Preliminary
Application Information
The RT9701B is a high-side single N-Channel
MOSFET
switch with active-high enable input.
Input and Output
VIN (input) is the power supply connection to the circuitry
and the drain of the output MOSFET. VOUT (output) is
the source of the output MOSFET. In a typical circuit,
current flows through the switch from VIN to VOUT toward
the load. Both VOUT pins must be short on the board
and connected to the load and so do both VIN pins but
connected to the power source.
Thermal Shutdown
Thermal shutdown shuts off the output MOSFET if the
die temperature exceeds 130°C and 20°C of hysteresis
forces the switch turning off until the die temperature
drops to 110°C.
Soft Start
In order to eliminate the upstream voltage droop caused
by the large inrush current during hot-plug events, the
“
soft-start
”
feature effectively isolates power supplies
from such highly capacitive loads.
Under-voltage Lockout
UVLO prevents the
MOSFET switch from turning on until
input voltage exceeds 3V (typical). If input voltage drops
below 3V (typical), UVLO shuts
off the MOSFET switch.
Current Limiting and Short Protection
The current limit circuit is designed to protect the system
supply, the MOSFET switch and the load from damage
caused by excessive currents. The current limit threshold
is set internally to allow a minimum of 1.1A through the
MOSFET but limits the output current to approximately
1.5A typical. When the output is short to ground, it will
limit to a constant current 1A until thermal shutdown or
short condition removed.
Filtering
To limit the input voltage drop during hot-plug events,
connect a 1μF ceramic capacitor from VIN to GND.
However, higher capacitor values will further reduce the
voltage drop at the input.
DS9701B-04 March 2007
RT9701B
Connect a sufficient capacitor from VOUT to GND. This
capacitor helps to prevent inductive parasitics from pulling
VOUT negative during turn-off or EMI damage to other
components during the hot-detachment. It is also
necessary for meeting the USB specification during hot
plug-in operation. If RT9701B is implanted in device end
application, minimum 1μF capacitor from VOUT to GND
is recommended and higher capacitor values are also
preferred.
In choosing these capacitors, special attention must be
paid to the Effective Series Resistance, ESR, of the
capacitors to minimize the IR drop across the capacitor
ESR. A lower ESR on this capacitor can get a lower IR
drop during the operation.
Ferrite beads in series with all power and ground lines are
recommended to eliminate or significantly reduce EMI. In
selecting a ferrite bead, the DC resistance of the wire
used must be kept to a minimum to reduce the voltage
drop.
Reverse current preventing
The output MOSFET and driver circuitry are also designed
to allow the MOSFET source to be externally forced to a
higher voltage than the drain (VOUT > VIN
≥
0). To prevent
reverse current from such condition, disable the switch
(RT9701BCB) or connect VIN to a fixed voltage under
UVLO.
Layout and Thermal Dissipation
Place the switch as close to the USB connector as
possible. Keep all traces as short as possible to reduce
the effect of undesirable parasitic inductance.
Place the ot capacitor and ferrite beads asclose to the
USB connector as possible.
If ferrite beads are used, use wires with minimum
resistance and large solder pads to minimize connection
resistance.
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