LM3544 [TI]
4 通道、0.5A 负载、2.7-5.5V、90mΩ USB 电源开关;
| 型号： | LM3544 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 4 通道、0.5A 负载、2.7-5.5V、90mΩ USB 电源开关 开关 电源开关 |
| 文件： | 总17页 (文件大小：1124K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LM3544
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SNVS062C –AUGUST 2000–REVISED MAY 2013
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1
FEATURES
DESCRIPTION
The LM3544 is a quad high-side power switch that is
an excellent choice for use in Root, Self-Powered and
Bus- Powered USB (Universal Serial Bus) Hubs.
Independent port enables, flag signals to alert USB
controllers of error conditions, controlled start-up in
hot-plug events, and short circuit protection all satisfy
USB requirements.
•
•
•
•
•
•
Compatible with USB1.1 and USB 2.0
90mΩ (typ.) High-Side MOSFET Switch
500mA Continuous Current per Port
7 ms Fault Flag Delay Filters Hot-Plug Events
Industry Standard Pin Order
Short Circuit Protection with Power-Saving
Current Foldback
The LM3544 accepts input voltages between 2.7V
and 5.5V. The Enable logic inputs, available in active-
high and active-low versions, can be powered off any
voltage in the 2.7V to 5.5V range. The LM3544 limits
the continuous current through a single port to 1.25A
(max.) when it is shorted to ground.
•
•
•
•
•
•
•
Thermal Shutdown Protection
Undervoltage Lockout
Recognized by UL and Nemko CB
Input Voltage Range: 2.7V to 5.5V
5μA Maximum Standby Supply Current
16-Pin SOIC Package
The low on-state resistance of the LM3544 switches
ensures the LM3544 will satisfy USB voltage drop
requirements, even when current through a switch
reaches 500 mA. Thus, High-Powered USB
Functions, Low-Powered USB functions, and Bus-
Powered USB Hubs can all be powered off a Root or
Self-Powered USB Hub containing the LM3544.
Ambient Temperature Range: −40°C to 85°C
APPLICATIONS
•
USB Root, Self-Powered, and Bus-Powered
Hubs
Added features of the LM3544 include current fold-
back to reduce power consumption in current
overload conditions, thermal shutdown to prevent
device failure caused by high current overheating,
and undervoltage lockout to keep switches from
operating if the input voltage is below acceptable
levels.
•
•
USB Devices such as Monitors and Printers
General Purpose High Side Switch
Applications
Functional Diagram
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Copyright © 2000–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
LM3544
SNVS062C –AUGUST 2000–REVISED MAY 2013
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Connection Diagrams
Figure 1. LM3544-H
16-Pin SOIC
Figure 2. LM3544-L
16-Pin SOIC
Top View
Top View
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Typical Application Circuit
Figure 3. The LM3544 used in a Self-Powered or Root USB Hub
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ABSOLUTE MAXIMUM RATINGS(1)(2)
Voltage at INX and OUTX pins
Voltage at ENX(ENX) and FLAGX pins
Power Dissipation(3)
−0.3V to 6V
−0.3V to 5.5V
Internally Limited
150°C
Maximum Junction Temperature
Storage Temperature Range
−65°C to 150°C
260°C
Lead Temperature Range (Soldering, 5 sec.)
ESD Rating(4)
2 kV
(1) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
(2) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when
operating the device beyond its rated operating conditions.
(3) The maximum allowable power dissipation is a function of the Maximum Junction Temperature (TJMAX), Junction to Ambient Thermal
Resistance (θJA), and the Ambient Temperature (TA). The LM3544 in the 16-pin SOIC package has a TJMAX of 150°C and a θJA of
130°C/W. The maximum allowable power dissipation at any temperature is PMAX = (TJMAX − TA)/θJA. Exceeding the maximum allowable
power dissipation will cause excessive die temperature, and the part will go into thermal shutdown.
(4) The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
RECOMMENDED OPERATING CONDITIONS
Supply Voltage Range
2.7V to 5.5V
0 mA to 500 mA
−40°C to 125°C
Continuous Output Current Range (Each Output)
Junction Temperature Range
DC ELECTRICAL CHARACTERISTICS
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 5.0V, ENX = VIN (LM3544-H) or ENX = 0V (LM3544-L).
