Datasheet

 2016 Microchip Technology Inc. DS00002199A-page 45
KSZ8081RNA/RND
11.0 MAGNETIC - CONNECTION AND SELECTION
A 1:1 isolation transformer is required at the line interface. Use one with integrated common-mode chokes for designs
exceeding FCC requirements.
The KSZ8081RNA/RND design incorporates voltage-mode transmit drivers and on-chip terminations.
With the voltage-mode implementation, the transmit drivers supply the common-mode voltages to the two differential
pairs. Therefore, the two transformer center tap pins on the KSZ8081RNA/RND side should not be connected to any
power supply source on the board; instead, the center tap pins should be separated from one another and connected
through separate 0.1 µF common-mode capacitors to ground. Separation is required because the common-mode volt-
age is different between transmitting and receiving differential pairs.
Figure 11-1 shows the typical magnetic interface circuit for the KSZ8081RNA/RND.
FIGURE 11-1: TYPICAL MAGNETIC INTERFACE CIRCUIT
Table 11-1 lists recommended magnetic characteristics.
Table 11-2 is a list of compatible single-port magnetics with separated transformer center tap pins on the PHY chip side
that can be used with the KSZ8081RNA/RND.
TABLE 11-1: MAGNETICS SELECTION CRITERIA
Parameter Value Test Conditions
Turns Ratio 1 CT : 1 CT —
Open-Circuit Inductance (min.) 350 µH 100 mV, 100 kHz, 8 mA
Insertion Loss (max.) –1.1 dB 100 kHz to 100 MHz
HIPOT (min.) 1500 V
RMS
—
1
2
3
7
8
4
5
6
4 x 75
1000pF/2kV
RJ-45 CONNECTOR
CHASSIS GROUND
(2 x 0.1μF)
TXP
TXM
RXP
RXM
KSZ8081RNA/RND
SIGNAL GROUND