Maxim MAX3865 User manual

Design Note:
HFDN-18.0
Rev.1; 04/08
The MAX3865 Laser Driver with Automatic
Modulation Control
Maxim Integrated Products

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 2 of 9
The MAX3865 Laser Driver with
Automatic Modulation Control
1 Introduction
Laser diodes for telecommunication applications are
characterized by two principal parameters:
•The threshold current, Ith, which can be defined
as the minimum current through the laser diode
that will support stimulated emission of photons
(resulting in coherent optical output). Laser
current levels below the threshold current result
in low-level spontaneous emission (non-
coherent optical output). See Figure 1.
•The slope efficiency, S, is the gradient of optical
power output versus current input above the
threshold as defined in Equation (1).
.
)( )( currentinputlaserd outputpoweropticald
S=(1)
In actual use, the maximum and minimum drive
currents, Imax and Imin, should be chosen so that the
average optical power output, Pav, (see Equation (2))
is adequate for the application, and so that the
extinction ration, re, (Equation (3)) is as large as
possible:
2
minmax PP
Pav
+
=(2)
.
min
max
P
P
re=(3)
Attempting to obtain a large but controlled
extinction ratio is ultimately the source of many
problems with practical laser drivers. On the one
hand the laser must never operate below the
threshold current, because this will cause an
unpredictable start-up delay and a poor waveform
(due to relaxation oscillation), in addition to
increased noise and degraded laser spectral
properties (“chirp”). On the other hand, laser
characteristics are somewhat variable from one to
the next, and in any case they vary with temperature
and age.
For example, suppose we have set up a laser driver
with appropriate values of Imax and Imin to achieve
desired values for Pav and re, using the nominal laser
characteristic (a) in Figure 1. Now suppose the
actual laser characteristic is different from nominal.
We will consider two cases:
Case 1. The laser slope efficiency is reduced, but the
threshold is unchanged (this could be the result of
initial manufacturing tolerances, or of a temperature
change, or it could occur over a period of time as the
laser ages). Referring to characteristic (b) in Figure
1, it is obvious that the maximum, minimum, and
average optical power output levels are lower than
the intended set-up levels, while the extinction ratio
is unchanged. The system will probably remain
usable, but the signal-to-noise ratio and bit error rate
are degraded.
Figure 1. Characteristics of a typical dc-
coupled
communications laser diode. The horizontal
axis represents current flowing into the laser,
and the vertical axis represents optical
output power (or equivalently, monitor-
diode
current since this is proportional to optical
output power).

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 3 of 9
Case 2. The laser slope efficiency is unchanged but
the threshold current is greater than nominal, as in
Figure 1, characteristic (c). This is the disaster
situation in which the laser operation falls below
threshold. Other scenarios can be investigated, with
various combinations of laser slope efficiency and
threshold, but it is obvious that, for specified average
optical power output, the disaster situation becomes
more likely as the nominal extinction ratio is made
larger.
2 Optical Feedback
via a Monitor Diode
Many laser assemblies include a monitor photo
diode. Photo diodes are essentially linear in their
relation between optical power input and reverse-
biased current. Incident photons generate hole-
electron pairs in the diode, and increase its reverse
leakage current above the “dark” value. Thus,
monitor current is a measure of laser optical power
output and, by incorporating the laser and monitor
into a suitable feedback system, it should be possible
to control the optical output. Figure 2 shows the
general idea.
The desired laser optical output waveform is first
scaled by a factor that is the inverse of monitor gain,
Amonitor, where
.
)( )(
monitor outputpoweropticallaserd ntphotocurremonitord
A=(4)
This scaled waveform is used as input to a
“classical” feedback control system, in which the
forward path consists of a high-gain current
amplifier plus the laser acting in cascade, and the
monitor diode constitutes the feedback network.
Then, provided only that the loop gain is large at all
frequencies of concern, the actual optical output
waveform from the laser mimics the desired
waveform:
.gainloop monitorlaseramplifier AAA ××= (5)
Pmax and Pmin, hence Pav and re, are all controlled and
held constant despite variations in the laser
characteristic.
The trouble is that photo diodes have a limited
bandwidth; details vary, but typical diodes behave
much like a low-pass filter with a cutoff around
100MHz. Diodes can of course be manufactured
with larger bandwidths, but at increased cost and
with other problems. When the data rate is low
(perhaps up to 100Mbps), the system can be made to
work. However a number of things go wrong at high
data rates:
•Basically, the problems all stem from the fact
that the loop bandwidth must be of the same
order as the bit rate, for satisfactory reproduction
of the input waveform. Thus a 2.5Gbps data
waveform requires a bandwidth around 2.5GHz.
(2GHz or even 1.5GHz might be enough,
depending on the fidelity requirement, but the
order of magnitude is 2.5GHz.)
•The monitor diode contributes a dominant pole
at about 100MHz to the feedback loop.
Therefore the requirements on the high-gain
current amplifier become extreme: it is difficult
to stabilize the feedback loop.
Figure 2. Feedback control of a laser

