LM3544

SNVS062C – AUGUST 2000 – REVISED MAY 2013

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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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