RT9702/A
Preliminary
80mΩ, 500mA/1.1A High-Side Power Switches with Flag
General Description
The RT9702 and RT9702A are cost-effective, low
voltage, single N-Channel MOSFET high-side power
switches, optimized for self-powered and buspowered Universal Serial Bus (USB) applications.
The RT9702/A equipped with a charge pump
circuitry to drive the internal MOSFET switch; the
switch’s low RDS(ON), 80mΩ, meets USB voltage drop
requirements; and a flag output is available to
indicate fault conditions to the local USB controller.
Additional features include soft-start to limit inrush
current during plug-in, thermal shutdown to prevent
catastrophic switch failure from high-current loads,
under-voltage lockout (UVLO) to ensure that the
device remains off unless there is a valid input
voltage present, fault current is limited to typically
800mA for RT9702 in single port and 1.5A for
RT9702A in dual ports in accordance with the USB
power requirements, lower quiescent current as
25µA making this device ideal for portable
battery-operated equipment.
The RT9702/A is available in SOT-25 package
requiring minimum board space and smallest
components.
Ordering Information
RT9702/A
Package Type
B : SOT-25
Operating Temperature Range
C: Commercial Standard
1.1A Output Current
Features
Compliant to USB Specifications
Built-In (Typically 80mΩ) N-Channel MOSFET
Output Can Be Forced Higher Than Input
(Off-State)
Low Supply Current:
25µA Typical at Switch On State
0.1µA Typical at Switch Off State
Guaranteed 500mA/RT9702 and 1.1A/RT9702A
Continuous Load Current
Wide Input Voltage Ranges: 2V to 5.5V
Open-Drain Fault Flag Output
Hot Plug-In Application (Soft-Start)
1.7V Typical Under-Voltage Lockout (UVLO)
Current Limiting Protection
Thermal Shutdown Protection
Reverse Current Flow Blocking (no “body
diode)
Smallest SOT-25 Package Minimizes Board
Space
UL Approved – E219878
Applications
USB Bus/Self Powered Hubs
USB Peripherals
ACPI Power Distribution
PC Card Hot Swap
Notebook, Motherboard PCs
Battery-Powered Equipment
Hot-Plug Power Supplies
Battery-Charger Circuits
Pin Configurations
Part Number
Pin Configurations
500mA Output Current
Marking Information
For marking information, contact our sales
representative directly or through a RichTek
distributor located in your area, otherwise visit our
website for detail.
DS9702A-01
March 2003
RT9702/ACB
(Plastic SOT-25)
5
1
4
2
3
TOP VIEW
1. CE
2. GND
3. FLG
4. VIN
5. VOUT
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1
RT9702/A
Preliminary
Typical Application Circuit
Pull-Up Resistor (10k to 100kΩ)
(Optional)
Flag Transient Filtering
10kΩ
Supply Voltage 5V
1µF
+
Ov er-current
FLG
VIN
USB Controller
0.1µF
RT9702/A
VOUT
On
VBUS
CE
GND
10µF
+
+
D+
150µF
D−
GND
Off
Ferrite
Beads
Data
Note: A low-ESR 150µF aluminum electrolytic or tantalum between VOUT and GND is strongly recommended
to meet the 330mV maximum droop requirement in the hub VBUS. (see Application Information Section for
further details)
Pin Description
Pin Name
Pin Function
VIN
Supply Input
VOUT
Switch Output
GND
Common Ground
CE
Chip Enable Control Input
FLG
Open-Drain Fault Flag Output
Function Block Diagram
VIN
CE
Bias
Oscillator
Current
Limiting
UVLO
Charge
Pump
Thermal
Protection
Gate
Control
Output Voltage
Detection
VOUT
FLG
Delay
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2
DS9702A-01
March 2003
RT9702/A
Preliminary
Test Circuits
RFG
FLG
VIN
S1
VIN
CIN
+
I SUPPLY
V FLG
CIN
VIN
IOUT
RT9702/A
+
GND
A
CE
On
CE
On
I LEAK AGE
RT9702/A
VOUT
A
VOUT
FLG
VIN
A
VOUT
C OUT
+
RL
GND
IL
RL
Off
Off
RFG
VR DS(ON)
V
I OUT
VIN
VIN
CIN
+
FLG
V FLG
VOUT
VIN
VOUT
+
RT9702/A
CIN
VIN
C OUT
+
RT9702/A
VOUT
CE
On
CE
FLG
GND
GND
C OUT
VCE
+
IL
RL
Off
RFG
FLG
S2
V FLG
VOUT
VIN
VIN
C IN
+
I OUT
RT9702/A
A
VOUT
CE
GND
C OUT
+
S3
RL
IL
Note: Above test circuits reflected the graphs shown on “Typical Operating Characteristics” are as follows:
– Turn-On Rising & Falling Time vs. Temperature, Turn-On & Off Response, Flag Response
– Supply Current vs. Input Voltage & Temperature, Switch Off Supply Current vs. Temperature, Turn-Off Leakage Current vs.
