Manual
MAX3737
An external resistor at the MODBCOMP pin sets current
proportional to I
BIAS
. Open circuiting the MODBCOMP
pin can turn off the interaction between I
BIAS
and I
MOD
:
If I
MOD
must be increased from I
MOD1
to I
MOD2
to
maintain the extinction ratio at elevated temperature,
the required compensation factor is:
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
The temperature coefficient of thermal compensation
above T
TH
is set by R
MODTCOMP
. Leaving the MODT-
COMP pin open disables additional thermal compensation:
Current Compliance (I
MOD
≤ 60mA),
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
For:
V
DIODE
—Diode bias point voltage (1.2V typ)
R
L
—Diode bias point resistance (5Ω typ)
R
D
—Series matching resistor (20Ω typ)
For compliance:
Current Compliance (I
MOD
> 60mA),
AC-Coupled
For applications requiring modulation current greater
than 60mA, headroom is insufficient for proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3737’s modulation output
can be AC-coupled to the cathode of a laser diode. An
external pullup inductor is necessary to DC-bias the
modulation output at V
CC
. Such a configuration isolates
laser forward voltage from the output circuitry and
allows the output at OUT+ to swing above and below
the supply voltage (V
CC
). When AC-coupled, the
MAX3737 modulation current can be programmed up
to 85mA. Refer to Maxim Application Note HFAN 02.0:
Interfacing Maxim’s Laser Drivers to Laser Diodes for
more information on AC-coupling laser drivers to laser
diodes.
For compliance:
Determine C
APC
The APC loop filter capacitor C
APC
must be selected to
balance the requirements for fast turn-on and minimal
interaction with low frequencies in the data pattern. The
low-frequency cutoff is:
High-frequency noise can be filtered with an additional
cap C
MD
from the MD pin to ground:
The MAX3737 is designed so that turn-on time is faster
than 1ms for most laser gain values (η ✕ ρ
MON
).
Choosing a smaller value of C
APC
reduces turn-on
time. Careful balance between turn-on time and low-fre-
quency cutoff may be needed at low data rates for
some values of laser gain.
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3737 laser driver. If dice are used,
replace package parasitic elements with bondwire par-
asitic elements.
C
C
MD
APC
≈
4
CF
f kHz
APC
DB
MON
x
()
()
( )
µηρ≈××
68
3
11
VV
I
RR V
OUT C
MOD
DL+
=
×+
()
≥
C
-
2
075 .
VVV I RRI R V
OUT CC DIODE MOD D L BIAS L+
=
×+
()
×≥
- - - .07
TC
Rk
mA
C
MODTCOMP
=
+°
±
1
05
10
. ()
%
Ω
TC
M
kR
C
TH
TH TEMP
=°+
+
°±
.
.
%
_
-70
145
92
10
Ω
Ω
K
II
II
MOD MOD
BIAS BIAS
=
21
21
-
-
K
R
MODBCOMP
=
+
±
1700
1000
10%
ITCTTTT
ITT
MODT TH TH
MODT TH
=×
()
>
=≤
|
|
-
0
I
V
R
MODS
REF
MODSET
=×268
II KII
MOD MODS BIAS MODT
=+×+
Multirate Laser Driver with Extinction
Ratio Control
12 ______________________________________________________________________________________