RT9742

13

DS9742-00 July 2015

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Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

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.

Thermal Shutdown

Thermal protection limits the power dissipation in RT9742.

When the operation junction temperature exceeds 140

°C,

the OTP circuit starts the thermal shutdown function and

turns the pass element off. The pass element turn on again

after the junction temperature cools to 120

°C.

Power Dissipation

The junction temperature of the RT9742 series depend

on several factors such as the load, PCB layout, ambient

temperature and package type. The output pin of the

RT9742 can deliver the current of up to 3A (RT9742A/B),

2A (RT9742C/D/J), 1.5A (RT9742E/F/K/M) and 1A

(RT9742G/H/L) 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 125

°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

Although the devices are rated for 3A, 2A, 1.5A and 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 :

the die (125

temperature.

The junction to ambient thermal resistance (

23-5 package at recommended minimum footprint is

250

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.