MAX3261
Single +5V, Fully Integrated,
1.25Gbps Laser Diode Driver
10 ______________________________________________________________________________________
Reducing Power Consumption
The laser driver typically consumes 40mA of current
for internal functions. Typical load currents, such as
12mA of modulation current and 20mA of bias cur-
rent, bring the total current requirement to 72mA. If
this were dissipated entirely in the laser driver, it
would generate 360mW of heat. Fortunately, a sub-
stantial portion of this power is dissipated across the
laser diode. A typical laser diode will drop approxi-
mately 1.6V when forward biased. This leaves 3.4V at
the MAX3261’s OUT- terminal. It is safe to reduce the
output terminal voltage even further with a series
damping resistor. Terminal voltage levels down to
2.2V can be used without degrading the laser driver’s
high-frequency performance. Power dissipation can
be further reduced by adding a series resistor on the
laser driver’s OUT+ side. Select the series resistor so
the OUT+ terminal voltage does not drop below 2.2V
with the maximum modulation current.
__________Applications Information
Programming the MAX3261 Laser Driver
Programming the MAX3261 is best explained by an
example. Assume the following laser diode characteris-
tics:
Wavelength λ 780nm
Threshold Current I
TH
20mA at +25°C
(+0.35mA/°C temperature variation)
Monitor Responsivity ρ
mon
0.1A/W (monitor current /
average optical power into the
fiber)
Modulation Efficiency η 0.1mW/mA (worst case)
Now assume the communications system has the fol-
lowing requirements:
Average Power P
AVE
0dBm (1mW)
Extinction Ratio Er 6dB (Er = 4)
Temperature Range Tr 0°C to +70°C
1) Determine the value of IPINSET:
The desired monitor-diode current is (P
AVE
)(ρ
mon
) =
(1mW)(0.1A/W) = 100µA. The R
PINSET
vs. Monitor
Current graph in the Typical Operating Characteristics
shows that R
PINSET
should be 18kΩ.
2) Determine R
MODSET
:
The average power is defined as (P1 + P0) / 2, where
P1 is the average amplitude of a transmitted “one”
and P0 is the average amplitude of a transmitted
“zero.” The extinction ratio is P1/P0. Combining these
equations results in P1 = (2 x P
AVE
x Er) / (Er + 1) and
P0 = (2 x P
AVE
) / (Er + 1). In this example, P1 = 1.6mW
and P0 = 0.4mW. The optical modulation is 1.2mW. The
modulation current required to produce this output is
1.2mW / η = (1.2mW) / (0.1mA/mW) = 12mA. The
Typical Operating Characteristics show that R
MODSET
= 3.9kΩ yields the desired modulation current.
3) Determine the value of R
OSADJ
:
Using the Allowable R
OSADJ
vs. Modulation Current
graph in the Typical Operating Characteristics, a 5.6kΩ
resistor is chosen for 12mA of modulation current. The
maximum ROSADJ values given in the graph minimize
aberrations in the waveform and ensure that the driver
stage operates fully limited.
4) Determine the value of R
BIASSET
:
The automatic power control circuit can adjust the bias
current 40mA from the initial setpoint. This feature
makes the laser driver circuit reasonably insensitive to
variations of laser threshold from lot to lot. The bias set-
ting can be determined using one of two methods:
a) Set the bias at the laser threshold.
b) Set the bias at the midpoint of the highest and low-
est expected threshold values.
Method A is straightforward. In the second method, it is
assumed that the laser threshold will increase with age.
The lowest threshold current occurs at 0°C, when the
laser is new. The highest threshold current occurs at
+70°C, at the end of the product’s life. Assume the
laser is near the end of life when its threshold reaches
two-times its original value.
Lowest Bias Current:
I
TH
+ ∆I
TH
= 20mA + (0.35mA/°C)(-25°C) = 11.25mA
Highest Bias Current:
2 x I
TH
+ ∆I
TH
= 40mA + (0.35mA/°C)(+45°C) = 55.8mA
In this case, set the initial bias value to 34mA (which is
the midpoint of the two extremes). The adjustment
range of the MAX3261 maintains the average laser
power at either extreme.
The Typical Operating Characteristics show that
R
BIASSET
= 1.8kΩ delivers the required bias current.
