02068-DSH-001-C Datasheet, PDF (10/18 Pages) M/A-COM Technology Solutions, Inc. – 3.3 Volt Laser Driver IC for 155/622 Mbps

Functional Description
3.4
Mean Power Control Loop
To maintain constant average optical power, the CX02068 incorporates a control loop to compensate for the
changes in laser threshold current over temperature and lifetime. A monitor photodiode mounted in the laser
package is used to convert the optical power into a photocurrent. The MPC loop adjusts the laser bias current so
that the monitor current is matched to a reference current set by a single external resistor (R_MPCSET). The time
constant of the loop is then determined by C_MPC.
Figure 3-3. Typical bias current vs R_BIASSET (3.3V)
120
100
80
60
40
20
0
0
10000
20000
R_BIASSET (Ω )
30000
40000
Safety logic is provided in order to limit the maximum bias current. The bias current at which the safety logic trips
is set by an external resistor to ground (R_OCA) from the OCA_SET pin. When the bias current limit is reached alarm
flag OCA is asserted LOW. A loop failure alarm is also provided to detect when the bias current can no longer be
adjusted to achieve the desired average optical power.
Figure 3-4. Typical limiting current vs R_OCA (at 25 °C)
100
90
80
70
60
50
40
30
20
10
0
0
10000
20000
R_OCA (Ω )
30000
40000
02068-DSH-001-C
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