RT9741 Datasheet, PDF (8/11 Pages) Richtek Technology Corporation – High-Side Power Switches

RT9741
Although the devices are rated for 2A 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:
P_D(MAX) = ( T_{J (MAX)} − T_A ) / θ_JA
Where T_{J (MAX)} is the maximum junction temperature of
the die (100°C) and T_A is the
maximum ambient temperature. The junction to ambient
thermal resistance (θ_JA) for SOT-23-3 at recommended
minimum footprint is 243.3°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.
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.
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 1μF low-ESR ceramic capacitor from V_IN to GND, 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.
Output Filter Capacitor
A low-ESR 150μF aluminum electrolytic or tantalum
between V_OUT and GND is strongly recommended to meet
the 330mV maximum droop requirement in the hub V_BUS
(Per USB 2.0, output ports must have a minimum 120μF
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
transients. Ferrite beads in series with V_BUS, the ground
line and the 0.1μF bypass capacitors at the power
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is a registered trademark of Richtek Technology Corporation.
DS9741-01 March 2015