MAX3646 Maxim Integrated Products, MAX3646 Datasheet - Page 8

• Manufacturer Part Number: MAX3646
• Description: 155Mbps to 622Mbps Sff/sfp Laser Driver
• Manufacturer: Maxim Integrated Products

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Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compen-
sating the modulation current for reduced slope effi-
ciency (h) of laser over time and temperature:
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) need-
ed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note HFAN-02.21 for details:
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Figure 4. Functional Diagram
8
_______________________________________________________________________________________
R
R
BC_MON
PC_MON
TX_DISABLE
SHUTDOWN
TX_FAULT
PC_MON
BC_MON
IN+
IN-
R
PULL
P
P
AVG
I
P-P
= 45kΩ
I
BIAS
82
MD
1
= I
= η x I
R
TH_TEMP
TH_TEMP
MD
T > T
/ρ
INPUT BUFFER
POWER DETECTOR
MOD
SAFETY LOGIC
MON
TH
V
T
CC
AND
R
MODTCOMP
MODTCOMP
xTC
ENABLE
ENABLE
I
I
MOD
BIAS
R
MODSET
MODSET
MAX3646
DATA
PATH
x268
V
MODBCOMP
BG
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power.
Slope efficiency decreases more rapidly as tempera-
ture increases. The MAX3646 provides additional tem-
perature compensation as temperature increases past
a user-defined threshold (T
R
xK
MODBCOMP
APCFILT1
I
BIAS
C
APC
APCFILT2
K = ∆I
V
x1/2
CC
I
x1
APCSET
MOD
V
BG
TH
I
MOD
).
/ ∆I
SHUTDOWN
OUT-
OUT+
BIAS
APCSET
MD
BIAS
R
APCSET
R
I
D
BIAS
I
MD
V
C
CC
MD