MAX3262 Maxim, MAX3262 Datasheet - Page 9

• Manufacturer Part Number: MAX3262
• Description: Single +5V / Fully Integrated / 155Mbps Laser Diode Driver
• Manufacturer: Maxim

heat. Fortunately, a substantial portion of this power is
dissipated across the laser diode. A typical laser diode
drops approximately 1.6V when forward biased. This
leaves 3.4V at the MAX3263’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.
Programming the MAX3263 is best explained by an
example. Assume the following laser diode characteris-
tics:
Wavelength
Threshold Current
Monitor Responsivity
Modulation Efficiency
Now assume the communications system has the fol-
lowing requirements:
Figure 5. Typical Laser Interface with Bias Compensation
_____________Applications Information
MAX3263
Programming the MAX3263 Laser Driver
IBIASOUT
OUT+
OUT-
IPIN
0.1 F
_______________________________________________________________________________________
FERRITE
10 H
BEAD
18
I
TH
mon
PHOTO-
DIODE
18
1300nm
20mA at +25°C(+0.35mA/
°C temperature variation)
0.1A/W (monitor current /
average optical power
into the fiber)
0.1mW/mA (worst case)
+5V
LASER
SHUNT RC
25
AS CLOSE TO THE
LASER CATHODE AS
POSSIBLE
0.01 F
AS CLOSE TO THE
LASER ANODE AS
POSSIBLE
Single +5V, Fully Integrated,
155Mbps Laser Diode Driver
C
Average Power
Extinction Ratio
Temperature Range
1) Determine the value of IPINSET:
The desired monitor-diode current is (P
(1mW)(0.1A/W) = 100µA. The R
Current graph in the Typical Operating Characteristics
show that R
2) Determine R
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 equa-
tions results in P1 = (2 x P
(2 x P
P0 = 0.4mW. The optical modulation is 1.2mW. The
modulation current required to produce this output is
1.2mW /
Typical Operating Characteristics show that R
= 3.9kΩ yields the desired modulation current.
3) Determine the value of R
Using the Allowable R
Current graph in the Typical Operating Characteristics ,
a 5.6kΩ resistor is chosen for 12mA of modulation cur-
rent. The maximum R
minimize aberrations in the waveform and ensure that
the driver stage operates fully limited.
4) Determine the value of R
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:
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:
Highest Bias Current:
A) Set the bias at the laser threshold.
B) Set the bias at the midpoint of the highest and low-
I
2 x I
TH
est expected threshold values.
+ I
AVE
TH
+ I
) / (Er + 1). In this example, P1 = 1.6mW and
TH
PINSET
= (1.2mW) / (0.1mA/mW) = 12mA. The
TH
= 20mA + (0.35mA/°C)(-25°C) = 11.25mA
MODSET
= 40mA + (0.35mA/°C)(+45°C) = 55.8mA
should be 18kΩ.
OSADJ
:
OSADJ
P
Er
Tr
AVE
AVE
OSADJ
BIASSET
values given in the graph
x Er) / (Er + 1) and P0 =
Range vs. Modulation
:
:
PINSET
0dBm (1mW)
6dB (Er = 4)
0°C to +70°C
AVE
vs. Monitor
)(
MODSET
mon
) =
9