Max3737 – Rainbow Electronics MAX3737 User Manual
Page 12
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 (5
Ω typ)
R
D
—Series matching resistor (20
Ω 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
C
F
f
kHz
APC
DB
MON
x
(
)
(
)
(
)
µ
η
ρ
≈
Ч
Ч
68
3
1
1
V
V
I
R
R
V
OUT
C
MOD
D
L
+
=
×
+
(
)
≥
C
-
2
0 75
.
V
V
V
I
R
R
I
R
V
OUT
CC
DIODE
MOD
D
L
BIAS
L
+
=
Ч
+
(
)
Ч
≥
-
-
-
.
0 7
TC
R
k
mA
C
MODTCOMP
=
+
°
±
1
0 5
10
.
(
)
%
Ω
T
C
M
k
R
C
TH
TH TEMP
=
° +
+
° ±
.
.
%
_
- 70
1 45
9 2
10
Ω
Ω
K
I
I
I
I
MOD
MOD
BIAS
BIAS
=
2
1
2
1
-
-
K
R
MODBCOMP
=
+
±
1700
1000
10%
I
TC
T T
T
T
I
T
T
MODT
TH
TH
MODT
TH
=
×
(
)
>
=
≤
|
|
-
0
I
V
R
MODS
REF
MODSET
=
×
268
I
I
K
I
I
MOD
MODS
BIAS
MODT
=
+
×
+
Multirate Laser Driver with Extinction
Ratio Control
12
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