RT9741

8

DS9741-01 March 2015

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Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

current, but the application may limit the amount of output

current based on the total power dissipation and the

ambient temperature. The final operating junction

temperature for any set of conditions can be estimated

by the following thermal equation:

the die (100

maximum ambient temperature. The junction to ambient

thermal resistance (

minimum footprint is 243.3

Universal Serial Bus (USB) & Power Distribution

The goal of USB is to enable device from different vendors

to interoperate in an open architecture. USB features

include ease of use for the end user, a wide range of

workloads and applications, robustness, synergy with the

PC industry, and low-cost implementation. Benefits

include self-identifying peripherals, dynamically attachable

and reconfigurable peripherals, multiple connections

(support for concurrent operation of many devices), support

for as many as 127 physical devices, and compatibility

with PC Plug-and-Play architecture.

The Universal Serial Bus connects USB devices with a

USB host: each USB system has one USB host. USB

devices are classified either as hubs, which provide

additional attachment points to the USB, or as functions,

which provide capabilities to the system (for example, a

digital joystick). Hub devices are then classified as either

Bus-Power Hubs or Self-Powered Hubs.

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 poly-fuse or poly-switch) have slow

trip times, high on-resistance, and lack the necessary

circuitry for USB-required fault reporting.

The faster trip time of the RT9741 power distribution allows

designers to design hubs that can operate through faults.

The RT9741 provides low on-resistance to meet voltage

regulation and fault notification requirements.

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

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