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RT9741AGV

P1-P3P4-P6P7-P9P10-P11
RT9741
7
DS9741-01 March 2015 www.richtek.com
©
Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The RT9741 is a single N-MOSFET high-side power
switches, optimized for self-powered and bus-powered
Universal Serial Bus (USB) applications. The RT9741 is
equipped with a charge pump circuitry to drive the internal
N-MOSFET switch; the switch's low R
DS(ON)
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.
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 input the voltage exceeds
approximately 1.7V. If input voltage drops below
approximately 1.3V, UVLO turns off the MOSFET switch.
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
through the switch of the RT9741A and 1A for RT9741C
respectively. When a heavy load or short circuit is applied
to 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 RT9741.
When the operation junction temperature exceeds 120°C,
the OTP circuit starts the thermal shutdown function and
turns the pass element off. The pass element turns on
again after the junction temperature cools to 80°C. The
RT9741 lowers its OTP trip level from 120°C to 100°C
when output short circuit occurs (V
OUT
< 1V) as shown in
Figure 1.
Figure 1. Short Circuit Thermal Folded Back Protection
when Output Short Circuit Occurs (Patent)
V
OUT
Short to GND
1V
V
OUT
I
OUT
Thermal
Shutdown
OTP Trip Point
120 C
100 C
100 C
80 C
IC Temperature
Power Dissipation
The junction temperature of the RT9741 series depends
on several factors such as the load, PCB layout, ambient
temperature and package type. The output pin of the
RT9741 can deliver the current of up to 2A (RT9741A) and
1A (RT9741C) respectively over the full operating junction
temperature range. However, the maximum output current
must be decreased at higher ambient temperature to
ensure the junction temperature does not exceed 100°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
RT9741
8
DS9741-01 March 2015www.richtek.com
©
Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
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
RT9741
9
DS9741-01 March 2015 www.richtek.com
©
Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
connector pins are recommended for EMI and ESD
protection. The bypass capacitor itself should have a low
dissipation factor to allow decoupling at higher frequencies.
Voltage Drop
The USB specification states a minimum port-output
voltage in two locations on the bus, 4.75V out of a Self-
Powered Hub port and 4.40V out of a Bus-Powered Hub
port. As with the Self-Powered Hub, all resistive voltage
drops for the Bus-Powered Hub must be accounted for to
guarantee voltage regulation.
The following calculation determines V
OUT(MIN)
for multiple
ports (N
PORTS
) ganged together through one switch (if using
one switch per port, N
PORTS
is equal to 1) :
V
OUT(MIN)
= 4.75V − [I
I
x ( 4 x R
CONN
+ 2 x R
CABLE
)] −
(0.1A x N
PORTS
x R
SWITCH
) − V
PCB
Where
R
CONN
= Resistance of connector contacts (two contacts
per connector)
R
CABLE
= Resistance of upstream cable wires (one 5V
and one GND)
R
SWITCH
= Resistance of power switch (typical 100mΩ for
RT9741A and 120mΩ for RT9741C)
V
PCB
= PCB voltage drop
The USB specification defines the maximum resistance
per contact (R
CONN
) of the USB connector to be 30mΩ
and the drop across the PCB and switch to be 100mV.
This basically leaves two variables in the equation: the
resistance of the switch and the resistance of the cable. If
the hub consumes the maximum current (I
I
) of 500mA,
the maximum resistance of the cable is 90mΩ. The
resistance of the switch is defined as follows :
R
SWITCH
= { 4.75V − 4.4V − [ 0.5A x ( 4 x 30mΩ + 2 x
90mΩ) ] − V
PCB
}
( 0.1A x N
PORTS
)
= (200mV − V
PCB
)
( 0.1A x N
PORTS
)
If the voltage drop across the PCB is limited to 100mV,
the maximum resistance for the switch is 250mΩ for four
ports ganged together. The RT9741, with its maximum
120mΩ on resistance can fit the demand of this
requirement.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA
where T
J(MAX)
is the maximum junction temperature, T
A
is
the ambient temperature, and θ
JA
is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θ
JA
, is layout dependent. For
SOT-23-3 package, the thermal resistance, θ
JA
, is
243.3°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at T
A
= 25°C
can be calculated by the following formula :
P
D(MAX)
= (125°C − 25°C) / (243.3°C/W) = 0.41W for
SOT-23-3 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance, θ
JA
. The derating curve in Figure 2 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Figure 2. Derating Curve of Maximum Power Dissipation
0.0
0.1
0.2
0.3
0.4
0.5
0 25 50 75 100 125
Ambient Temperature (°C)
Maximum Power Dissipation (W) 1
Four-Layer PCB
P1-P3P4-P6P7-P9P10-P11

RT9741AGV

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IC POWER SWITCH USB SOT-23-3
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