MIC3003 Datasheet, PDF (26/75 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
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 on in the MIC3003’s
default state as shipped from the factory. When the APC
is on, the value in the selected APCSETx register is
added to the signed compensation value taken from the
APC look-up table and loaded into the V_BIAS DAC. When
the APC is off, the V_BIAS DAC may be written directly via
the VBIAS 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.
MIC3003
Figure 6. Programmable Feedback Resistor
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. The MODREF
bit 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 MIC3003’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 the 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 the selected MODSETx register, and loaded
into the V_MOD DAC. When the APC is off, the
compensation value in VMOD is loaded directly into the
V_MOD DAC, bypassing the look-up table entirely. This
provides for direct modulation control for setup and
calibration.
Figure 5. MIC3003 APC and Modulation Control
Block Diagram
November 2009
Figure 7. Transmitter Configurations
Supported by MIC3003
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M9999-111209-C
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