MIC3000_07 Datasheet, PDF (17/68 Pages) Micrel Semiconductor – FOM Management IC

MIC3000
Laser Diode Bias Control
The MIC3000 can be configured to generate a constant bias
current using electrical feedback, or regulate average trans-
mitted optical power using a feedback signal from a monitor
photodiode, see Figure 3. An operational amplifier is used to
control laser bias current via the V_BIAS output. The VBIAS pin
can drive a maximum of ±10mA. An external bipolar transistor
provides current gain. The polarity of the op amp’s output is
programmable BIASREF in OEMCFG1 in order to accommo-
date either NPN or PNP transistors that drive common anode
and common cathode laser, respectively. Additionally, the
polarity of the feedback signal is programmable for use with
either common-emitter or emitter-follower transistor circuits.
Furthermore, the reference level for the APC circuit is select-
able 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 4.
The APCCAL bit in OEMCAL0 is used to turn the APC
function on and off. It will be turned off in the MIC3000’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.
VOUT
DAC
VDD-OUT
GAIN
APC
Look-Up
INV
Table
APCSET
Temp
Sensor
RFB[2:0]
VDD
BIASREF
VBIAS
COMP
FB
VMOD
Look-Up
Table
MODSET
DAC
VOUT
VDD-OUT
MODREF
VMOD
VMOD–
Figure 3. MIC3000 APC and Modulation Control
Block Diagram
Micrel
APC Op-Amp
FB
R7 R6 R5 R4 R3 R2 R1
51.2k 25.6k 12.8k 6.4k 3.2k 1.6k 0.8k
V_DD
RFB[2:0] ⁷
BIASREF
Figure 4. Programmable Feedback Resistor
Laser Modulation Control
As shown in Figure 3, 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 MIC3000’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 6.
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 deter-
mines the DACs response to higher or lower numeric values.
V_DD
Common-cathode with Common-anode with
monitor photodiode monitor photodiode
V_DD
Common-anode
Common-cathode
Figure 5. Transmitter Configurations
Supported by MIC3000
October 2004
17
M9999-101204