RT9724
8
DS9724-02 July 2012www.richtek.com
©
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.
Applications Information
The RT9724 is a single N-MOSFET high-side power
switches with enable input, optimized for self-powered and
bus-powered Universal Serial Bus (USB) applications. The
RT9724 is equipped with a charge pump circuitry to drive
the internal N-MOSFET switch; the switch's low R
DS(ON)
,
100mΩ, meets USB voltage drop requirements.
Input and Output
V
IN
(input) is the power source connection to the internal
circuitry and the drain of the MOSFET. V
OUT
(output) is
the source of the MOSFET. In a typical application, current
flows through the switch from V
IN
to V
OUT
toward the load.
If V
OUT
is greater than V
IN
, current will flow from V
OUT
to
V
IN
since the MOSFET is bidirectional when on.
Unlike a normal MOSFET, there is no parasitic body diode
between drain and source of the MOSFET, the RT9724
prevents reverse current flow if V
OUT
is externally forced
to a higher voltage than V
IN
when the chip is disabled
(V
EN
< 0.8V).
Chip Enable Input
The switch will be disabled when the EN pin is in a logic
low condition. During this condition, the internal circuitry
and MOSFET will be turned off, reducing the supply current
to 0.1μA typical. Floating the EN may cause unpredictable
operation. EN should not be allowed to go negative with
respect to GND. The EN pin may be directly tied to V
IN
to
keep the part on.
Soft Start for Hot Plug-In Applications
In order to eliminate the upstream voltage droop caused
by the large inrush current during hot-plug events, the
“soft-start” feature effectively isolates the power source
from extremely large capacitive loads, satisfying the USB
voltage droop requirements.
Under Voltage Lockout
Under Voltage Lockout (UVLO) prevents the MOSFET
switch from turning on until the input voltage exceeds
approximately 1.75V. If input voltage drops below
approximately 1.7V, UVLO turns off the MOSFET switch.
Under-voltage detection functions only when the switch
is enabled.
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 of typically 2A.
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.
D
G
S
D
G
S
Normal MOSFET RT9724
