
Micrel, Inc.
KSZ8842-PMQL/PMBL
October 2007
114
M9999-100207-1.5
Selection of Isolation Transformers
A 1:1 isolation transformer is required at the line interface. An isolation transformer with integrated common-mode choke
is recommended for exceeding FCC requirements.
Table 18 gives recommended transformer characteristics.
Parameter
Value
Test Condition
Turns ratio
1 CT : 1 CT
Open-circuit inductance (min)
350
H
100mV, 100kHz, 8mA
Leakage inductance (max)
0.4
H
1MHz (min)
Inter-winding capacitance (max)
12pF
D.C. resistance (max)
0.9
Insertion loss (max)
1.0dB
0MHz – 65MHz
HIPOT (min)
1500Vrms
Table 18. Transformer Selection Criteria
Magnetic Manufacturer
Part Number
Auto MDI-X
Number of Port
Pulse
H1102
Yes
1
Pulse (low cost)
H1260
Yes
1
Transpower
HB726
Yes
1
Bel Fuse
S558-5999-U7
Yes
1
Delta
LF8505
Yes
1
LanKom
LF-H41S
Yes
1
TDK (Mag Jack)
TLA-6T718
Yes
1
Table 19. Qualified Single Port Magnetics
Selection of Reference Crystal
Characteristics
Value
Units
Frequency
25
MHz
Frequency tolerance (max)
±50
ppm
Load capacitance (max)
20
pF
Series resistance
25
Table 20. Typical Reference Crystal Characteristics