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RT9711AGB

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P17
RT9711A/B/C/D
10
DS9711A/B/C/D-03 April 2011www.richtek.com
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 RT9711 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 130°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 t
D
is typically 10ms.
Under-Voltage Lockout
Under-voltage lockout (UVLO) prevents the MOSFET
switch from turning on until input voltage exceeds
approximately 1.7V. If input voltage drops below
approximately 1.3V, UVLO turns off the MOSFET switch,
FLG will be asserted accordingly. 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 2.5A
through the switch of RT9711A/B and 1A for RT9711C/D
respectively. 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
Applications Information
The RT9711A/B/C/D are single N-MOSFET high-side
power switches with enable input, optimized for self-
powered and bus-powered Universal Serial Bus (USB)
applications. The RT9711 series are 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
RT9711A/B/C/D prevents reverse current flow if V
OUT
being
externally forced to a higher voltage than V
IN
when the
output disabled (V
EN
< 0.8V or V
EN
> 2V).
D
G
S
D
G
S
Normal MOSFET RT9711A/B/C/D
Chip Enable Input
The switch will be disabled when the EN/EN pin is in a
logic low/high condition. During this condition, the internal
circuitry and MOSFET are turned off, reducing the supply
current to 0.1μA typical. Floating the EN/EN may cause
unpredictable operation. EN should not be allowed to go
negative with respect to GND. The EN/EN pin may be
directly tied to V
IN
(GND) to keep the part on.
RT9711A/B/C/D
11
DS9711A/B/C/D-03 April 2011 www.richtek.com
limit threshold is exceeded the device enters constant
current mode until the thermal shutdown occurs or the
fault is removed.
And for SOP-8 and MSOP-8 packages, the thermal
resistance θ
JA
is 160°C/W. The maximum power
dissipation at T
A
= 25°C can be calculated by following
formula :
P
D(MAX)
= (125°C − 25°C) / 250°C/W = 0.4W for
SOT-23-5 and TSOT-23-5 packages
P
D(MAX)
= (125°C − 25°C) / 160°C/W = 0.625W for
SOP-8 and MSOP-8 packages
The maximum power dissipation depends on operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance θ
JA
. For RT9711A/B/C/D packages, the Figure
1 of derating curves allows the designer to see the effect
of rising ambient temperature on the maximum power
allowed.
Figure 1. Derating Curves for RT9711A/B/C/D Package
Universal Serial Bus (USB) & Power Distribution
The goal of USB is to be enabled 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 implement- ation. 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
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 25 50 75 100 125
Ambient Temperature
Maximum Power Dissipation (W)
Single Layer PCB
(°C)
SOP-8, MSOP-8
SOT-23-5, TSOT-23-5
Thermal Shutdown
Thermal shutdown is employed to protect the device from
damage if the die temperature exceeds approxi- mately
130°C. If enabled, the switch automatically restarts when
the die temperature falls 20°C. The output and FLG signal
will continue to cycle on and off until the device is disabled
or the fault is removed.
Power Dissipation
The junction temperature of the RT9711 series depend on
several factors such as the load, PCB layout, ambient
temperature and package type. The output pin of
RT9711A/B/C/D can deliver the current of up to 1.5A
(RT9711A/B), and 0.6A (RT9711C/D) respectively over the
full operating junction temperature range. However, the
maximum output current must be derated 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
switch as below.
P
D
= R
DS(ON)
x I
OUT
2
Although the devices are rated for 1.5A and 0.6A 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 operation junction
temperature 125°C, T
A
is the ambient temperature and the
θ
JA
is the junction to ambient thermal resistance.
The junction to ambient thermal resistance θ
JA
is layout
dependent. For SOT-23-5 and TSOT-23-5 packages, the
thermal resistance θ
JA
is 250°C/W on the standard JEDEC
51-3 single-layer thermal test board.
RT9711A/B/C/D
12
DS9711A/B/C/D-03 April 2011www.richtek.com
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 polyfuse or polyswitch) have slow
trip times, high on-resistance, and lack the necessary
circuitry for USB-required fault reporting.
The faster trip time of the RT9711A/B/C/D power
distribution allow designers to design hubs that can operate
through faults. The RT9711A/B/C/D have low on-resistance
and internal fault-reporting circuitry that help the designer
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 6.5V
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
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 (see Figure 7-47 of Universal
Serial Specification Revision 2.0 ).
The following calculation determines V
OUT (MIN)
for multi-
ple 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
(80mΩ typical for RT9711A/B/C/D)
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P17

RT9711AGB

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IC PWR SWITCH USB 1.5A SOT23-5
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