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 4 of 9
•Recall that the overall response of a feedback
system has a zero at each pole of its feedback
network. Therefore, the overall gain of the
feedback part of Figure 2 rises above 100MHz,
and its output becomes extremely noisy. Another
way of looking at it is that the feedback becomes
ineffectual above 100MHz, so the system output
includes all the noise of the high-gain amplifier
running without feedback.
2.1 Automatic Power Control
Something useful can be achieved by abandoning
the quest for large loop bandwidth, and replacing the
high-gain amplifier by an integrator:
τ
s
A1
amplifier . (6)
When
montiorlaser
11 outputopticaldesired outputopticalactual AAs τ +
=, (7)
the system becomes a low-pass filter. If the
integrator time constant τis chosen long enough, the
average optical power output Pav becomes equal to
the desired average output, independent of the detail
of the data pattern in the modulation waveform.
However, all the high-frequency information in the
modulation waveform is filtered out, so Pmax, Pmin,
and the difference between them are not controlled.
Automatic power control (APC) is achieved, but not
automatic modulation control (AMC).
Laser driver systems incorporating APC are quite
common. The user can program the average optical
power output, and this will be maintained
automatically despite variations in the laser.
However, peak-peak optical output and extinction
ratio are not controlled, and must be set up and
tweaked for each individual laser.
3 Automatic Modulation Control
via a Pilot Tone
Maxim’s MAX3865, achieves automatic modulation
control by adding a small pilot tone to the laser
current. This pilot tone is a square wave with a
frequency of about 1MHz (low enough that it can
pass through the monitor diode without attenuation).
Figure 3 shows a simplified block diagram. The
control loops for laser bias (average) current and
modulation (peak-peak) current are separated.
The automatic power control (APC) feedback loop
holds the bias or average current in the monitor
diode constant, and is basically the same as Figure 2.
The desired (pre-scaled) bias current enters at the
center-left in Figure 3. (The pre-scalar is omitted
Figure 3. Conceptual block diagram of a laser driver with APC and AMC

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 5 of 9
from Figure 3 for simplicity.) The DC current (or
bias) in the monitor diode is subtracted from the
entering pre-scaled bias current. The resulting
difference or bias error is integrated, and applied to
the laser via a suitable high-current output stage. In
the steady state, the bias error must be zero, or else
the integrator output would be changing, which
denies the steady state. Therefore the bias feedback
must be equal to the pre-scaled bias current input.
Average monitor-diode current is controlled, hence
average optical power output from the laser is
controlled.
To understand the operation of the automatic
modulation control (AMC) function, suppose
initially that the modulation current Imod (located
near the center of Figure 3) is known. Then the
peak-peak data or modulation current in the laser is
known, and the pilot-tone current is known. At the
output of the monitor diode, the modulation
component of current is near zero because of the
restricted bandwidth, but the pilot- tone component
is not similarly restricted and is a true indication of
the pilot-tone component of optical output. The
desired pilot-tone current enters Figure 3 at the top-
left (again, the pre-scalar of Figure 2 is omitted for
simplicity), and from this current, the pilot-tone
feedback current is subtracted. The resulting
difference or pilot-tone error is integrated and
becomes Imod. As with the bias control loop, in the
steady state, the pilot-tone feedback current must be
equal to the pilot-tone input.
4 Detailed Description
Figure 4 is a block diagram of the MAX3865,
simplified only in that the current-scaling factors
Figure 4. Block diagram of the MAX3865 laser driver, which includes both APC and AMC
Control Circuits
20

