RT9742
13
DS9742-07 June 2017 www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
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 RT9742 power distribution allows
designers to design hubs that can operate through faults.
The RT9742 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 10μF low-ESR ceramic capacitor from V
IN
to GND,
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 7V
of the absolute maximum supply voltage even for a short
duration.
Output Filter Capacitor
A low-ESR 150μF aluminum electrolytic or tantalum
between VOUT and GND is strongly recommended to
meet the 330mV maximum droop requirement in the hub
V
BUS
(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
BUS
, the ground 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.
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 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
OUT(MIN)
for multi-
ple ports (N
PORTS
) ganged together through one switch (if
using one switch per port, N
PORTS
is equal to 1) :
V
OUT (MIN)
= 4.75V − [ I
I
x ( 4 x R
CONN
+ 2 x R
CABLE
) ] −
(0.1A x N
PORTS
x R
SWITCH
) − V
PCB
Where
R
CONN
= Resistance of connector contacts
(two contacts per connector)
R
CABLE
= Resistance of upstream cable wires
(one 5V and one GND)
R
SWITCH
= Resistance of power switch
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
)
