SMSC LAN91C111
The receive input must be terminated with the correct termination resistance to meet the input impedance and return loss requirements of IEEE 802.3. In addition, the receive TP inputs must be attenuated. Both the termination and attenuation is accomplished with four external resistors in series across the TPI± inputs. Each resistor should be 25% of the total series resistance, and the total series resistance should be equal to the characteristic impedance of the cable (100 W for UTP, 150 W for STP). It is also recommended that a 0.01 F capacitor be placed between the center of the series resistor string and VDD to provide an AC ground for attenuating common mode signal at the input.
To minimize common mode input noise and to aid in meeting susceptibility requirements, it may be necessary to add a common mode choke on the receive input. Common mode bundle termination may be required and can be achieved by connecting the unused pairs in the RJ45 connector are connected to chassis ground through 75 W resistors and a 1000 pF capacitor. A common mode AC ground return path can be designed by connecting the center tap with a 75W resistor and capacitor to chassis ground.
To minimize noise pickup into the receive path in a system or on a PCB, loading on TPI should be minimized and both inputs should be loaded equally.
7.1.3Magnetics
For the suggested Magnetics for the Twisted Pair interface refer to Application Note 8.13, “Suggested Magnetics”. The latest revision is available on the SMSC web site , or contact your SMSC representative.
7.2RBIAS
7.2.1RBIAS pin
The LAN91C111 RBIAS pin is used to set transmit current level. An external resistor connected between this pin and ground will set the output current for the TP transmits outputs.
An 11Kohm resistor should be connected between the RBIAS pin and ground.
7.2.2TP Transmit Output Current Set
The TPO± output current level is set with an external resistor connected between the RBIAS pin and GND. This output current is determined from the following equation, where R is the value of RBIAS:
Iout = (11K/R) Iref
Where
Iref = 40mA (100Mbps, UTP)
=32.6mA (100Mbps, STP)
=100mA (10Mbps, UTP)
=81.6mA (10Mbps, STP)
RBIAS should typically be an 11 kW 1% resistor to meet IEEE 802.3 specified levels. Once RBIAS is set for the 100Mbps and UTP modes as shown by the equation above, Iref is then automatically changed inside the device when the 10Mbps mode or UTP120/STP150 modes are selected.
Keep the RBIAS resistor as close to the RBIAS and GND pins as possible to reduce noise pickup into the transmitter. Because the TP output is a current source, capacitive and inductive loading can reduce the output voltage from the ideal level. Thus, in actual application, it might be necessary to adjust the value of the output current to compensate for external loading. One way to adjust the TP output level is to change the value of the external resistor connected to RBIAS. This value is PCB design dependant, most be verified by designer.
SMSC AN 9.6 | 43 | Revision 1.0 |
APPLICATION NOTE