VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 3.125Gb/s
Laser Diode Driver with Automatic Power Control
VSC7938
Applications Information
The following is a typical design example for the VSC7938 assuming 3.3V operation with APC.
Select a Laser
The Table 5 provides specifications for a typical communication-grade laser capable of operating at
2.5Gb/s.
Table 6: Typical Laser Characteristics
Symbol
Parameter
Value
Units
λ
Wavelength
1310
nm
mW
PAVE
Ith
ρMON
η
Average Optical Output Power
Threshold Current
6
6
mA
Laser to Monitor Transfer
Laser Slope Efficiency
Operating Temperature Range
0.04
mA/mW
mW/mA
°C
0.4
TC
-40 to +85
Select Resistor for APCSET
The monitor diode current is estimated by IMD = PAVE * ρMON = 6mW * 0.04mA/mW = 0.24mA. The IMD
vs. RAPCSET in Typical Operating Characteristics shows the resistor at APCSET should be 5kΩ.
Select Resistor for MODSET
To ensure some minimum extinction ratio over temperature and lifetime, assume an optimal extinction ratio
of 20 (13dB) at 25°C. The modulation current may be calculated from the following equation:
I
= P / η= 2 * P
* (r -1) / (r +1) / η = 2 * 6mA * (20-1) / (20 + 1) / 0.4 = 27.1mA
MOD
p-p
AVE e e
The graph of IMODSET vs. RMODSET in Typical Operating Characteristics shows the resistor for MODSET
should be 8.5kΩ.
Select Resistor for BIASMAX
The maximum threshold current at +85°C and end of life must be determined. A graph of a typical laser’s
I
versus TC reveals a maximum threshold current of 30mA at 85°C. Therefore, the maximum bias can be
th
approximated by:
IBIASMAX = ITH-MAX + IMOD / 2 = 30mA + 27.1mA / 2 = 43.6mA
The graph of IBIASMAX vs. RBIASMAX in Typical Operating Characteristics shows the resistor for BIASMAX
should be 5kΩ.
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
G52349-0, Rev 3.0
01/20/01
Page 13
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
