RT9715CA

10

DS9715CA-00 April 2015

www.richtek.com

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Copyright 2015 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 RT9715CA power distribution

allows designers to design hubs that can operate through

faults. The RT9715CA 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 1

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 6V

of the absolute maximum supply voltage even for a short

duration.

Output Filter Capacitor

A low-ESR 150

μF aluminum electrolytic or tantalum

(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

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 4.40V 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).

Where

(two contacts per connector)

(one 5V and one GND)

(90m

Ω typical for RT9715CA)

The USB specification defines the maximum resistance

Ω

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.

the maximum resistance of the cable is 90m

Ω.

The resistance of the switch is defined as follows :

90m

= (200mV

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 RT9715CA, with its maximum

110m

Ω on-resistance over temperature, can fit the demand

of this requirement.