LM3544M-H National Semiconductor, LM3544M-H Datasheet - Page 9

• Manufacturer Part Number: LM3544M-H
• Description: Quad Port USB Power Distribution Switch and Over-Current Protection
• Manufacturer: National Semiconductor

Functional Description
In Figure 3 , 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 4 , 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.
Both First-Stage and Second-Stage Thermal-Shutdown
FIGURE 3. Thermal Shutdown Characteristics when
FIGURE 4. Thermal Shutdown Characteristics when
only the First-Stage Thermal-Shutdown Mode is
Modes are Needed
Needed
(Continued)
10120825
10120826
9
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 V
Pulling the flag pins to voltages higher than V
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 V
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.
Application Information
Output Filtering
The schematic in Figure 1 showed 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 V
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
bus
and ground pins to further reduce EMI
IN
.
IN
will forward
www.national.com
IN
pins.