Temperature
– On-Resistance vs. Input Voltage & Temperature
– CE Threshold Voltage vs. Input Voltage & Temperature, Flag Delay Time vs. Input Voltage & Temperature, UVLO Threshold
vs. Temperature, UVLO at Rising & Falling
– Current Limit vs. Input Voltage/Temperature, Short Circuit Current Response, Short Circuit Current vs. Temperature, Inrush
Current Response, Soft-start Response, Ramped Load Response, Current Limit Transient Response, Thermal Shutdown
Response
DS9702A-01
March 2003
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3
RT9702/A
Preliminary
Absolute Maximum Ratings (Note 1)
Supply Voltage
Chip Enable Input Voltage
Flag Voltage
Power Dissipation, PD @ TA = 25°C
SOT-25
Package Thermal Resistance
SOT-25, θJA
Junction Temperature
Lead Temperature (Soldering, 10 sec.)
Storage Temperature Range
ESD Susceptibility (Note 2)
HBM (Human Body Mode)
MM (Machine Mode)
6.5V
−0.3V to 6.5V
6.5V
0.25W
250°C/W
150°C
260°C
−65°C to 150°C
8kV
800V
Recommended Operating Conditions (Note 3)
Supply Input Voltage
Chip Enable Input Voltage
Junction Temperature Range
2V to 5.5V
0V to 5.5V
−20°C to 100°C
Electrical Characteristics
(VIN = 5V, CIN = COUT = 1µF, TA = 25°C, unless otherwise specified)
Parameter
Switch On Resistance
Symbol
RT9702
RT9702A
IOUT = 500mA
IOUT = 1.1A
Min
Typ
Max
Units
--
80
100
mΩ
ISW_ON
switch on, VOUT = Open
--
25
45
ISW_OFF
switch off, VOUT = Open
--
0.1
1
VIL
VIN = 2V to 5.5V, switch off
--
--
0.8
V
Logic-High Voltage VIH
VIN = 2V to 5.5V, switch on
2.0
--
--
V
Supply Current
CE Threshold
RDS(ON)
Test Conditions
Logic-Low Voltage
µA
CE Input Current
ICE
VCE = 0V to 5.5V
--
0.01
--
µA
Output Leakage Current
ILEAKAGE
VCE = 0V, RLOAD = 0Ω
--
0.5
10
µA
Output Turn-On Rise Time
TON_RISE
10% to 90% of VOUT rising
--
400
--
µS
ILIM
RLOAD = 1Ω
0.5
0.8
1.1
1.1
1.5
2.0
ISC_FB
VOUT = 0V, measured prior to
thermal shutdown
--
0.8
--
--
1.0
--
FLAG Output Resistance
RFLG
ISINK = 1mA
--
20
400
Ω
FLAG Off Current
IFLG_OFF
VFLG = 5V
--
0.01
1
µA
FLAG Delay Time (Note 4)
tD
From fault condition to FLG
assertion
2
10
15
mS
Current Limit
Short Circuit FoldBack Current
RT9702
RT9702A
RT9702
RT9702A
A
A
To be continued
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4
DS9702A-01
March 2003
RT9702/A
Preliminary
Parameter
Symbol
Test Conditions
Min
Typ
Max
Units
Under-voltage Lockout
VUVLO
VIN increasing
1.3
1.7
--
V
Under-voltage Hysteresis
∆VUVLO
VIN decreasing
--
0.1
--
V
Thermal Shutdown Protection
TSD
--
130
--
°C
Thermal Shutdown Hysteresis
∆TSD
--
20
--
°C
Note 1. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device.
