VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 3.125Gb/s
Laser Diode Driver with Automatic Power Control
VSC7939
Modulation and Bias Current Monitors
The VSC7939 provides monitoring of the modulation and bias currents vias BIASMON and MODMON.
These pins sink a current proportional to the actual modulation and bias currents. MODMON sinks approxi-
mately 1/28th of the amount of modulation current and BIASMON sink approximately 1/35th of the amount of
the bias current. These pins should be tied through a pull-up resistor to V . The resistors must be chosen such
CC
that the voltage at MODMON is greater than V - 1.0V and the voltage at BIASMON is greater than V
-
CC
CC
1.6V.
Enable/Disable
Two pins are provided to allow either ENABLE or DISABLE control. If ENABLE is used, connect DIS-
ABLE to ground. Is DISABLE is used, leave ENABLE floating. Both modulation and bias currents are turned
off when ENABLE is low or DISABLE is high. Typically, ENABLE or DISABLE responds within approxi-
mately 250ns.
Controlling the Modulation Current
The output modulation current may be determined from the following equation where P is the peak-to-
p-p
peak optical power, P
is the average power, r is the extinction ratio, and η is the laser slope efficiency:
AVE
e
I
= P / η= 2 * P
* (r -1) / (r +1) / η
MOD
p-p
AVE e e
A resistor at MODSET controls the output bias current. Graphs of IMODSET vs. RMODSET in Typical Operat-
ing Characteristics for both 3.3V and 5V operation describe the relationship between the resistor at MODSET
and the output modulation current at 25°C. After determining the desired output modulation current, use the
graph to determine the appropriate resistor value at MODSET.
Controlling the Bias Current
A resistor at BIASMAX should be used to control the output bias current. Graphs of IBIASMAX vs. RBIASMAX
in Typical Operating Characteristics for both 3.3V and 5V operation describe the relationship between the
resistor at BIASMAX and the output bias current at 25°C. If the APC is not used, the appropriate resistor value
at BIASMAX is determined by first selecting the desired output bias current, and then using the graph to deter-
mine the appropriate resistor value at BIASMAX. When using APC, BIASMAX sets the maximum allowed
bias current. After determining the maximum end-of-life bias current at 85°C for the laser, refer to the graph of
IBIASMAX vs. RBIASMAX in Typical Operating Characteristics to select the appropriate resistor value.
Controlling the APC Loop
To select the resistor at APCSET, use the graph of IMD vs. RAPCSET in Typical Operating Characteristics.
The graph relates the desired monitor current to the appropriate resistance value at APCSET. IMD may be calcu-
late from the desired optical average power, PAVE,, and the laser-to-monitor transfer, ρMON, for a specific laser
using the following equation:
IMD = PAV E * ρMON
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
Page 10
G52350-0, Rev 3.2
02/26/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
