Datasheet

Programming Modulation Current
The modulation current output of the MAX3740A is con-
trolled by a resistor (R
MODSET
) placed between
MODSET and ground. The R
MODSET
resistor controls
the amount of current being sourced to the VCSEL. The
modulation current is given by the following:
It is important to note that the modulation current being
sourced by the MAX3740A is affected by the load
impedance of the VCSEL. The Modulation Current vs.
R
MODSET
graph in the Typical Operating Characteristics
shows the current into a 50Ω electrical load.
Programming Bias Current
The bias current output of the MAX3740A is controlled
by a resistor (R
BIASSET
) placed between BIASSET and
ground. In open-loop operation the R
BIASSET
controls
the bias current level of the VCSEL. In closed-loop
operation the R
BIASSET
controls the maximum bias cur-
rent provided by the APC. The bias current is given by
the following:
The Bias Current vs. R
BIASSET
graph is also shown in
the Typical Operating Characteristics .
Photodiode Selection
To ensure stable operation of the APC circuit, the time
constant of the MD node should be shorter than the
APC time constant. (t
APC
= 5µs if C
APC
= 0.047µF).
For typical I
PD
= 400µA, R
PWRSET
= 500Ω, select a
photodiode with capacitance less than 500pF.
Programming Modulation-Current Tempco
Compute the required modulation tempco from the
slope efficiency of the laser at T
A
= +25°C and at a
higher temperature. Then select the value of R
TC
from
the Typical Operating Characteristics . For example,
suppose a laser has a slope efficiency (SE) of
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
From the Typical Operating Characteristics , the value
of R
TC
, which offsets the tempco of the laser, is 9kΩ. If
modulation temperature compensation is not desired,
short TC1 and TC2.
Programming the APC Loop
Program the average optical power by adjusting
R
PWRSET
. To select the resistance, determine the
desired monitor current to be maintained over tempera-
ture and lifetime. See the Monitor Diode Current vs.
R
PWRSET
graph in the Typical Operating Characteristics
section, and select the value of R
PWRSET
that corre-
sponds to the required current.
Input Termination Requirements
The MAX3740A data inputs are SFP MSA compatible.
On-chip 100Ω differential input impedance is provided
for optimal termination (Figure 4). Because of the on-chip
biasing network, the MAX3740A inputs self-bias to the
proper operating point to accommodate AC-coupling.
Laser tempco
SE SE
SE
E
ppm C
=
−
×−
×
=− °
()
()
/
85 25
25
85 25
16
2380
t
MD
t
APC
MD MD
RC
s
ns≤×≤
μ
=
20
5
20
250,
II
I
R
BIAS BIASSET
BIAS
BIASSET
=
()
×
=
+
⎛
⎝
⎜
⎞
⎠
⎟
×
34
12
200
34
.
II
R
RR
I
R
R
RR
MOD MODSET
OUT
OUT LOAD
MOD
MODSET
OUT
OUT LOAD
=
()
×
[]
×
+
⎡
⎣
⎢
⎤
⎦
⎥
=
+
⎛
⎝
⎜
⎞
⎠
⎟
×
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
×
+
⎡
⎣
⎢
⎤
⎦
⎥
+
+
+
+
30
1
200
30
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
12 ___________________________________________________
MAX3740A
IN+
IN-
1nH
1nH
0.5pF
0.5pF
V
CC
50Ω
50Ω
16kΩ
24kΩ
V
CC
V
CC
PACKAGE
Figure 4. Simplified Input Structure