These are stress ratings only, and functional operation of the device at these or any other conditions beyond those
indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
Note 2. Devices are ESD sensitive. Handling precaution recommended. The human body model is a 100pF capacitor
discharged through a 1.5kΩ resistor into input and output pins.
Note 3. The device is not guaranteed to function outside its operating conditions.
Note 4. The FLAG delay time is input voltage dependent, see “Typical Operating Characteristics” graph for further details.
DS9702A-01
March 2003
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5
RT9702/A
Preliminary
Typical Operating Characteristics (U.U.T: RT9702ACB, unless otherwise indicated)
2
Supply Curent vs. Input Voltage
40
VIN = VCE = 5V
CIN = 33µF
RL = Open
30
CIN = 33µF
COUT = 33µF
RL = Open
VIN = VCE = 5V
35
30
Supply Current
Supply Current (µA)
35
Supply Current vs. Temperature 2
40
25
20
15
25
20
15
10
10
5
5
0
0
2
2.5
3
3.5
4
4.5
5
-40
5.5
-20
0
20
Current Limit vs. Input Voltage
2
60
80
100
Current Limit vs. Temperature
2.4
2.2
2
Current Limit (A)
Current Limit (A)
5
CIN = 33µF
COUT = 33µF
RL = 1Ω
S2 = On
S3 = Off
1.8
40
120
Temperature (°C)
Input Voltage (V)
1.6
1.4
1.8
5
VIN = 5V
CIN = COUT = 33µF
RL = 1Ω
S2 = On
S3 = Off
1.6
1.4
1.2
1
0.8
1.2
0.6
0.4
1
2
2.5
3
3.5
4
4.5
5
-40
5.5
-20
0
On-Resistance vs. Input Voltage
160
140
120
100
80
60
40
20
60
80
100
120
120
3
VIN = 5V
CIN = COUT = 33µF
IOUT = 1.1A
100
80
60
40
20
0
0
2
2.5
3
3.5
4
4.5
Input Voltage (V)
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6
40
On-Resistance vs. Temperature
160
On-Resistance (mΩ)
On-Resistance (mΩ)
3
CIN = COUT = 33µF
IOUT = 1.1A
140
20
Temperature(°C)
Input Voltage (V)
5
5.5
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
DS9702A-01
March 2003
RT9702/A
Preliminary
Short Circuit Current Response
Short Circuit Current vs. Temperature 5
5
2
Short Circuit Current (A)
IOUT
(1A/DIV)
VOUT
(5V/DIV)
VIN = 5V
CIN = 33µF
COUT = 0.1µF
S2 = S3 = On
1.8
1.6
1.4
1.2
1
0.8
0.6
-40
Time (10ms/DIV)
0
2.4
CE Threshold Voltage (V)
1.6
1.2
0.8
0.4
0
40
60
80
100
120
2
VIN = 5V
CIN = COUT = 33µF
IL = 100mA
1.6
1.2
0.8
0.4
0
2
2.5
3
3.5
4
4.5
5
5.5
-40
-20
0
Input Voltage (V)
Turn-On Rising Time vs. Temperature
450
60
140
Turn-Off Falling Time (µs)
540
40
80
100
120
Turn-Off Falling Time vs. Temperature 1
1
VIN = 5V
CIN = 33µF
COUT = 1µF
RL = 30Ω
S1 = On
630
20
Temperature(°C)
720
Turn-On Rising Time (µs)
20
CE Threshhold Voltage vs. Temperature 4
4
CIN = COUT = 33µF
IL = 100mA
2
-20
Temperature(°C)
CE Threshold Voltage vs. Input Voltage
2.4
CE Threshold Voltage (V)
VIN = 5V
CIN = COUT = 33µF
S2 = S3 = On
360
270
180
VIN = 5V
CIN = 33µF
COUT = 1µF
RL = 30Ω
S1 = On
120
100
80
60
40
20
90
0
0
-40
-20
0
20
40
60
Temperature(°C)
DS9702A-01
March 2003
80
100
120
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
