August 2003 27 KS8737
KS8737 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
choke is recommended for exceeding FCC requirements. The following table gives recommended transformer characteristics.
Characteristics Name Value Test Condition
Turns Ratio 1 CT : 1 CT
Open-Circuit Inductance (min.) 350µH 100mV, 100kHz, 8mA
Leakage Inductance (max.) 0.4µH 1MHz (min.)
Inter-Winding Capacitance (max.) 12pF
D.C. Resistance (max.) 0.9Ω
Insertion Loss (max.) 1.0dB 0MHz – 65MHz
HIPOT (min.) 1500Vrms
Note 1. The IEEE 802.3u standard for 100BaseTX assumes a transformer loss of 0.5 dB. 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 Value Units
Frequency 25.00000 MHz
Frequency Tolerance (max.) ±100 ppm
Load Capacitance (max.) 20 pF
Series Resistance (max.) 25 Ω
