M02066 Datasheet, PDF (14/20 Pages) M/A-COM Technology Solutions, Inc. – 3.3 Volt Laser Driver IC for Applications to 3 Gbps Data Sheet

Functional Description
3.8
Current Monitors
The M02066 features bias and modulation current monitor outputs. The BIAS_MON output sinks a current equal to
nominally 1/60 of the laser bias current (I_BIAS). The MOD_MON output sinks a current equal to nominally 1/60 of the
laser peak to peak modulation current (I_MOD). BIAS_MON and MOD_MON should be connected through a pull-up
resistor to V_CC. Choose a pull-up resistor value that ensures a voltage at BIAS_MON greater than V_CC-1.6V and a
voltage at MOD_MON greater than V_CC - 1.0V. These pins should be tied to V_CC if not used.
Figure 3-5. Typical Eye Diagram
2.5 Gbps optical eye @ -3 dBm
(1.87 GHz filter)
3.9
Temperature Compensation
The reduction in slope efficiency of typical laser diodes caused by increased temperature can be compensated by
utilising the TC_START and TC_SLOPE features of the M02066. Under closed loop conditions the average optical
power will be maintained by the Mean Power Control loop, compensating for factors such as temperature and age.
However, as the laser slope efficiency reduces with increasing temperature the laser diode will require a greater
peak to peak current swing in order to maintain the same peak to peak optical output swing. To this end the
M02066 senses temperature and can be programmed to increase the modulation current amplitude accordingly.
Two external resistors are used to program the temperature compensation.
The temperature (TC_START) at which the compensation begins to take effect is set by a resistor (RTC_START) from
the TC_START pin to ground. The rate of increase of modulation current with temperature (beyond TC_START) is set
by a resistor (RTC_SLOPE) from the TC_SLOPE pin to ground.
As a safety feature, the M02066 limits the increased modulation current to twice the initial (ambient) modulation
current.
The effects of the two programming resistors are shown schematically in the Temperature Compensation graph
below.
If using the TQFP package; only TC_SLOPE is programmable, TC_START is connected to 0 V.
02066-DSH-001-D
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