VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
SONET/SDH 3.125Gb/s
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 within approxi-
mately 250ns.
Slow-Start
For laser safety, the VSC7938 offers a slow-start mechanism via the SLWSTRT pin which provides delay
for enabling the laser diode. To disable Slow-Start, leave SLWSTRT open. An external capacitor to ground sets
the delay by the following equation:
τ
ENABLE
(ns) = C
SLWSTRT
(pF) * 20 + 250ns
Controlling the Modulation Current
The output modulation current may be determined from the following equation where P
p-p
is the peak-to-
peak optical power, P
AVE
is the average power, r
e
is the extinction ratio, and
η
is the laser slope efficiency:
I
MOD
= P
p-p
/
η
= 2 * P
AVE
* (r
e
-1) / (r
e
+1) /
η
A resistor at MODSET controls the output bias current. Graphs of I
MODSET
vs. R
MODSET
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 I
BIASMAX
vs. R
BIASMAX
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
I
BIASMAX
vs. R
BIASMAX
in
Typical Operating Characteristics
to select the appropriate resistor value.
Controlling the APC Loop
To select the resistor at APCSET, use the graph of I
MD
vs. R
APCSET
in
Typical Operating Characteristics.
The graph relates the desired monitor current to the appropriate resistance value at APCSET. I
MD
may be calcu-
late from the desired optical average power, P
AVE,
, and the laser-to-monitor transfer,
ρ
MON
, for a specific laser
using the following equation:
I
MD
= P
AVE
*
ρ
MON
G52349-0, Rev 3.0
01/20/01
©
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 9
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