MIC3002 Datasheet, PDF (26/65 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibr ation

Micrel, Inc.
MIC3002
Furthermore, the reference level for the APC circuit is
selectable to accommodate electrical, i.e., current
feedback, or optical feedback via a monitor photodiode.
Finally, any one of seven different internal feedback
resistors can be selected. This internal resistor can be
used alone or in parallel with an external resistor. This wide
range of adjustability (50:1) accommodates a wide range
of photodiode current, i.e., wide range of transmitter output
power. The APC operating point can be kept near the mid-
scale value of the APC DAC, insuring maximum SNR,
maximum effective resolution for digital diagnostics, and
the widest possible DAC adjustment range for temperature
compensation, etc. See Figure 6.
The APCCAL bit in OEMCAL0 is used to turn the APC
function on and off. It will be turned off in the MIC3002’s
default state as shipped from the factory. When APC is on,
the value in the selected APCSETx register is added to the
signed value taken from the APC look-up table and loaded
into the V_BIAS DAC. When APC is off, the V_BIAS DAC may
be written directly via the V_BIAS register, bypassing the
look-up table entirely. This provides direct control of the
laser diode bias during setup and calibration. In either
case, the V_BIAS DAC setting is reported in the APCDAC
register. The APCCFG bits determine the DACs response
to higher or lower numeric values.
Laser Modulation Control
As shown in Figure 5, a temperature-compensated DAC is
provided to set and control the laser modulation current via
an external laser driver circuit. MODREF in OEMCFG0
selects whether the V_MOD DAC output swings up from
ground or down from V_DD. If the laser driver requires a
voltage input to set the modulation current, the MIC3002’s
V_MOD output can drive it directly. If a current input is required,
a fixed resistor can be used between the driver and the
V_MOD output. Several different configurations are possible as
shown in Figure 8.
When APC is on, i.e., the APCCAL bit in OEMCAL0 is set to
0, the value corresponding to the current temperature is
taken from the MODLUT look-up table, added to MODSET,
and loaded into the V_MOD DAC. When APC is off, the value
in V_MOD is loaded directly into the V_MOD DAC, bypassing the
look-up table entirely. This provides for direct modulation
control for setup and calibration. The MODREF bit
determines the DACs response to higher or lower numeric
values.
Figure 5. MIC3002 APC and Modulation Control
Block Diagram
Figure 7. Transmitter Configurations
Supported by MIC3002
Figure 6. Programmable Feedback Resistor
July 2007
26
M9999-073107-B
[e-mail] or [phone]