Micrel, Inc. KS8721BL/SL
June 2009
33
M9999-062509-1.3
Selection of Isolation Transformers
(1)
One simple 1:1 isolation transformer is needed at the line interface. An isolation transformer with integrated common-
mode choke is recommended for exceeding FCC requirements. The following table gives recommended transformer
characteristics.
Parameter Value Test Condition
Turns ratio 1 CT : 1 CT
Open-circuit inductance (min)
350
μH
100mV, 100kHz, 8mA
Leakage inductance (max)
0.4
μH
1MHz (min)
Inter-winding capacitance (max) 12pF
D.C. resistance (max)
0.9
Ω
Insertion loss (max) 1.0dB 0MHz – 65MHz
HIPOT (min) 1500Vrms
Note:
1. The IEEE 802.3u standard for 100BASE-TX assumes a transformer loss of 0.5dB. For the transmit line transformer, insertion loss of up to 1.3dB
can be compensated or by increasing the line drive current by means of reducing the ISET resistor value. Please select the transformer that
supports auto-MDI/MDI-X.
Selection of Reference Crystal
An oscillator or crystal with the following typical characteristics is recommended.
Characteristics Value Units
Frequency 25 MHz
Frequency tolerance (max)
±100
ppm
Load capacitance (max) 20 pF
Series resistance (max) 40
Ω
Single Port Magnetic Manufacturer Part Number Auto MDIX Number of Ports
Pulse H1102 Yes 1
Bel Fuse S558-5999-U7 Yes 1
YCL PT163020 Yes 1
Transpower HB726 Yes 1
Delta LF8505 Yes 1
LanKom LF-H41S Yes 1
Integrated Transformers
Pulse J0011D21 Yes 1
Pulse J00-0061 Yes 1
Table 9. Qualified Transformer Lists
