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.

_____________Applications Information

Programming the MAX3263 Laser Driver

Programming the MAX3263 is best explained by an

example. Assume the following laser diode characteris-

tics:

Wavelength

λ

1300nm

Threshold Current

20mA at +25°C(+0.35mA/

°C temperature variation)

Monitor Responsivity

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

0dBm (1mW)

Extinction Ratio

Er

6dB (Er = 4)

Temperature Range

Tr

0°C to +70°C

1) Determine the value of IPINSET:

Current graph in the

Typical Operating Characteristics

Ω.

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-

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

= 3.9k

Ω yields the desired modulation current.

Current graph in the

Typical Operating Characteristics,

a 5.6k

Ω resistor is chosen for 12mA of modulation cur-

minimize aberrations in the waveform and ensure that

the driver stage operates fully limited.

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:

Highest Bias Current:

Single +5V, Fully Integrated,

155Mbps Laser Diode Driver

_______________________________________________________________________________________

9

MAX3263

+5V

OUT+

OUT-

IPIN

≥0.1µF

IBIASOUT

25

Ω

18

Ω

18

Ω

FERRITE

BEAD

LASER

SHUNT RC

PHOTO-

DIODE

0.01

µF

AS CLOSE TO THE

LASER ANODE AS

POSSIBLE

AS CLOSE TO THE

LASER CATHODE AS

POSSIBLE

C

10

µH

Figure 5. Typical Laser Interface with Bias Compensation