Max3738, Η x i – Rainbow Electronics MAX3738 User Manual
Page 8
MAX3738
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:
P
AVG
= I
MD
/ ρ
MON
P
P-P
= η x I
MOD
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:
K = ∆I
MOD
/ ∆I
BIAS
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
TH
).
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
8
_______________________________________________________________________________________
MAX3738
IN+
IN-
OUT-
OUT+
INPUT BUFFER
DATA
PATH
I
MOD
V
BG
R
MODSET
MODSET
V
BG
R
APCSET
APCSET
I
APCSET
C
APC
APCFILT1
APCFILT2
I
BIAS
BIAS
MD
V
CC
I
BIAS
ENABLE
x1
TX_FAULT
TX_DISABLE
SAFETY LOGIC
AND
POWER DETECTOR
R
BC_MON
BC_MON
V
CC
R
PC_MON
PC_MON
R
D
V
CC
I
MD
C
MD
SHUTDOWN
SHUTDOWN
x268
R
PULL
= 45kΩ
T
xK
xTC
T > T
TH
I
BIAS
I
MOD
ENABLE
x1/2
MODTCOMP
R
MODTCOMP
TH_TEMP
R
TH_TEMP
MODBCOMP
R
MODBCOMP
I
BIAS
82
I
MD
1
Figure 4. Functional Diagram