RT9711GS datasheet(12/15 Pages) RICHTEK

12 / 15 page

RT9711

Preliminary

DS9711-04 March 2007

www.richtek.com

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 RT9711 power distribution allow

designers to design hubs that can operate through faults.

The RT9711 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 1

μF low-ESR ceramic capacitor from VIN 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 6.5V

of the absolute maximum supply voltage even for a short

duration.

Output Filter Capacitor

A low-ESR 150

μF aluminum electrolytic or tantalum

between VOUT and GND is strongly recommended to meet

the 330mV maximum droop requirement in the hub VBUS

(Per USB 2.0, output ports must have a minimum 120

μF

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 VBUS, the ground

line and the 0.1

μF 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 4.4V 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 VOUT (MIN) for multi-

ple ports (NPORTS) ganged together through one switch (if

using one switch per port, NPORTS is equal to 1) :

VOUT (MIN) = 4.75V

− [ II x ( 4 x RCONN + 2 x RCABLE ) ] −

(0.1A x NPORTS x RSWITCH )

− VPCB

Where

RCONN : Resistance of connector contacts

(two contacts per connector)

RCABLE : Resistance of upstream cable wires

(one 5V and one GND)

RSWITCH : Resistance of power switch

(80m

Ω typical for RT9711)

VPCB : PCB voltage drop

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