SYMBOL
PARAMETER
On Resistance
TEST CONDITIONS
VIN = 5V, IOUTX = 0.5A
MIN
TYP
90
MAX
125
UNIT
RON
mΩ
VIN = 3.3V, IOUTX = 0.5A
3.0V ≤ VIN ≤ 5.5V
95
130
IOUT
OUTX Continuous Output Current
OUTX Leakage Current
0.5
A
ILEAK-OUT
ENX = 0 (ENX = VIN); TJ = 25°C
0.01
1
μA
μA
ENX = 0 (ENX = VIN); − 40 ≤ TJ ≤
10
125°C
ISC
OUTX Short-Circuit Current(1)
OUTX Connected to GND
0.8
2.0
0.1
1.25
3.2
0.3
1
A
A
OCTHRESH Overcurrent Threshold
VL_FLAG FLAGX Output-Low Voltage
ILEAK-FLAG FLAGX Leakage Current
I(FLAGX) = 10 mA
V
2.7 ≤ VFLAG ≤ 5.5V
ENX/ENX = 0V or ENX/ENX = VIN
2.7V ≤ VIN ≤ 5.5V
μA
μA
V
ILEAK-EN
VIH
ENX Input Leakage Current
EN/EN Input Logic High
EN/EN Input Logic Low
-0.5
0.5
2.4
VIL
4.5V ≤ VIN ≤ 5.5V
0.8
0.4
V
2.7V ≤ VIN ≤ 4.5V
V
VUVLO
IDDON
Under-Voltage Lockout Threshold
Operational Supply Current
1.8
V
ENX = VIN (ENX = 0 ); TJ = 25°C
375
600
μA
μA
ENX = VIN (ENX = 0 ); −40°C ≤ TJ ≤
800
125°C
IDDOFF
Shutdown Supply Current
ENX = 0 (ENX = VIN); TJ = 25°C
1
μA
μA
−40°C ≤ TJ ≤ 125°C
5
(1) Thermal Shutdown will protect the device from permanent damage.
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AC ELECTRICAL CHARACTERISTICS
Limits are for TJ = 25°C and VIN = 5.0V.
SYMBOL
PARAMETER
OUTX Rise Time(1)
OUTX Fall Time(2)
Turn-on Delay(3)
Turn-off Delay(4)
Flag Delay(5)
TEST CONDITIONS
CL = 33 μF, ILOAD = 500 mA
CL = 33 μF, ILOAD = 500 mA
CL = 33 μF, ILOAD = 500 mA
CL = 33 μF, ILOAD = 500 mA
IFLAG = 10 mA
MIN
TYP
1.5
0.9
2.9
0.7
7
MAX
UNIT
ms
tr
tf
ms
tON
tOFF
tF
ms
ms
ms
(1) Time for OUTX to rise from 10% to 90% of its enabled steady-state value after ENX (ENX) is asserted.
(2) Time for OUTX to fall from 10% to 90% of its enabled steady-state value after ENX (ENX) is deasserted.
(3) Time between ENX rising through VIH (ENX falling through VIL) and OUTX rising through 90% of its enabled steady-state voltage.
(4) Time between ENX falling through VIL (ENX rising through VIH) and OUTX falling through 10% of its enabled steady-state voltage.
(5) Time between ENX rising through VIN (ENX falling through VIN) and FLAGX falling through 0.3V when OUTX is connected to GND.
PIN DESCRIPTIONS
Pin Number
Pin Name
Pin Function
2, 6
IN 1, 2
Supply Inputs: These pins are the inputs to the power switches and the supply input for
the IC. In most applications they are connected together externally and to a single input
voltage supply.
1, 5
GND 1, 2
Grounds: Must be connected together and to a common ground.
Switch Outputs: These pins are the outputs of the high side switches.
Enable (Inputs): Active-high (or active-low) logic enable inputs.
15, 14, 11, 10
3, 4, 7, 8
OUT 1, 2, 3, 4
LM3544-H: EN 1, 2, 3, 4
(LM3544-L: EN 1, 2, 3, 4)
16, 13, 12, 9
FLAG 1, 2, 3,4
Fault Flag (Outputs): Active-low open drain outputs. Indicates over-current, UVLO or
thermal shutdown. See APPLICATION INFORMATION for more details.