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7
RT9702/A
Preliminary
Turn-Off Leakage Current vs. Temperature 2
Switch Off Supply Current vs. Temperature 2
VIN = 5V
CIN = COUT = 33µF
VCE = 0V
RL = Open
0.8
0.6
0.4
Turn-off Leakage Current (µA)
Switch Off Supply Current (µA)
1
0.2
0
-0.2
-0.4
-0.6
-0.8
3.5
3
2.5
VIN = 5V
CIN = COUT = 33µF
VCE = 0V
RL = 0Ω
2
1.5
1
0.5
0
-1
-40
-20
0
20
40
60
80
100
-40
120
-20
0
20
FLAG Delay Time vs. Input Voltage
24
60
80
100
FLAG Delay Time vs. Temperature
16
15
120
4
VIN = VCE = 5V
CIN = COUT = 33µF
14
Delay Time (ms)
Delay Time (ms)
4
VCE = 5V
CIN = COUT = 33µF
20
40
Temperature(°C)
Temperature (°C)
16
12
8
13
12
11
10
9
4
8
7
0
2
2.5
3
3.5
4
4.5
5
-40
5.5
-20
0
UVLO Threshold vs. Temperature 4
60
80
Inrush Current Response
CIN = COUT = 33µF
RL = 1kΩ
VIN = 5V
CIN = 33µF
RL = 1Ω
S2 = On
S3 = Off
2.5
100
120
5
COUT = 1000µF
COUT = 220µF
2
1.5
IOUT
(1A/DIV)
UVLO Threshold (V)
3
40
Temperature(°C)
Input Voltage (V)
3.5
20
1
0.5
COUT = 1µF
0
-40
-20
0
20
40
60
Temperature(°C)
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8
80
100
120
Time (10ms/DIV)
DS9702A-01
March 2003
RT9702/A
Preliminary
1
VOUT
(1V/DIV)
VIN = 5V
CIN = 33µF
COUT = 1µF
RL = 30Ω
S1 = On
VIN = 5V
RL = 30Ω
S1 = Off
Time (100µs/DIV)
CIN = 33µF
COUT = 1µF
Time (100µs/DIV)
4
UVLO at Rising
4
UVLO at Falling
VIN = 5V
CIN = 33µF
COUT = 1µF
RL = 30Ω
VIN
VIN = 5V
CIN = 33µF
COUT = 1µF
RL = 30Ω
VOUT
Time (1ms/DIV)
Soft-Start Response
VOUT
VIN
(1V/DIV) (1V/DIV)
VIN
(1V/DIV)
VIN
VOUT
(1V/DIV)
1
Turn-Off Response
IL
VCE
VOUT
(0.5A/DIV) (5V/DIV) (5V/DIV)
VCE
(5V/DIV)
Turn-On Response
5
VOUT
Time (10ms/DIV)
Ramped Load Response
5
RL = 1Ω
VIN = 5V
Time (50µs/DIV)
DS9702A-01
March 2003
CIN = 33µF
COUT = 1µF
S2 : Off
On
S3 = Off
IL
(0.5A/DIV)
IL
(0.5A/DIV)
VOUT
(5V/DIV)
VCE
(5V/DIV)
4.9V
1.1A V = 5V
IN
CIN = 33µF
COUT = 1µF
RL : 1kΩ 1Ω
Time (10ms/DIV)
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9
RT9702/A
Preliminary
Flag Response (Enable into Short Circuit)
1
1
Flag Response
VOUT
VFLG
(5V/DIV) (5V/DIV)
VCE
VFLG
(5V/DIV) (5V/DIV)
RT9702CB
12ms (tD)
IL
(1A/DIV)
IL
(0.5A/DIV)
CIN = 33µF
COUT = 1µF
S1 = On
RL = 0Ω
12ms (tD)
CIN = 0.1µF
COUT = 33µF
S1 = On
RL = 1Ω
Time (2.5ms/DIV)
Time (10ms/DIV)
Time (5µs/DIV)
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10
5
Thermal Shutdown Response
VCE
IOUT(1A/DIV) IOUT(1A/DIV)
Current Limit (5V/DIV)
Short
CIN = 0.1µF
COUT = 33µF
VIN = 5V
RL = 1Ω
S2 = On
S3 = Off
IOUT
(1A/DIV)
VTRIGGER
(5V/DIV)
Current Limit Transient Response
5
S2 = On
S3 = Off
RL = 1Ω
S3 = On
CIN = 33µF
COUT = 1µF
Time (50ms/DIV)
DS9702A-01
March 2003
Preliminary
RT9702/A
Applications Information
The RT9702 and RT9702A are single N-Channel
MOSFET high-side power switches with active-high
enable input, optimized for self-powered and
bus-powered Universal Serial Bus (USB) applications.
