LAN9514_12 Datasheet, PDF (18/52 Pages) SMSC Corporation – USB 2.0 Hub and 10/100 Ethernet Controller

2.2.2
USB 2.0 Hub and 10/100 Ethernet Controller
Datasheet
Port Power Control Using a Poly Fuse
When using the LAN9514/LAN9514i with a poly fuse, an external diode must be used (See Figure 2.4).
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.
5V
Poly Fuse
PRTCTL4
PRTCTL3
5V
Poly Fuse
USB
Device
5V
Poly Fuse
LAN9514/
LAN9514i
PRTCTL5
PRTCTL2
USB
Device
5V
Poly Fuse
USB
Device
USB
Device
Figure 2.4 Port Power Control with Poly Fuse
Revision 1.2 (02-29-12)
18
DATASHEET
SMSC LAN9514/LAN9514i