M02066_17 Datasheet, PDF (12/20 Pages) M/A-COM Technology Solutions, Inc. – 3.3 Volt Laser Driver IC for Applications to 3 Gbps

Functional Description
3.4
Mean Power Control Loop
To maintain constant average optical power, the M02066 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 voltage at PD is matched to an on-chip reference voltage. The external resistor (R_MPCSET) sets the optical
power.
R_MPCSET = 1.28/I_PIN
I_PIN is the mean current from the monitor photodiode at the required mean laser power level (see laser
datasheet).The time constant of the loop is determined by C_MPC. In some applications the internal capacitance at
C_MPC may be sufficient and an external C_MPC will not be required.
Table 3-1.
C_MPC
0
1 nF
10 nF
100 nF
MPC Loop Bandwidth
6 dB cutoff frequency
17 MHz
100 kHz
10 kHz
1 kHz
(at 6 dB cut off frequency) vs C_MPC (for nominal process)
Figure 3-3 shows the bias current vs R_MPCSET over the full operating temperature range.
Figure 3-3. Bias Current vs. R_MPCSET
120
100
80
60
40
20
0
0
5000 10000 15000 20000 25000 30000 35000
R_MPCSET Ω
-40
0
25
50
85
Symbols shown indicate °C
NOTE:
This curve will scale with monitor photodiode current transfer efficiency.
02066-DSH-001-D
Mindspeed Technologies^®
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