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TYPICAL PERFORMANCE CHARACTERISTICS
Figure 4. RON vs Input Voltage
Figure 5. RON vs Junction Temperature
Figure 6. Quiescent Current, Output(s) Enabled vs Junction
Temperature
Figure 7. Quiescent Current, Output(s) Disabled vs Junction
Temperature
Figure 8. Quiescent Current, Output(s) Enabled vs Input
Voltage
Figure 9. Quiescent Current, Output(s) Disabled vs Input
Voltage
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
125
Figure 10. Short-Circuit Output Current vs Junction
Temperature(1)
Figure 11. Over-Current Threshold vs Junction
Temperature(1)
Figure 12. Under-Voltage Lockout (UVLO) Threshold vs
Junction Temperature
Figure 13. Turn-On Delay vs Input Voltage (CIN = 33 μF,
COUT = 33 μF)
Figure 14. Turn-Off Delay vs Input Voltage (CIN = 33 μF,
COUT = 33 μF)
Figure 15. Fault Flag Delay Time vs Junction Temperature
(1) Output is shorted to Ground through a 100 mΩ resistor.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Figure 16. Turn-On/Turn-Off Response with 47Ω/33μF Load
Figure 17. Turn-On/Turn-Off Response with 10Ω/33μF Load
Figure 18. Enable Into a Short(2)
Figure 19. Short Connected to Enabled Device(2)
Figure 20. Over-Current Response with Ramped Load on
OUT1 and Fixed Load on OUT2(3)
Figure 21. Inrush Current to Downstream Device when
LM3544 is Enabled(4)
(2) Output is shorted to Ground through a 100 mΩ resistor.
(3) Output is shorted to Ground through a 100 mΩ resistor.
(4) Load is two capacitors and one resistor in parallel to model an actual USB load condition. The first capacitor has a value of 33 μF to
model the LM3544 output capacitor. The second capacitor has a value of 10 μF to model the maximum allowable input capacitance of
the downstream device. The resistor is a 47Ω resistor to model the maximum allowable input resistance of the downstream device.
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FUNCTIONAL DESCRIPTION
POWER SWITCHES
The power switches that comprise the four ports of the LM3544 are N-Channel MOSFETs. They have a typical
onstate drain-to-source resistance of 90 mΩ when the input voltage is 5 V. When enabled, each switch will
supply a 500 mA minimum current to its load. In the unlikely event that a switch is enabled and the output
voltage of that switch is pulled above the input voltage, the bi-directional nature of the switch results in current to
flow from the output to the input. When a switch is disabled, current flow through the switch is prevented in both
directions.
CHARGE PUMP AND DRIVER
The gate voltages of the high-side NFET power switches are supplied by an internal charge-pump and driver
circuit combination. The charge pump is a low-current, switched capacitor circuit that efficiently generates
voltages above the LM3544 input supply. The charge pump output is used to supply a transconductance
amplifier driver circuit that controls the gate voltages of the power switches. Rise and fall times on the gates are
typically kept between 2 ms and 4 ms to limit large current surges and associated electromagnetic interference
(EMI).
ENABLE (ENX OR ENX)
The LM3544 comes in two versions: an active-high enable version, LM3544-H, and an active-low enable version,
LM3544-L. In the LM3544-H, the ENX pins are active-high logic inputs that, when asserted, turn on the
associated power supply switch(es). Power supply switches are controlled by the ENX active-low logic inputs in
the LM3544-L. With all four ports disabled on either version of the LM3544, less than 5 μA of supply current is
consumed. Both types of enable inputs, active-high and active-low, are TTL and CMOS logic compatible.
INPUT AND OUTPUT
The power supply to the control circuitry and the drains of the power-switch MOSFETs are connected to the two
input pins, IN1 and IN2. These two pins are connected externally in most standard applications. The two ground
nodes GND1 and GND2 must be connected externally in all applications. Pins OUT1, OUT2, OUT3, and OUT4
are connections to the source nodes of the power-switch MOSFETs. In a typical application circuit, current flows
through the switches from IN1 and IN2 to OUTX toward the load.
UNDERVOLTAGE LOCKOUT (UVLO)
Undervoltage Lockout (UVLO) prevents the MOSFET switches from turning on until the input voltage exceeds a
typical value of 1.8V.
