Single +5V, Fully Integrated,

1.25Gbps Laser Diode Driver

10

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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

20mA at +25°C

(+0.35mA/°C temperature variation)

Monitor Responsivity

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

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

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

= 3.9k

Ω yields the desired modulation current.

graph in the

Typical Operating Characteristics, a 5.6k

Ω

resistor is chosen for 12mA of modulation current. The

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:

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

Ω delivers the required bias current.