MAX3738 Maxim Integrated Products, MAX3738 Datasheet - Page 9

• Manufacturer Part Number: MAX3738
• Description: 155Mbps to 4.25Gbps SFF/SFP Laser Driver with Extinction Ratio Control
• Manufacturer: Maxim Integrated Products

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 (η) of laser over time and temperature:
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) needed
to maintain peak-to-peak laser power as bias current
Figure 4. Functional Diagram
R
R
BC_MON
PC_MON
TX_DISABLE
SHUTDOWN
TX_FAULT
PC_MON
BC_MON
155Mbps to 4.25Gbps SFF/SFP Laser Driver
IN+
IN-
R
P
PULL
_______________________________________________________________________________________
AVG
P
I
P-P
= 45kΩ
I
BIAS
MD
82
1
= I
= η x I
R
TH_TEMP
TH_TEMP
MD
T > T
/ ρ
INPUT BUFFER
POWER DETECTOR
MOD
SAFETY LOGIC
TH
MON
V
T
CC
R
AND
MODTCOMP
MODTCOMP
xTC
ENABLE
ENABLE
I
I
MOD
BIAS
with Extinction Ratio Control
R
MODSET
MODSET
MAX3738
DATA
PATH
x268
V
MODBCOMP
REF
increases with temperature. Refer to Application Note
1119: HFAN-02.2.1: Maintaining the Extinction Ratio of
Optical Transmitters Using K-Factor Control for details:
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 MAX3738 provides additional tem-
perature compensation as temperature increases past
a user-defined threshold (T
xK
R
MODBCOMP
APCFILT1
I
BIAS
C
APC
APCFILT2
K = ΔI
x1/2
V
CC
I
x1
APCSET
MOD
V
REF
TH
I
MOD
).
/ ΔI
SHUTDOWN
OUT-
OUT+
BIAS
APCSET
MD
BIAS
www.DataSheet4U.com
R
APCSET
R
I
D
BIAS
I
MD
V
C
CC
MD
9