MAX3668EHJ-T Maxim Integrated Products, MAX3668EHJ-T Datasheet - Page 8

• Manufacturer Part Number: MAX3668EHJ-T
• Description: Laser Driver +3.3V, 622Mbps SDH-S ONET Laser Driver wi
• Manufacturer: Maxim Integrated Products

+3.3V, 622Mbps SDH/SONET
Laser Driver with Automatic Power Control
When the MAX3668’s APC feature is used, program the
average optical power by adjusting the APCSET resis-
tor. To select this resistor, determine the desired moni-
tor current to be maintained over temperature and life.
Refer to the Monitor Diode Current vs. APC Set Resistor
graph in the Typical Operating Characteristics and
select the value of R
required current.
To minimize optical output aberrations due to the laser
parasitic inductance, an RC shunt network may be
used (see Typical Operating Circuit). If R
the laser diode resistance, the recommended total
resistance for R
al lasers are R
C
the output waveform. A bypass capacitor should also
be placed as close to the laser anode as possible for
best performance.
When transmitting NRZ data with long strings of consec-
utive identical digits (CID), LF droop can contribute to
pattern-dependent jitter. To minimize this pattern-depen-
dent jitter, two external components must be properly
chosen: capacitor C
time constant; and AC-coupling capacitor C
To filter out noise effects and guarantee loop stability,
the recommended value for C
in an APC loop bandwidth of 20kHz. Consequently, the
pattern-dependent jitter associated with an APC loop
time constant can be ignored.
The time constant associated with the DC blocking
capacitor on I
important that this time constant produce minimum
droop for long consecutive bit streams.
Referring to Figure 4, the droop resulting from long time
periods without transitions can be represented by the
following equation:
AC coupling of I
that is equal to P
P
ensure a droop of less than 12% (6% relative to P
this equation can be solved for τ as follows:
8
p-p
FILT
_______________________________________________________________________________________
equates to a 12% droop relative to P
should be experimentally adjusted to optimize
Interfacing with the Laser Diode
FILT
MOD
Pattern-Dependent Jitter (PDJ)
D
(100% - DROOP) = e
τ =
AVG
MOD
+ R
= 20Ω and C
Programming the APC Loop
APC
ln
. An overall droop of 6% relative to
will have an effect on PDJ. It is
L
APCSET
results in a discharge level for τ
(1 - 0.12)
is 10Ω. Starting values for coaxi-
, which dominates the APC loop
-t
APC
that corresponds to the
= 7.8t
FILT
is 0.1µF. This results
-t
τ
= 5pF. R
L
D
.
represents
FILT
AVG
p-p
. To
and
),
If t
sition, the time constant associated with the DC block-
ing capacitor needs to be longer than:
R
estimated value of R
Assuming R
with C
The MAX3668 data inputs are PECL-compatible.
However, it is not necessary to drive the MAX3668 with
a standard PECL signal. As long as the specified com-
mon-mode voltage and differential voltage swings are
met, the MAX3668 will operate properly.
The total power dissipation of the MAX3668 can be esti-
mated by the following:
where I
MAX
cal laser forward voltage.
The following is an example of how to set up the
MAX3668.
A communication-grade laser should be selected for
622Mbps applications. Assume the laser output aver-
age power is P
Figure 4. Droop
FILT
P
1
p-p
, I
equals 80 consecutive unit intervals without a tran-
τ
AC
MOD
D
can be ignored for C
BIAS
= 1.0µF, τ
P
≥ R
AVG
P = V
is the modulation current, and V
AC
D
+ I
is the maximum bias current set by R
R
= 5Ω, and r
C
Input Termination Requirement
AC
MOD
Applications Information
CC
D
Calculate Power Consumption
AVE
AC
= 7.8 (80 bits) (1.6ns/bit) = 1.0µs
= 20Ω   (R
×
AC
(V
= 0dBm, the minimum extinction
= 6.7µs.
I
R
CC
CC
AC
is:
τ << τ
LASER
+ (V
- 20Ω
= 6.7Ω
AC
FILT
CC
τ = ∞
D
t
1
= 5Ω:
×
+ r
<< C
- V
I
MOD
LASER
f
τ
)
AC
×
D
Select Laser
I
/ 2)
, therefore the
BIAS
)
f
is the typi-
DROOP
t
BIAS-