LM3544

SNVS062C – AUGUST 2000 – REVISED MAY 2013

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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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