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 three ports are disabled (port

3 is not shown in the figure) and all three fault-flag signals

are asserted. Just prior to time index 2.5 ms, the device

temperature falls below 135˚C, all three ports activate, and

all three 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 LM3543 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

LM3543 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 LM3543 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 LM3543 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 LM3543 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 LM3543. The USB specification requires 120

µF at the output of each hub. A three-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

LM3543 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

LM3543 and the downstream USB port are recommended.

Beads should also be placed between the ground node of

the

LM3543

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

LM3543 to ensure the voltage drop on this node is less than

330 mV during a heavy-load hot-plug event. A 33 µF, 16V

10125825

FIGURE 3. Thermal Shutdown Characteristics when

only the First-Stage Thermal-Shutdown Mode is

Needed

10125826

FIGURE 4. Thermal Shutdown Characteristics when

Both First-Stage and Second-Stage Thermal-Shutdown

Modes are Needed

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