Datasheet

If PC board space is not a constraint, Maxim recom-
mends enabling the transmitter with a reset-pulse gen-
erator, such as the MAX809, which generates a reset
signal after V
CC
reaches 4.5V (Figure 6). This method
ensures that the transmitter starts correctly, even if the
supply ramps very slowly.
__________________Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
0.5ns or better for 622Mbps applications. The voltage
swing at the OUT+ pin affects the output waveform,
and is largely determined by the laser resistance,
inductance, and modulation current. To obtain the
MAX3766’s AC specifications, the output voltage at
OUT+ must remain above V
CC
- 2.5V at all times.
An approximation for the minimum voltage at OUT+ is
given by the following equation (Table 1):
Select a laser that meets the output voltage criteria. A
high-efficiency laser requires low modulation current
and generates low voltage swing at OUT+. Laser pack-
age inductance can be reduced by trimming leads.
Typical package leads have inductance of 25nH per
inch (1nH/mm). A compensation filter network can also
be used to reduce ringing, edge speed, and voltage
swing.
V = V- V-I
OUT(MIN) CC(MIN) LASER MOD
()
++
⎛
⎝
⎜
⎞
⎠
⎟
RR
L
t
LD
r
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
______________________________________________________________________________________ 11
MAX3766*
MAX809MEUR-T
SAFETY
C
SAFETY
RESET
V
CC
V
CC
OR
ENABLE
ENABLE
*IN LATCHED SHUTDOWN
CONFIGURATION
Figure 6. Reset Pulse Generator
Table 1. Output Voltage Approximation
Minimum power supplyV
CC(MIN)
4.5V
Laser dynamic resistanceR
L
3Ω
Laser forward voltage at operating powerV
LASER
1.3V
Any damping resistance or line termination in series with the laser
(but not in series with BIAS)
R
D
10Ω
20% to 80% rise time of the laser modulation current, filtered by a
compensation network
t
r
300ps
(20% to 80%)
Total series inductance of laser, laser package, and board traces to the
MAX3766
L 6nH
Laser modulation currentI
OUT
30mA
VARIABLE
Approximation for the lowest voltage at the OUT+ pinV
OUT(MIN)
DESCRIPTION TYPICAL VALUE
2.2V