RT9712

10

DS9712-04 November 2012

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Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

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 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 RT9712A/B/C/D power

distribution allow designers to design hubs that can operate

through faults. The RT9712A/B/C/D provide 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 0.1

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.

Figure 1. Current Limit

Short Circuit

Fold-back Current

Output Current (A)

Current Limit

Trigger Point

When the load current reaches the trigger point (typically

3.5A for RT9712A/B and 2.8A for RT9712C/D) or short

circuit is applied to the output, the device will enter constant

current mode until the thermal shutdown occurs or the

fault is removed.