VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 3.125Gb/s
VSC7940
Laser Diode Driver with Automatic Power Control
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 to 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 describe the relationship between the resistor at MODSET and the output modulation cur-
rent 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 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 determine the appropriate resistor value at
BIASMAX. When using APC, BIASMAX sets the maximum allowed bias current. After determining the max-
imum end-of-life bias current at 85°C for the laser, refer to the graph of IBIASMAX vs. RBIASMAX in Typical Oper-
ating 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
Laser Diode Interface
An RC shunt network should be placed at the laser output interface. The sum of the resistor placed at the
output and the laser diode resistance should be 25Ω. For example, if the laser diode has a resistance of 5Ω, a
20Ω resistor should be placed in series with the laser. For optimal performance, a bypass capacitor should be
placed close to the laser anode.
A “snubber network” consisting of a capacitor CF and resistor RF should be placed at the laser output to
minimize reflections from the laser (see Block Diagram). Suggested values for these components are 80Ω and
2pF, respectively, however, these values should be adjusted until an optical output waveform is obtained.
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
G52357-0, Rev 3.2
05/11/01
Page 9
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
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