RT9742
12
DS9742-07 June 2017www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Current Limiting and Short-Circuit Protection
The current limit circuitry prevents damage to the MOSFET
switch and the hub downstream port but can deliver load
current up to the current limit threshold. When a heavy
load or short circuit is applied to an enabled switch, a
large transient current may flow 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.
Thermal Shutdown
Thermal protection limits the power dissipation in the
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 the current limit
threshold 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 current
and the R
DS(ON)
of the switch as below.
P
D
= R
DS(ON)
x I
OUT
2
Although the devices are rated for 3A, 2.5A, 2A, 1.5A, 1A
and 0.5A , 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 :
P
D (MAX)
= ( T
J (MAX)
- T
A
) / θ
JA
Where T
J (MAX)
is the maximum junction temperature of
the die (125°C) and T
A
is the maximum ambient
temperature.
The junction to ambient thermal resistance (θ
JA
) for TSOT-
23-5 package at recommended minimum footprint is
203°C/W (θ
JA
is layout dependent).
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.