If the input voltage drops below the UVLO threshold, the MOSFET switches are opened and fault flags are
activated. UVLO flags function only when one or more of the ports is enabled. If a port is enabled in a UVLO
condition, flags corresponding to the enabled port and its dual (port 1 is paired with port 2, port 3 is paired with
port 4) are asserted.
CURRENT LIMIT AND FOLD-BACK
The current limit circuit is designed to protect the system supply, the LM3544 switches, and the load from
potential damage resulting from excessive currents. If a direct short occurs on an output of the LM3544, the input
capacitor(s) rapidly discharge through the part, activating current limit circuitry. The threshold for activating
current limiting is 2.0A (typ.). Protection is achieved by momentarily opening the MOSFET switch and then
gradually turning it on. Turn-on is halted when the current through the switch reaches the current-limit level of
1.0A (typ.) The current is held at this level until either the excessive load/short is removed or the part overheats
and thermal shutdown occurs (see THERMAL SHUTDOWN). The fault flag of a switch is asserted whenever the
switch is current limiting.
If a port on the LM3544 is enabled into a short condition, the output current of that port will rise to the current-
limit level and hold there.
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When a port is in a current-limit condition, the LM3544 senses the output voltage on that port and, if it is less
than 1.0V (typ.), will reduce the output current through that port. This operation is shown in Figure 22, below. The
current reduction, or foldback, reduces power dissipation through the overloaded MOSFET switch. An additional
advantage of the foldback feature is the reduction of power required from the source supply when one or more
output ports are shorted.
Figure 22. Short-Circuit Output Current (with Foldback) vs. Output Voltage
THERMAL SHUTDOWN
The LM3544 is internally protected against excessive power dissipation by a two-stage thermal protection circuit.
If the device temperature rises to approximately 145°C, the thermal shutdown circuitry turns off any switch that is
current limited. Non-overloaded switches continue to function normally. If the die temperature rises above 160°C,
all switches are turned off and all four fault flag outputs are activated. Hysteresis ensures that a switch turned off
by thermal shutdown will not be turned on again until the die temperature is reduced to 135°C. Shorted switches
will continue to cycle off and on, due to the rising and falling die temperature, until the short is removed.\
The thermal shutdown function is shown graphically in Figure 23 and Figure 24.
Figure 23. Thermal Shutdown Characteristics
when only the First-Stage Thermal-Shutdown Mode is Needed
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Figure 24. Thermal Shutdown Characteristics when both
First-Stage and Second-Stage Thermal-Shutdown Modes are Needed
In Figure 23, port 1 is enabled into a short. When this occurs, the MOSFET switch of port 1 repeatedly opens
and closes as the device temperature rises and falls between 145°C and 135°C. In this example, the device
temperature never rises above 160°C. The second stage thermal shutdown is not used and port 2 remains
operational.
When port 1 is enabled into a short in the example illustrated in Figure 24, the device temperature immediately
rises above 160°C. A higher ambient temperature or a larger number of shorted outputs can cause the junction
temperature to increase, resulting in the difference in behavior between the current example and the previous
one. When the junction temperature reaches 160°C, all four ports are disabled (ports 3 and 4 are not shown in
the figure) and all four fault-flag signals are asserted. Just prior to time index 2.5 ms, the device temperature falls
below 135°C, all four ports activate, and all four fault flags are removed. The short condition remains on port 1,
however. For the remainder of the example, the device temperature cycles between 135°C and 145°C, causing
port 1 to repeatedly turn on and off but allowing the un-shorted ports to function normally.
SOFT START
When a power switch is enabled, high levels of current will flow instantaneously through the LM3544 to charge
the large capacitance at the output of the port. This is likely to exceed the over-current threshold of the device, at
which point the LM3544 will enter its current-limit mode. The amount of current used to charge the output
capacitor is then set by the current-limit circuitry. The device will exit the current-limit mode when the current
needed to continue to charge the output capacitor is less than the LM3544 current-limit level.
FAULT FLAG
The fault flags are open-drain outputs, each capable of sinking up to a 10 mA load current to typically 100 mV
above ground.
A parasitic diode exists between the flag pins and VIN pins. Pulling the flag pins to voltages higher than VIN will
forward bias this diode and will cause an increase in supply current. This diode will also clamp the voltage on the
flag pins to a diode drop above VIN.
