RT9715 RICHTEK [Richtek Technology Corporation], RT9715 Datasheet - Page 10

• Manufacturer Part Number: RT9715
• Description: 90m?, 2A/1.5A/1.1A/0.7A High-Side Power Switches with Flag
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9715
The faster trip time of the RT9715 power distribution allows
designers to design hubs that can operate through faults.
The RT9715 provides low on-resistance and internal fault-
reporting circuitry to meet voltage regulation and fault
notification requirements.
Because the devices are also power switches, the designer
of self-powered hubs has the flexibility to turn off power to
output ports. Unlike a normal MOSFET, the devices have
controlled rise and fall times to provide the needed inrush
current limiting required for the bus-powered hub power
switch.
Supply Filter/Bypass Capacitor
A 1uF low-ESR ceramic capacitor from V
at the device is strongly recommended to prevent the input
voltage drooping during hot-plug events. However, higher
capacitor values will further reduce the voltage droop on
the input. Furthermore, without the bypass capacitor, an
output short may cause sufficient ringing on the input (from
source lead inductance) to destroy the internal control
circuitry. The input transient must not exceed 6V of the
absolute maximum supply voltage even for a short duration.
Output Filter Capacitor
A low-ESR 150uF aluminum electrolytic or tantalum
between V
the 330mV maximum droop requirement in the hub V
(Per USB 2.0, output ports must have a minimum 120uF
of low-ESR bulk capacitance per hub). Standard bypass
methods should be used to minimize inductance and
resistance between the bypass capacitor and the
downstream connector to reduce EMI and decouple voltage
droop caused when downstream cables are hot-insertion
transients. Ferrite beads in series with V
line and the 0.1uF bypass capacitors at the power connector
pins are recommended for EMI and ESD protection. The
bypass capacitor itself should have a low dissipation factor
to allow decoupling at higher frequencies.
Voltage Drop
The USB specification states a minimum port-output voltage
in two locations on the bus, 4.75V out of a Self-Powered
Hub port and 4.40V out of a Bus-Powered Hub port. As
with the Self-Powered Hub, all resistive voltage drops for
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10
OUT
and GND is strongly recommended to meet
IN
BUS
to GND, located
, the ground
BUS
the Bus-Powered Hub must be accounted for to guarantee
voltage regulation (see Figure 7-47 of Universal Serial
Specification Revision 2.0 ).
The following calculation determines V
ple ports (N
using one switch per port, N
V
Where
R
R
R
V
The USB specification defines the maximum resistance
per contact (R
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 RT9715, with its maximum
100mΩ on-resistance over temperature, can fit the demand
of this requirement.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum operation junction temperature. The maximum
power dissipation depends on the thermal resistance of IC
package, PCB layout, the rate of surroundings airflow and
temperature difference between junction to ambient. The
OUT (MIN)
PCB
CONN
CABLE
SWITCH
SWITCH
= PCB voltage drop
= Resistance of connector contacts
= Resistance of upstream cable wires
= (200mV − V
= Resistance of power switch
= { 4.75V − 4.4V − [ 0.5A x ( 4
= 4.75V − [ I
90mΩ) ] − V
(90mΩ typical for RT9715)
(two contacts per connector)
(one 5V and one GND)
PORTS
(0.1A x N
CONN
) ganged together through one switch (if
) of the USB connector to be 30mΩ and
PORTS
PCB
I
PCB
x ( 4
)
}
x R
÷
÷
PORTS
x
( 0.1A x N
( 0.1A x N
R
SWITCH
CONN
is equal to 1) :
DS9715-03 April 2011
+ 2
) − V
OUT (MIN)
PORTS
PORTS
x
30mΩ + 2 x
x
PCB
R
I
CABLE
) of 500mA,
)
)
for multi-
) ] −