Datasheet

BIAS Register/APC Control, 3-Wire Mode
A Maxim laser driver controls its laser bias current DAC
using the APC loop within the device. The APC loop’s
feedback to the device is the monitor diode (MON2)
current, which is converted to a voltage using an exter-
nal resistor. The feedback is sampled by a comparator
and compared to a digital set-point value. The output of
the comparator has three states: up, down, or no-oper-
ation. The no-operation state prevents the output from
excessive toggling once steady state is reached. As
long as the comparator output is in either the up or
down states, the bias is adjusted by writing increment
and decrement values to the Maxim laser driver
through the BIASINC register.
The device has an LUT to allow the APC set point to
change as a function of temperature to compensate for
tracking error (TE). The APC LUT has 36 entries that
determine the APC setting in 4°C windows between
-40°C and +100°C.
MODULATION Control
A Maxim laser driver controls the laser modulation
using the internal temperature-indexed LUT within the
device. The modulation LUT is programmed in 2°C
increments over the -40°C to +102°C range to provide
temperature compensation for the laser’s modulation.
The modulation is updated after each temperature con-
version using the 3-wire interface that connects to the
Maxim laser driver. A Maxim laser driver include a 9-bit
DAC. The modulation LUT is 8 bits.
Figure 1 demonstrates how the 8-bit LUT controls the
9-bit DAC with the use of a temperature control bit
DS1878
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Table 1. Acronyms
ACRONYM DEFINITION
ADC Analog-to-Digital Converter
AGC Automatic Gain Control
APC Automatic Power Control
APD Avalanche Photodiode
ATB Alarm Trap Bytes
BM Burst Mode
DAC Digital-to-Analog Converter
DFB Distributed Feedback Laser
LDD Laser Diode Driver
LOS Loss of Signal
LUT Lookup Table
NV Nonvolatile
QT Quick Trip
TE Tracking Error
TIA Transimpedance Amplifier
ROSA Receiver Optical Subassembly
SEE Shadowed EEPROM
SFF Small Form Factor
SFF-8472
Document Defining Register Map of SFPs
and SFFs
SFP Small Form Factor Pluggable
SFP+ Enhanced SFP
TOSA Transmit Optical Subassembly
TXP Transmit Power
VCSEL Vertical Cavity Self-Emitting Laser
SFP+ Controller with Digital LDD Interface
MOD LUT
LOADED TO [7:0]
MOD LUT
LOADED TO [7:0]
MODTI
8
7
6
5
4
3
2
1
0
MODTI
MODTC = 0
TEMPERATURE (°C)
-40 +102
TEMPERATURE (°C)
-40 +102
MODTC = 1
MOD LUT
LOADED TO [8:1]
(DAC BIT 0 = 0)
MOD LUT
LOADED TO [8:1]
(DAC BIT 0 = 0)
MAX3798 DAC BIT
8
7
6
5
4
3
2
1
0
MAX3798 DAC BIT
Figure 1. Modulation LUT Loading to a Maxim Laser Driver MOD DAC