The fault flag is active (pulled low) when any of the following conditions are present: under-voltage, current-limit,
or thermal-shutdown.
The LM3544 has an internal delay in reporting fault conditions that is typically 7 ms in length. In start-up, the
delay gives the device time to charge the output capacitor(s) and exit the current-limit mode before a flag signal
is set. This delay also prevents flag signal glitches from occurring when brief changes in operating conditions
momentarily place the LM3544 into one of its three error conditions. If an error condition still exists after the delay
interval has elapsed, the appropriate fault flag(s) will be asserted (pulled low) until the error condition is removed.
In most applications, the 7 ms internal flag delay eliminates the need to extend the delay with an external RC
delay network.
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APPLICATION INFORMATION
OUTPUT FILTERING
The schematic in Figure 3 shows a typical application circuit for the LM3544. The USB specification requires 120
μF at the output of each hub. A four-port hub with 33 μF tantalum capacitors at each port output meets the
specification. These capacitors provide short-term transient current to drive downstream devices when hot-plug
events occur. Capacitors with low equivalent-series-resistance should be used to lower the inrush current flow
through the LM3544 during a hot-plug event.
The rapid change in currents seen during a hot plug event can generate electromagnetic interference (EMI). To
reduce this effect, ferrite beads in series between the outputs of the LM3544 and the downstream USB port are
recommended. Beads should also be placed between the ground node of the LM3544 and the ground nodes of
connected downstream ports. In order to keep voltage drop across the beads to a minimum, wire with small DC
resistance should be used through the ferrite beads. A 0.01 μF - 0.1 μF ceramic capacitor is recommended on
each downstream port directly between the Vbus and ground pins to further reduce EMI effects.
POWER SUPPLY FILTERING
A sizable capacitor should be connected to the input of the LM3544 to ensure the voltage drop on this node is
less than 330 mV during a heavy-load hot-plug event. A 33 μF, 16V tantalum capacitor is recommended. The
input supply should be further bypassed with a 0.01 μF - 0.1 μF ceramic capacitor, placed close to the device.
The ceramic capacitor reduces ringing on the supply that can occur when a short is present at the output of a
port.
EXTENDING THE FAULT FLAG DELAY
While the 7 ms (typical) internal delay in reporting flag conditions is adequate for most applications, the delay can
be extended by connecting external RC filters to the FLAG pins, as shown in Figure 25.
Figure 25. Typical Circuit for Lengthening the Internal Flag Delay
POWER DISSIPATION AND JUNCTION TEMPERATURE
A few simple calculations will allow a designer to calculate the approximate operating temperature of the LM3544
for a given application. The large currents possible through the low resistance power MOSFET combined with
the high thermal resistance of the SOIC package, in relation to power packages, make this estimate an important
design step. Begin the estimate by determining RON at the expected operating temperature using the graphs in
the Typical Performance Characteristics section of this datasheet. Next, calculate the power dissipation through
the switch with Equation 1.
PD = RON * IDS
2
(1)
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NOTE
The equation for power dissipation neglects the portion that comes from LM3544
quiescent current, because this value will almost always be insignificant.
Using this figure, determine the junction temperature with Equation 2:
TJ = PD * θJA + TA
where
•
θJA = SOIC Thermal Resistance: 130°C/W and TA = Ambient Temperature (°C).
(2)
Compare the calculated temperature with the expected temperature used to estimate RON. If they do not
reasonably match, re-estimate RON using a more appropriate operating temperature and repeat the calculations.
Reiterate as necessary.
PCB LAYOUT CONSIDERATIONS
In order to meet the USB requirements for voltage drop, droop and EMI, each component used in this circuit
must be evaluated for its contribution to the circuit performance. These principles are illustrated in Figure 26. The
following PCB layout rules and guidelines are recommended:
1. Place the switch as close to the USB connector as possible. Keep all Vbus traces as short as possible and
use at least 50-mil, 1 ounce copper for all Vbus traces. Solder plating the traces will reduce the trace
resistance.
2. Avoid vias as much as possible. If vias are used, use multiple vias in parallel and/or make them as large as
possible.
3. Place the output capacitor and ferrite beads as close to the USB connector as possible.
4. If ferrite beads are used, use wires with minimum resistance and large solder pads to minimize connection
resistance.
