RT9702PB datasheet(12/16 Pages) RICHTEK

12 / 16 page

RT9702/A

DS9702/A-11

June 2007

www.richtek.com

Thermal Shutdown

Thermal shutdown is employed to protect the device from

damage if the die temperature exceeds approxi- mately

130

°C. If enabled, the switch automatically restarts when

the die temperature falls 20

°C. The output and FLG

signal will continue to cycle on and off until the device is

disabled or the fault is removed.

Power Dissipation

The device s junction temperature depends on several

factors such as the load, PCB layout, ambient temperature

and package type. The output pin of RT9702/A can deliver

a current of up to 500mA, and 1.1A respectively over the

full operating junction temperature range. However, the

maximum output current must be derated at higher

ambient temperature to ensure the junction temperature

does not exceed 100

°C. With all possible conditions, the

junction temperature must be within the range specified

under operating conditions. Power dissipation can be

calculated based on the output current and the RDS(ON) of

switch as below.

PD = RDS(ON) x IOUT2

Although the devices are rated for 500mA and 1.1A of

output 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:

PD (MAX) = ( TJ (MAX) - TA ) /

θJA

Where TJ (MAX) is the maximum junction temperature of

the die (100

°C) and TA is the maximum ambient

temperature. The junction to ambient thermal resistance

(

θJA)for SOT-23-5 andTSOT-23-5 package at recommended

minimum footprint is 250

°C/W (θJA is layout dependent).

Universal Serial Bus (USB) & Power Distribution

The goal of USB is to be enabled 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 implement- ation. 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 RT9702/A power distribution

allow designers to design hubs that can operate through

faults. The RT9702/A have low on-resistance and internal

fault-reporting circuitry that help the designer to meet

voltage regulation and fault notification requirements.

until the current limit circuitry responds. Once this current

limit threshold is exceeded the device enters constant

current mode until the thermal shutdown occurs or the

fault is removed.

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