SMSC LAN9512 - Manual (Page 17)

USB Hub with Integrated 10/100 Ethernet Controller
Datasheet
SMSC LAN9512 17 Revision 1.0 (04-20-09)
DATASHEET
2.1.2 Port Power Control Using a Poly Fuse
When using theLAN9512 with a poly fuse, an external diode must be used (See Figure 2.3 ). 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 3.3 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 0 volts. The anode of the diode will be at 0.7
volts, and the Schmidt trigger input will register this as a low, resulting in an overcurrent detection. The
open drain output does not interfere.
Figure 2.3 Port Power Control with Poly Fuse
USB
Device
PRTCTL3
5V
Poly Fuse
LAN9512
USB
Device
PRTCTL2
5V
Poly Fuse