The USB specification defines the maximum resistance
per contact (R
CONN
) of the USB connector to be 30mΩ
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
I
) of 500mA,
the maximum resistance of the cable is 90mΩ.
The resistance of the switch is defined as follows :
R
SWITCH
= { 4.75V − 4.4V − [ 0.5A x ( 4 x 30mΩ + 2 x
90mΩ) ] − V
PCB
}
÷
( 0.1A x N
PORTS
)
= (200mV − V
PCB
)
÷
( 0.1A x N
PORTS
)
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 RT9711A/B/C/D, with its
maximum 100mΩ on-resistance over temperature, easily
meets this requirement.
Layout Considerations
For best performance of the RT9711 series, the following
guidelines muse be strictly followed :
` Input and output capacitors should be placed close to
the IC and connected to ground plane to reduce noise
coupling.
` The GND should be connected to a strong ground plane
for heat sink.
` Keep the main current traces as possible as short and
wide.
GND
EN
V
IN
V
OUT
The input and output capacitors should be
placed as close as possible to the IC.
GND
FLG
V
IN
Figure 2. PCB Layout Guide
