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High Speed Inter-Chip USB 2.0 Hub and Flash Media Controller
Datasheet
SMSC USB4640/USB4640i 23 Revision 1.0 (06-01-09)
DATASHEET
Port Power control using a Poly Fuse
When using the USB4640/USB4640i with a poly fuse, an external diode must be used (See
Figure 6.2). When disabling port power, the USB4640/USB4640i 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 USB4640/USB4640i output driver is disabled, and the pull-up resistor is enabled which creates an
open drain output. This open drain output condition means that the pull-up resistor is providing 3.3
volts 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 zero volts. The anode of the diode will be at 0.7 volts, and
the Schmitt trigger input will register this as a low resulting in an over-current detection. The open drain
output does not interfere.
Figure 6.2 Port Power Control with a 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.
Figure 6.3 Port Power with Ganged Control with Poly Fuse
USB4640/40i
USB
Device
5 V
PRTCTL3
USB
Device
5V
PRTCTL2
USB
Device
Poly Fuse
5 V
USB
Device
PRTCTL2
PRTCTL3
USB4640/40i