The RT9702/A equipped with a charge pump circuitry to
drive the internal NMOS switch; the switch’s low
RDS(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
VIN (input) is the power source connection to the
internal circuitry and the drain of the MOSFET. VOUT
(output) is the source of the MOSFET. In a typical
application, current flows through the switch from VIN to
VOUT toward the load. If VOUT is greater than VIN,
current will flow from VOUT to VIN 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
RT9702/A prevents reverse current flow if VOUT being
externally forced to a higher voltage than VIN when the
output disabled (VCE < 0.8V).
D
S
D
S
G
G
Normal MOSFE T
RT9702/A
Chip Enable Input
The switch will be disabled when the CE 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 CE pin is 0.8V. A minimum
guaranteed voltage of 2V at the CE pin will turn the
RT9702/A back on. Floating the input may cause
unpredictable operation. CE should not be allowed to
go negative with respect to GND. The CE pin may be
directly tied to VIN to keep the part on.
DS9702A-01
March 2003
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 RT9702/A provides a FLG signal pin which is an
N-Channel open drain MOSFET output. This open
drain output goes low when VOUT < VIN – 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, tD, 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 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 800mA through the switch of RT9702 and
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11
RT9702/A
Preliminary
1.5A for RT9702A respectively. When a heavy load or
Universal Serial Bus (USB) & Power Distribution
short circuit is applied to an enabled switch, a large
transient current may flow until the current limit circuitry
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 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.
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 shutdown is employed to protect the device
from damage if the die temperature exceeds approximately 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 device’s junction temperature depends on several
factors such as the load, PCB layout, ambient
temperature and package type. The output pin of
RT9702/A can deliver a current of up to 500mA, and
1.1A 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 RDS(ON)
of switch as below.
PD = RDS(ON) x IOUT2
Although the devices are rated for 500mA and 1.1A 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:
PD (MAX) = ( TJ (MAX) − TA ) / θJA
Where TJ (MAX) is the maximum junction temperature of
the die (100°C) and TA is the maximum ambient
temperature. The junction to ambient thermal
resistance (θJA) for SOT-25 package at recommended
minimum footprint is 250°C/W (θJA is layout
dependent).
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12
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 polyfuse or polyswitch) have
slow trip times, high on-resistance, and lack the
necessary circuitry for USB-required fault reporting.
DS9702A-01
March 2003
Preliminary
The faster trip time of the RT9702/A power distribution
allow designers to design hubs that can operate
through faults. The RT9702/A 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 VIN 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.
RT9702/A
Fault Flag Filtering (Optional)
The transient inrush current to downstream
capacitance may cause a short-duration error flag,
which may cause erroneous over-current reporting. A
simple 1mS RC low-pass filter (10KΩ and 0.1µF) in the
flag line (see Typical Application Circuit) eliminates
short-duration transients.
