Ultra Fast USB 2.0 Multi-Format Flash Media Controller/USB Hub Combo

Port Power Control Using a Poly Fuse

When using the USB2640/USB2641 with a poly fuse, an external diode must be used (See Figure 6.2). When disabling port power, the driver will drive a '0'. This procedure will have no effect since the external diode will isolate the pin from the load. When port power is enabled, the output driver is disabled, and the pull-up resistor is enabled which creates an open drain output. This means that the pull-up resistor is providing 3.3V to the anode of the diode. If there is an over-current situation, the poly fuse will open. This will cause the cathode of the diode to go to 0V. The anode of the diode will be at 0.7V, and the Schmitt trigger input will register this as a low resulting in an over-current detection. The open drain output does not interfere.

USB2640/

USB2641

5V

PRTCTL3

USB

Device

5V

PRTCTL2

USB

Device

Figure 6.2 Port Power Control with Single Poly Fuse and Multiple Loads

When using a single poly fuse to power all devices, note that for the ganged situation, all power control pins must be tied together.

5V

PRTCTL3

Poly Fuse

USB2640/

USB2641

PRTCTL2

USB

USB

 

Device

Device

Figure 6.3 Port Power with Ganged Control with Poly Fuse

SMSC USB2640/USB2641

25

Revision 2.0 (10-03-08)

 

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