rt9706 Richtek Technology Corporation, rt9706 Datasheet - Page 12

• Manufacturer Part Number: rt9706
• Description: 500ma High-side Power Switch With Flag
• Manufacturer: Richtek Technology Corporation

RT9706
A Bus-Powered Hub draws all of the power to any internal
functions and downstream ports from the USB connector
power pins. The hub may draw up to 500mA from the
upstream device. External ports in a Bus-Powered Hub
can supply up to 100mA per port, with a maximum of four
external ports.
Self-Powered Hub power for the internal functions and
downstream ports does not come from the USB, although
the USB interface may draw up to 100mA from its
upstream connect, to allow the interface to function when
the remainder of the hub is powered down. The hub must
be able to supply up to 500mA on all of its external
downstream ports. Please refer to Universal Serial
Specification Revision 2.0 for more details on designing
compliant USB hub and host systems.
Over-Current protection devices such as fuses and PTC
resistors (also called polyfuse or polyswitch) have slow
trip times, high on-resistance, and lack the necessary
circuitry for USB-required fault reporting.
The faster trip time of the RT9706 power distribution allow
designers to design hubs that can operate through faults.
The RT9706 have low on-resistance and internal fault-
reporting circuitry that help the designer 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 1μF low-ESR ceramic capacitor from V
located 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 6.5V
of the absolute maximum supply voltage even for a short
duration.
www.richtek.com
12
IN
to GND,
Output Filter Capacitor
A low-ESR 150μF 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 120μF
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 0.1μF 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.
Fault Flag Filtering (Optional)
The transient inrush current to downstream capacitance
may cause a short-duration error flag, which may cause
erroneous over-current reporting. A simple 1ms RC low-
pass filter (10kΩ and 0.1μF) in the flag line (see Typical
Application Circuit) eliminates short-duration transients.
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 4.4V out of a Bus-Powered Hub
port. As with the Self-Powered Hub, all resistive voltage
drops for 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
(two contacts per connector)
R
(one 5V and one GND)
OUT (MIN)
CONN
CABLE
: Resistance of connector contacts
: Resistance of upstream cable wires
= 4.75V
OUT
PORTS
(0.1A x N
and GND is strongly recommended to meet
) ganged together through one switch (if
−
[ I
PORTS
I
x ( 4 x R
x R
PORTS
SWITCH
DS9706-02 March 2007
CONN
is equal to 1) :
)
+ 2 x R
OUT (MIN)
−
BUS
V
PCB
, the ground
CABLE
for multi-
) ]
BUS
−