RT9721A/B
9
DS9721A/B-02 April 2011 www.richtek.com
Applications Information
The RT9721A/B are single N-MOSFET high-side power
switch with active-high enable input, optimized for self-
powered and bus-powered Universal Serial Bus (USB)
applications. The RT9721A/B series equipped with a charge
pump circuitry to drive the internal N-MOSFET switch; the
switch's low R
DS(ON)
, 80mΩ, meets USB voltage drop
requirements; and a flag output is available to indicate fault
conditions to the local USB controller.
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 a parasitic body
diode between drain and source of the MOSFET, the
RT9721A/B prevent reverse current flow if V
OUT
being
externally forced to a higher voltage than V
IN
when the
chip is disabled (V
EN
< 0.8V).
D
G
S
D
G
S
Normal MOSFET RT9721A/B
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 are turned off, reducing the supply current
to 0.1μA typical. The maximum guaranteed voltage for a
logic low at the EN pin is 0.8V. A minimum guaranteed
voltage of 2V at the EN pin will turn the RT9721A/B on.
Floating the input may cause unpredictable operation. EN
should not be allowed to go negative with respect to GND.
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.
Fault Flag
The RT9721A/B series provides a FLG signal pin which is
an N-Channel open drain MOSFET output. This open drain
output goes low when V
OUT
< V
IN
− 1V, current limit or the
die temperature exceeds 150°C approximately. The FLG
output is capable of sinking a 10mA load to typically 200mV
above ground. The FLG pin requires a pull-up resistor, this
resistor should be large in value to reduce energy drain. A
100kΩ pull-up resistor works well for most applications. In
the case of an over-current condition, FLG will be asserted
only after the flag response delay time, t
D
, has elapsed.
This ensures that FLG is asserted only upon valid over-
current conditions and that erroneous error reporting is
eliminated.
For example, false over-current conditions may occur
during hot-plug events when extremely large capacitive
loads are connected and causes a high transient inrush
current that exceeds the current limit threshold. The FLG
response delay time tD is typically 12ms.
Under-Voltage Lockout
Under-voltage lockout (UVLO) prevents the MOSFET switch
from turning on until input voltage exceeds approximately
2.15V. If input voltage drops below approximately 2.05V,
UVLO turns off the MOSFET switch, FLG will be asserted
accordingly. Under-voltage detection functions only when
the switch is enabled.
Current Limiting 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 700mA
through the switch of RT9721A/B series. 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.
