MAX3667ECJ Maxim Integrated Products, MAX3667ECJ Datasheet - Page 9

• Manufacturer Part Number: MAX3667ECJ
• Description: Laser Driver Integrated Circuits (ICs)
• Manufacturer: Maxim Integrated Products

Since the PDJ adjusts with changes in loop gain, it is
important to choose capacitor values that are as large
as is physically possible. Since each capacitor repre-
sents a different pole, for stability reasons, C
be kept substantially smaller than C
mended that the value of C
smaller than C
The time constant associated with the DC-blocking
capacitor on IMOD can also have an effect on PDJ. It is
important that this time constant produce minimum
droop for long consecutive bit streams.
Referring to Figure 3, the droop resulting from long time
periods without transitions can be represented by the
following equation:
APC operation assures that the discharge level for τ is
P
to a 12% droop relative to P
less than 12% (6% relative to P
be solved for τ as follows:
If t
transition, then the time constant associated with the
DC-blocking capacitor needs to be longer than:
The estimated value of R
Assuming R
with C
When operating without APC, be sure to configure the
MAX3667 as follows:
1) Disconnect APC from BIASSET.
2) Force a voltage of 1V to 2V at APC to prevent the
3) Disconnect the monitor diode.
4) Pull up the MD pin to V
5) Pull down the COMP pin to ground through a 30kΩ
Remember that the bias-current output is programmed
by adjusting the BIASSET resistor when the APC loop is
disconnected.
AVG
τ AC ≥ R
1
OTA from saturating.
resistor.
equals 100 consecutive unit intervals without a
. An overall droop of 6% relative to P
AC
R
AC
Operation Without APC (Open Loop)
= 1µF, τ
AC
FILT
= 31Ω   R
C
COMP
AC
[100% - DROOP] = e
= 22Ω, R
+3.3V, 622Mbps SDH/SONET Laser Driver
τ =
AC
= 7.8 (100 bits) (1.6ns/bit) = 1.25µs
_______________________________________________________________________________________
.
= 5.2µs.
ln
R
[1 - 0.12]
FILT
AC
AC
DAMP
-t
CC
= 5.2Ω
  (R
is:
AVG
through a 5kΩ resistor.
APC
= 4.7Ω, and r
DAMP
. To ensure a droop of
P-P
= 7.8t
), this equation can
be set 1000 times
-t
τ
COMP
+ r
LASER
. It is recom-
with Automatic Power Control
LASER
P-P
APC
)
equates
should
= 4Ω:
The MAX3667 is equipped with output current limiting
and short-circuit protection. In +3.3V operation, I
limited to approximately 170mA open loop, and I
is limited to approximately 140mA (see Typical
Operating Characteristics). In +5.0V operation, I
limited to approximately 300mA, and I
to approximately 140mA.
If BIASSET is shorted to ground, I
limited. If either APCSET or MODSET is shorted to
ground, the MAX3667 output is turned off. Note that, in
5V operation, the I
300mA. Care should be taken if the MAX3667 is being
used with a laser diode that is sensitive to this current
level.
Adding damping resistance in series with the laser
diode (typically 3Ω to 5Ω) raises the load resistance,
reduces the load frequency dependence, and
improves output aberrations. A series damping resistor
of 4.7Ω is suggested for the MAX3667.
Series inductance at the cathode of the laser results in
high-frequency loading (V
put aberrations. Because of reduced headroom, the
output performance of the transmitted eye diagram can
be significantly impacted during 3.3V operation.
Assuming that laser package series inductance can not
be completely eliminated, a compensation network is
required. With a laser diode load of approximately 4Ω
and 4nH, a series damping resistor of 4.7Ω, and a cou-
pling capacitor of 0.1µF, a shunt R-C compensation
network of 22Ω and 0.01µF is recommended (see
Typical Operating Circuit). These values may need to
be adjusted depending on the style of laser used. Note
that it is important to place the compensation network
as close to the load as possible.
Figure 3. Droop
P
p-p
P
AVG
BIAS
and Laser Compensation
τ << τ
current limit is approximately
L
Interface Suggestions
Output Current Limits
= Ldi/dt) and increased out-
AC
τ = ∞
t
1
BIAS
τ
AC
becomes current
QMOD
DROOP
is limited
BIAS
BIAS
QMOD
t
is
is
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