Design procedure – Rainbow Electronics MAX3795 User Manual
Page 12
current. For appropriate R
TC
and R
MODSET
values, see
the Typical Operating Characteristics.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
90ps or better for 4.25Gbps applications. Use a high-
efficiency laser that requires low modulation current and
generates a low-voltage swing. Trim the leads to reduce
laser package inductance. The typical package leads
have inductance of 25nH per inch (1nH/mm). This
inductance causes a large voltage swing across the
laser. A compensation filter network can also be used to
reduce ringing, edge speed, and voltage swing.
Programming Modulation Current
A resistor (R
MODSET
) placed between the MODSET pin
and ground controls the modulation current out of the
MAX3795 to the VCSEL. The modulation current is
given by the following:
It is important to note that the load impedance of the
VCSEL affects the modulation current being sourced by
the MAX3795. The Modulation Current vs. R
MODSET
graph in the Typical Operating Characteristics shows the
current into a 50
Ω load. Capacitance at the MODSET pin
should be
≤20pF.
Programming Bias Current
The bias current output of the MAX3795 is controlled by
a resistor (R
BIASSET
) placed between the BIASSET pin
and ground. In open-loop operation, BIASSET controls
the bias current level of the VCSEL. In closed-loop
operation (APC); the R
BIASSET
controls the maximum
allowed bias current. The open-loop bias current is
given by the following:
The Bias Current vs. R
BIASSET
graph in the Typical
Operating Characteristics shows the current into a 50
Ω
load. Capacitance at the BIASSET pin should be
≤20pF.
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 9k
Ω. 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,
and select the value of R
PWRSET
that corresponds to the
required current.
Laser tempco
SE
SE
SE
ppm C
=
−
Ч
−
Ч
=
°
−
(
)
(
)
/
85
25
25
6
85 25
10
2380
I
R
BIAS
BIASSET
=
+
×
1 2
200
34
.
I
R
R
R
R
MOD
MODSET
OUT
OUT
LOAD
=
+
Ч
Ч
+
1
200
40
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
12
______________________________________________________________________________________
MAX3795
IN+
IN-
1nH
1nH
0.5pF
0.5pF
15pF
V
CC
50
Ω
50
Ω
1k
Ω
V
CC
V
CC
PACKAGE
Figure 4. Simplified Input Structure