Max3737, Detailed description – Rainbow Electronics MAX3737 User Manual
Page 8
MAX3737
Detailed Description
The MAX3737 laser driver consists of three main parts: a
high-speed modulation driver, biasing block with ERC,
and safety circuitry. The circuit design is optimized 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 MAX3737 is optimized for driving a 15
Ω
load. The minimum instantaneous voltage required at
OUT+ is 0.7V for modulation current 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 due to the series
damping resistor and the equivalent series resistance
(ESR) of the laser diode should equal 15
Ω. To further
damp aberrations caused by laser diode parasitic induc-
tance, an RC shunt network may be necessary. Refer to
Maxim Application Note HFAN 02.0: Interfacing Maxim’s
Laser Drivers to Laser Diodes for more information.
At data rates of 2.7Gbps, any capacitive load at the
cathode of a laser diode degrades optical output perfor-
mance. 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 parasitics from 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:
P
AVG
+ P
P-P
/ 2P
AVG
- P
P-P
Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compen-
sating the modulation current for reduced slope effi-
ciency (
η) of the laser over time and temperature:
P
P-P
=
η x I
MOD
P
I
AVG
MD
MON
=
ρ
Multirate Laser Driver with Extinction
Ratio Control
8
_______________________________________________________________________________________
100mV (MIN)
1200mV (MAX)
200mV
P-P
(MIN),
2400mV (MAX)
I
MOD
TIME
SINGLE ENDED
DIFFERENTIAL
VOLTAGE
CURRENT
V
IN
+
V
IN
-
I
OUT
+
(V
IN
+) - (V
IN
-)
Figure 1. Required Input Signal and Output Polarity
Z
0
= 30
Ω
30
Ω
0.5pF
30
Ω
30
Ω
OSCILLOSCOPE
OUT-
OUT-
V
CC
I
OUT
+
V
CC
MAX3737
Z
0
= 30
Ω
75
Ω
Z
0
= 50
Ω
50
Ω
OUT+
OUT+
Figure 2. Test Circuit for Characterization
VOLTAGE
SUPPLY
C1
0.1
µF
C3
0.1
µF
C2
10
µF
L1
1
µH
OPTIONAL
TO LASER
DRIVER V
CC
OPTIONAL
SOURCE
NOISE
HOST BOARD
FILTER DEFINED BY SFP MSA
MODULE
Figure 3. Supply Filter