MAX3738 Maxim Integrated Products, MAX3738 Datasheet - Page 8

• Manufacturer Part Number: MAX3738
• Description: 155Mbps to 4.25Gbps SFF/SFP Laser Driver with Extinction Ratio Control
• Manufacturer: Maxim Integrated Products

155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
The MAX3738 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).
The output stage is composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3738 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
Figure 1. Required Input Signal and Output Polarity
Figure 3. Supply Filter
8
(V
CURRENT
D
VOLTAGE
IN+
_______________________________________________________________________________________
) is required. The combined resistance damping
) - (V
I
OUT+
V
V
IN-
IN+
IN-
)
High-Speed Modulation Driver
Detailed Description
SINGLE ENDED
DIFFERENTIAL
VOLTAGE
SUPPLY
TIME
SOURCE
NOISE
C1
0.1μF
100mV (min)
1200mV (max)
200mV (min)
2400mV (max)
I
MOD
FILTER DEFINED BY SFP MSA
HOST BOARD
C2
10μF
1μH
L1
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
Application Note 274: HFAN-02.0: Interfacing Maxim
Laser Drivers with Laser Diodes for more information.
At high data rates, 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 asso-
ciated with the pin by using an inductor to isolate the
BIAS pin parasitics 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
MAX3738
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
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AVG
30Ω
75Ω
Z
0
= 50Ω
- P
P-P
OSCILLOSCOPE
)
50Ω