VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Features
SONET/SDH 3.125Gb/s
Laser Diode Driver with Automatic Power Control
Applications
• SDH/SONET at 622Mb/s, 1.244Gb/s,
2.488Gb/s, 3.125Gb/s
• Power Supply: 3.3V or 5V
±
5%
• AC-Coupled to Laser Diode
• Programmable Modulation Current: 5mA to 60mA
• Programmable Bias Current: 1mA to 100mA
• Enable/Disable Control
• Typical Rise/Fall Times of 60ps
• Automatic Optical Average Power Control
• Supply Current of 33mA at 3.3V
•
Full-Speed Fibre Channel (1.062Gb/s)
General Description
The VSC7938 is a single 3.3V or 5V supply laser diode driver specially designed for SDH/SONET applica-
tions up to 3.125Gb/s. External resistors set a wide range of bias and modulation currents for driving the laser.
Data and clock inputs accept differential PECL signals. The automatic power control (APC) loop maintains a
constant average optical power over temperature and lifetime. The dominant pole of the APC loop can be con-
trolled with an external capacitor. Other features include enable/disable control, short-circuit protection for the
modulation and bias inputs, short rise and fall times, programmable slow-start circuit to set laser turn-on delay,
and failure-monitor output to indicate when the APC loop is unable to maintain the average optical power. The
VSC7938 is available in die form or in a 48-pin TQFP package.
Block Diagram
3.3V Operation
V
CC
LATCH
IOUT+
IOUT-
MUX
DATA+
DATA-
CLK+
CLK-
SET
L
P
C
D
C
F
R
F
V
CC
BIAS
D
Q
CLR
Q
ENABLE
APC
DISABLE
MD
FAIL
SLWSTRT
MODSET
BIASMAX
CAPC
APCSET
1nF
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
L
P
Page 1
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
