VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 3.125Gb/s
VSC7940
Laser Diode Driver with Automatic Power Control
Select Resistors for MODMON and BIASMON
Assuming the modulation and bias currents never exceed 120mA, the following equations provide values
for the resistor at MODMON, R , and the resistor at BIASMON, R
:
BIASMON
MODMON
RMODMON = 1V * 28 / 120mA = 233Ω
RBIASMON = 1.6V * 35 / 120mA = 467Ω
Standard values for these values are R = 232Ω and R
= 464Ω. A voltage of 4.8V at
BIASMON
MODMON
MODMON would indicate a modulation current of:
I
MOD = (5.2V - 4.8V) * 28 / 232mA = 48mA
Wire Bonding
For best performance, gold ball-bonding techniques are recommended. Wedge bonding is not recom-
mended. For best performance and to minimize inductance keep wire bond lengths short.
PCB Layout Guidelines
Use high frequency PCB layout techniques with solid ground planes to minimize crosstalk and EMI. Keep
high speed traces as short as possible for signal integrity. The output traces to the laser diode must be short to
minimize inductance. Short output traces will provide best performance.
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
G52357-0, Rev 3.2
05/11/01
Page 13
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
