Detailed description – Rainbow Electronics MAX3646 User Manual
Page 7
Detailed Description
The MAX3646 laser driver consists of three main parts:
a high-speed modulation driver, biasing block with
ERC, and safety circuitry. The circuit design is opti-
mized for high-speed, low-voltage (+3.3V) operation
(Figure 4).
High-Speed Modulation Driver
The output stage is composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3646 is optimized for driving a 15Ω
load. The minimum instantaneous voltage required at
OUT- is 0.7V for modulation currents up to 60mA and
0.75V for currents from 60mA to 85mA. Operation
above 60mA can be accomplished by AC-coupling or
with sufficient voltage at the laser to meet the driver
output voltage requirement.
To interface with the laser diode, a damping resistor
(R
D
) is required. The combined resistance damping
resistor and the equivalent series resistance (ESR) of
the laser diode should equal 15Ω. To further damp
aberrations caused by laser diode parasitic induc-
tance, an RC shunt network may be necessary. Refer to
Maxim Application Note HFAN 0.0:
Interface Maxim’s
Laser Driver to Laser Diode for more information.
Any capacitive load at the cathode of a laser diode
degrades optical output performance. Because the
BIAS output is directly connected to the laser cathode,
minimize the parasitic capacitance associated with the
pin by using an inductor to isolate the BIAS pin para-
sitics form the laser cathode.
Extinction Ratio Control
The extinction ratio (r
e
) is the laser on-state power
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
r
e
= (2P
AVG
+ P
P-P
) / (2P
AVG
- P
P-P
)
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
_______________________________________________________________________________________
7
V
IN+
V
IN-
200mV (min)
2400mV (max)
100mV (min)
1200mV (max)
I
MOD
VOLTAGE
CURRENT
TIME
SINGLE ENDED
DIFFERENTIAL
(V
IN+
) - (V
IN-
)
I
OUT+
Figure 1. Required Input Signal and Output Polarity
MAX3646
30Ω
30Ω
30Ω
75Ω
50Ω
OUT+
OUT-
V
CC
V
CC
OSCILLOSCOPE
Z
0
= 30Ω
Z
0
= 30Ω
Z
0
= 50Ω
0.5pF
I
OUT+
Figure 2. Test Circuit for Characterization
L1
1µH
C1
0.1µF
C3
0.1µF
C2
10µF
VOLTAGE
SUPPLY
SOURCE
NOISE
OPTIONAL
OPTIONAL
FILTER DEFINED BY SFP MSA
HOST BOARD
MODULE
TO LASER
DRIVER V
CC
Figure 3. Supply Filter