MAX3646 Maxim Integrated Products, MAX3646 Datasheet - Page 7

• Manufacturer Part Number: MAX3646
• Description: 155Mbps to 622Mbps Sff/sfp Laser Driver
• Manufacturer: Maxim Integrated Products

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Figure 1. Required Input Signal and Output Polarity
Figure 3. Supply Filter
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
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
resistor and the equivalent series resistance (ESR) of
(V
CURRENT
D
VOLTAGE
IN+
) is required. The combined resistance damping
) - (V
I
OUT+
V
V
IN-
IN+
IN-
4).
)
Laser Driver with Extinction Ratio Control
High-Speed Modulation Driver
_______________________________________________________________________________________
Detailed Description
SINGLE ENDED
DIFFERENTIAL
VOLTAGE
SUPPLY
TIME
SOURCE
NOISE
155Mbps to 622Mbps SFF/SFP
C1
0.1µF
100mV (min)
1200mV (max)
200mV (min)
2400mV (max)
I
MOD
FILTER DEFINED BY SFP MSA
HOST BOARD
C2
10µF
1µH
L1
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.
The extinction ratio (r
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
Figure 2. Test Circuit for Characterization
MAX3646
C3
0.1µF
r
e
OUT+
OUT-
= (2P
OPTIONAL
OPTIONAL
30Ω
AVG
V
0.5pF
I
OUT+
CC
MODULE
V
+ P
CC
e
DRIVER V
Extinction Ratio Control
30Ω
TO LASER
) is the laser on-state power
P-P
Z
Z
0
0
= 30Ω
= 30Ω
) / (2P
CC
AVG
30Ω
75Ω
Z
0
= 50Ω
- P
P-P
OSCILLOSCOPE
)
50Ω
7