VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 3.125Gb/s
VSC7940
Laser Diode Driver with Automatic Power Control
Table 5: Pin Identifications
Pin Name
Pin Number
Description
10, 15, 22,
23, 27
GND
Ground
1, 4, 7, 16, 18,
21, 25, 32
VCC
Power Supply
DATA+
DATA-
CLK+
2
3
5
6
8
Positive Data Input (PECL)
Negative Data Input (PECL)
Positive Clock Input (PECL). Connect to VCC if LATCH function is not used.
Negative Clock Input (PECL). Leave unconnected if LATCH function is not used.
Latch Input (TTL/CMOS). Connect to VCC for data retiming and GND for direct data.
CLK-
LATCH
Enable Input (TTL/CMOS). If used, connect DISABLE to GND. Connect to VCC for
normal operation and GND to disable laser bias and modulation currents.
ENABLE
9
Disable Input (TTL/CMOS). If used, leave ENABLE pin floating. Connect to GND for
normal operation and VCC to disable laser bias and modulation currents.
DISABLE
BIASMON
MODMON
10
11
12
Bias Current Monitor. Sink current source that is proportional to the laser bias current.
Modulation Current Monitor. Sink current source that is proportional to the laser
modulation current.
FAIL
13
14
17
19
20
Output (TTL/CMOS). When low, indicates APC failure.
No effect on device operation.
APCFILT
BIAS
Laser Bias Current Output
OUT+
OUT-
Positive Modulation-Current Output. IMOD flows when input data is high.
Negative Modulation-Current Output. IMOD flows when input data is low.
Monitor Diode Input. Connect to monitor photodiode anode. Connect capacitor to GND to
filter high-speed AC monitor photocurrent.
MD
24
CAPC
26
28
Capacitor to GND sets dominant pole of the APC feedback loop.
Do not connect.
RESERVED
Resistor to GND sets desired average optical power. If APC is not used, connect 100kΩ
resistor to GND.
APCSET
29
30
31
MODSET
BIASMAX
Connect resistor to GND to set desired modulation current.
Connect resistor to GND to set maximum bias current. The APC function can subtract from
this value, but cannot add to it.
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
G52357-0, Rev 3.2
05/11/01
Page 7
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
