KS8721B/BT
Micrel
Selection of Isolation Transformer(Note 1)
One simple 1:1 isolation transformer is needed at the line interface. An isolation transformer with integrated common-mode
chokeisrecommendedforexceedingFCCrequirements. Thefollowingtablegivesrecommendedtransformercharacteristics.
Characteristics Name
Turns Ratio
Value
Test Condition
1 CT : 1 CT
350µH
0.4µH
Open-Circuit Inductance (min.)
Leakage Inductance (max.)
Inter-Winding Capacitance (max.)
D.C. Resistance (max.)
Insertion Loss (max.)
HIPOT (min.)
100mV, 100 KHz, 8 mA
1MHz (min.)
12pF
0.9Ω
1.0dB
0MHz to 65MHz
1500Vrms
Note 1. The IEEE 802.3u standard for 100BaseTX assumes a transformer loss of 0.5dB. For the transmit line transformer, insertion loss of up to
1.3dB can be compensated by increasing the line drive current by means of reducing the ISET resistor value.
Selection of Reference Crystal
An oscillator or crystal with the following typical characteristics is recommended.
Characteristics Name
Frequency
Value
25.00000
±100
20
Units
MHz
ppm
pF
Frequency Tolerance (max.)
Load Capacitance (max.)
Series Resistance (max.)
25
Ω
Single Port
Magnetic Manufacturer
Number
of Ports
Part Number
Auto MDIX
Yes
Pulse
H1102
1
1
1
1
1
1
Bel Fuse
YCL
S558-5999-U7
PT163020
HB726
Yes
Yes
Transpower
Delta
Yes
LF8505
Yes
LanKom
LF-H41S
Yes
Table 8. Qualified Transformer Lists
KS8721B/BT
30
August 2003
MICREL [ MICREL SEMICONDUCTOR ]