Figure 26. Self-Powered Hub Connections and Per-Port Voltage Drop
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TYPICAL APPLICATIONS
ROOT AND SELF-POWERED USB HUBS
The LM3544 has been designed primarily for use in root and self-powered USB hubs. In this application, the
switches of the LM3544 are used to connect the power source of the hub to the power bus used by downstream
devices and to protect the hub from dangerously excessive loads and shorts to ground. A high-power bus-
powered function, low-power bus-powered function, or a bus-powered hub can be driven through a single port of
the LM3544. A schematic of a circuit that uses the LM3544 for power-supply switching in a typical root or self-
powered hub was shown earlier in this datasheet in Figure 3.
Voltage drop requirements of USB power supplies require the power outputs of the root and self-powered hubs
to be no less than 4.75V. For this reason, it is recommended that a 5V power supply with a ±3% output voltage
tolerance is used in this application. Combining a 3% supply with a low resistance PCB design and the low on-
resistance of the LM3544 power switches will ensure that the hub power outputs meet the USB voltage drop
specification even with a 500mA load, the maximum allowed in the USB standard.
BUS-POWERED USB HUBS
The LM3544 is capable of performing the power supply switching functions required in Bus-Powered hubs. Use
here is very similar to the configuration used in root and self-powered hubs. With bus-powered hubs, however,
there is no internal power supply to drive the input pins of the LM3544. Instead, the input pins should be
connected to the power bus supplied by the upstream hub.
USB BUS-POWERED FUNCTIONS AND GENERAL IN-RUSH CURRENT LIMITING
APPLICATIONS
The LM3544 can be placed at the power-supply input of USB bus-powered functions, or other similar devices, to
protect them from high in-rush currents. If the current being delivered to the device were to exceed the 2.0A
over-current threshold (typ.) of the LM3544, switches in violation would open to protect the device from damage.
In addition to in-rush current limiting, the LM3544 can be used in high-power bus-powered functions to keep
current levels of the function in compliance during power-up. The USB specification requires the staged switching
of power when connecting high-power functions to the bus. When a high-power function is initially connected to
the bus, it must not draw more than one unit supply (100mA). After a connection is detected and enumerated,
and if the upstream device is capable of supplying the required power, the high power function may draw up to
five unit loads (500mA). With the proper control signals, the LM3544 can be used to achieve this staged power
connection. When the function is connected to the bus, one or more of the LM3544 switches can be closed to
connect bus power only to circuitry needed during the connection and enumeration process. If the function is to
be powered fully, remaining switches on the LM3544 can be closed to connect all blocks of the function to the
power bus. Figure 27 illustrates how the LM3544 can be connected for use in bus powered functions.
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Figure 27. Using the LM3544 in USB Bus-Powered Functions
WAKE-ON-USB AND REMOTE WAKE-UP APPLICATIONS
The LM3544 can be used in desktop and notebook PC based root hubs to switch the power connection of USB
devices between the main power source, used when the device is active, and a reduced power auxiliary supply.
The auxiliary supply, used when the device is in suspend mode, can dramatically decrease the power
consumption of a computer that contains devices that spend extended periods of time in suspend mode. This
application also works especially well with devices configured for remote wake-up capabilities (i.e.: using a
keystroke on a mouse or keyboard to awaken a suspended PC). A schematic showing an example of how the
LM3544 can be configured to switch the power connection of a device between two separate sources is shown in
Figure 28, below.
In the example, the logic signals EN and SRC control the power supply connections. If the EN signal is low, all
four LM3544 switches will be open and neither supply will be connected to the bus. If EN and SRC are both high,
switch 1 and switch 3 close, connecting the main power supply to the two bus power outputs. When EN is high
and SRC is low, the auxiliary supply is connected to the power outputs through switches 2 and 4.
In order to allow the power-switching circuit to smoothly transition from one supply to another, delay networks
must be placed before the enable pins. These delay networks account for the turn-on delay of the LM3544
switches by slowing the fall time of the enable signals. Rise times of these signals are not affected by the delay
networks. Glitch free transition is assured as long as both supplies remain within operating specifications for a
minimum of 5 ms after the logic signal SRC is toggled.
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LM3544
SNVS062C –AUGUST 2000–REVISED MAY 2013
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Figure 28. Using the LM3544 in a Wake-on-USB Application
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Product Folder Links: LM3544
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