LM3544

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SNVS062C – AUGUST 2000 – REVISED MAY 2013

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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