RT9702PB datasheet(13/16 Pages) RICHTEK

13 / 16 page

RT9702/A

DS9702/A-11

June 2007

www.richtek.com

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.

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 (10k

Ω 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 4.40V 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 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 • RCONN + 2 • 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 RT9702/A)

VPCB = PCB voltage drop

The USB specification defines the maximum resistance

per contact (RCONN) of the USB connector to be 30m

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 (II) of 500mA,

the maximum resistance of the cable is 90m

Ω.

The resistance of the switch is defined as follows:

RSWITCH = { 4.75V - 4.4V -[ 0.5A x ( 4

• 30m

Ω + 2 • 90mΩ

) ]-VPCB }

÷ ( 0.1A x NPORTS )

= (200mV - VPCB )

÷ ( 0.1A x NPORTS )

If the voltage drop across the PCB is limited to 100mV,

the maximum resistance for the switch is 250m

Ω for four

ports ganged together. The RT9702/A, with its maximum

100m

Ω on-resistance over temperature, easily meets this

requirement.

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.

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