VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Table 5: Pin Identifications
Pin Name
GND
V
CC
DATA+
DATA-
CLK+
CLK-
LATCH
ENABLE
DISABLE
SLWSTRT
FAIL
NC
APCFILT
BIAS
OUT+
OUT-
MD
CAPC
RESERVED
APCSET
MODSET
BIASMAX
SONET/SDH 3.125Gb/s
Laser Diode Driver with Automatic Power Control
Pin Number
1, 2, 7, 12, 16,
20, 23, 33, 34
36, 40, 42, 45
3, 6, 8, 11, 18, 24,
27, 32, 37, 48
4
5
9
10
13
14
15
17
19
21, 26, 28, 31, 39,
41
22
25
29
30
35
38
43
44
46
47
Ground
Power Supply
Positive Data Input (PECL)
Negative Data Input (PECL)
Description
Positive Clock Input (PECL). Connect to V
CC
if data retiming is not used.
Negative Clock Input (PECL). Leave unconnected if data retiming is not used.
Latch Input (TTL/CMOS). Connect to V
CC
for data retiming and GND for direct data.
Enable Input (TTL/CMOS). If used, connect DISABLE to GND. Connect to V
CC
for
normal operation and GND to disable laser bias and modulation currents.
Disable Input (TTL/CMOS). If used, leave ENABLE pin floating. Connect to GND for
normal operation and V
CC
to disable laser bias and modulation currents.
Connect capacitor to GND to delay turn on time of bias and modulation currents.
Output (TTL/CMOS). When low, it indicates APC failure.
No Connection. Leave these pins unconnected.
No effect on device operation.
Laser Bias Current Output
Positive Modulation-Current Output. I
MOD
flows when input data is high.
Negative Modulation-Current Output. I
MOD
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.
Capacitor to GND sets dominant pole of the APC feedback loop.
Do not connect.
Resistor to GND sets desired average optical power. If APC is not used connect 100kΩ
resistor to GND.
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 it cannot add to it.
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
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