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 VOUT (MIN) for multiple ports (NPORTS) ganged together through one switch
(if using one switch per port, NPORTS is equal to 1):
VOUT (MIN) = 4.75V – [ II x ( 4 • RCONN + 2 • RCABLE ) ]
( 0.1A x NPORTS x RSWITCH ) – VPCB
–
Where
Output Filter Capacitor
A low-ESR 150µF aluminum electrolytic or tantalum
between VOUT and GND is strongly recommended to
meet the 330mV maximum droop requirement in the
hub VBUS (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 VBUS, 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.
RCONN = Resistance of connector contacts
(two contacts per connector)
RCABLE = Resistance of upstream cable wires
(one 5V and one GND)
RSWITCH = Resistance of power switch
(80mΩ typical for RT9702/A)
VPCB = PCB voltage drop
The USB specification defines the maximum resistance
per contact (RCONN) 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 (II) of 500mA,
the maximum resistance of the cable is 90mΩ.
The resistance of the switch is defined as follows:
RSWITCH = { 4.75V – 4.4V – [ 0.5A x ( 4 • 30mΩ + 2
90mΩ ) ] –VPCB } ÷ ( 0.1A x NPORTS )
= (200mV – VPCB ) ÷ ( 0.1A x NPORTS )
DS9702A-01
March 2003
•
www.richtek.com
13
RT9702/A
Preliminary
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 RT9702/A, with its
maximum 100mΩ on-resistance over temperature,
easily meets this requirement.
PCB Layout
In order to meet the voltage drop, droop, and EMI
requirements, careful PCB layout is necessary. The
following guidelines must be considered:
• Keep all VBUS traces as short as possible and use at
least 50-mil, 2 ounce copper for all VBUS traces.
• Avoid vias as much as possible. If vias are necessary,
make them as large as feasible.
• Place a ground plane under all circuitry to lower both
resistance and inductance and improve DC and
transient performance (Use a separate ground and
power plans if possible).
ESD
Because USB is a hot insertion and removal system,
USB components (especially the connector pins) are
subject to electrostatic discharge (ESD) and should be
qualified to IEC801.2. The RT9702/A is designed to
withstand a 8kV human body mode, as defined in
MIL-STD-883C. The requirements in IEC801.2 are
much more stringent and require additional capacitors
for the RT9702/A to withstand the higher ESD energy.
Low-ESR 1µF ceramic bypass capacitors and output
capacitors should be placed as closely as possible to
the VIN and VOUT pins to increase the ESD immunity.
The RT9702/A may pass the requirements of IEC
1000-4-2 (EN 50082-1) level-4 for 15kV air discharge
and 8kV contact discharge tests when these capacitors
are added.
• Place cuts in the ground plane between ports to help
reduce the coupling of transients between ports.
• Locate the output capacitor and ferrite beads as close
to the USB connectors as possible to lower
impedance (mainly inductance) between the port and
the capacitor and improve transient load
performance.
• Locate the RT9702/A as close as possible to the
output port to limit switching noise.
• Locate the ceramic bypass capacitors as close as
possible to the VIN pins of the RT9702/A.
V BUS
VIN
V OUT
FLG
GND_ BUS
CE
GND
USB Controller
Board Layout
www.richtek.com
14
DS9702A-01
March 2003
RT9702/A
Preliminary
Package Information
H
D
L
B
C
b
A
A1
e
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.889
1.295
0.035
0.051
A1
0.000
0.152
0.000
0.006
B
1.397
1.803
0.055
0.071
b
0.356
0.559
0.014
0.022
C
2.591
2.997
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.102
0.254
0.004
0.010
L
0.356
0.610
0.014
0.024
SOT- 25 Surface Mount Package
DS9702A-01
March 2003
www.richtek.com
15
RT9702/A
Preliminary
RICHTEK TECHNOLOGY CORP.
RICHTEK TECHNOLOGY CORP.
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
8F-1, No. 137, Lane 235, Paochiao Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)89191466 Fax: (8862)89191465
Email: [email protected]
www.richtek.com
16
DS9702A-01
March 2003