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 6 of 9
which are shown localized as blocks #4, #8, #13 and
#16 are in fact distributed throughout the chip. The
maximum current in any external programming
resistor is about 200µA. The currents in blocks such
as the adders, multipliers and integrators are a few
microampéres, appropriate for realization in
integrated-circuit form. Only in the circuit blocks
which interface with the outside world are the
currents significantly large.
•maximum bias current output to laser = 100mA
•maximum modulation current output to laser =
60mA peak-peak
•maximum instantaneous current in monitor
diode = 1mA
Monitor pins BIAS_MON and MOD_MON provide
access to the actual bias and modulation currents,
with scale factors of 1/48 and 1/32 respectively.
The circuit has four modes of operation, which are
programmed by two TTL-compatible control bits:
(0,0) = shut down,
(0,1) = manual mode,
(1,0) = APC mode,
(1,1) = AMC mode.
In the shut-down mode, all laser currents are forced
to zero, but the rest of the circuit remains operative;
in particular, the various integrators are primed and
ready to go, once a working mode is selected. A
TTL-compatible active-low warning flag, , is
set under fault conditions.
In all modes, the input data can be routed directly to
the modulation output stage #10 via multiplexer #1,
or it can be routed via the re-timing latch #2. In the
latter case, jitter present on the data can be
eliminated if a reference clock is available. Re-
timing is enabled via a TTL-compatible active-high
control pin R-T_EN.
4.1 The Manual Mode
In the manual mode, the laser bias and modulation
currents are programmed directly via external
resistors or small current-output DACs. As noted,
the maximum current that flows in any programming
resistor is about 200µA.
Bias output current is programmed in #3 by
connecting a resistor to ground:
).(480
k2
V2.1
480
bias_max
bias_max
bias
RI
R
I
×=








Ω+
=(8)
where I(Rbias_max) is the current flowing through
Rbias_max.The rationale for the terminology Rbias_max is
explained in Section 4.2. Block #3 is protected
against short-circuits to ground or Vcc at the
programming pin and the whole chip shuts down if
the current is programmed too large. From #3, the
bias current flows via subtractor #5 (note that its
second input is off in this operating mode) to the
bias output amplifier #6. It then flows to the laser
cathode via a small inductor, which isolates the laser
from the output capacitance of the amplifier.
Similarly, peak-peak or modulation output current is
programmed in #7 by connecting a resistor to
ground:
).(320
k2
V2.1
320
maxmod_
maxmod_
mod
RI
R
I
×=








Ω+
=(9)
where I(Rmod_max) is the current flowing through
Rmod_max. From #7, the current flows via subtractor
#9 (its second input is off) to the modulation output
stage #10, and thence to the laser. In essence, the
modulation output stage consists of a differential
pair, with its bases driven by the data waveform and
with a DC tail current equal to the programmed
peak-peak modulation current.
With the arrangement in Figure 4, the
correspondences between the chip current outputs
Ibias and Imod and the laser currents Imax and Imin in
Figure 1 are:
,chiplaser biasmin II =(10)
.chipchiplaser modbiasmax III += (11)
4.1.1 Layout: Parasitic C and L
Voltage and current changes in the modulation
output stage and laser are extremely fast—up to
about 1010V/sec and 109A/sec. Therefore, substantial
currents flow to ground in even minute stray
capacitances, and substantial voltage drops occur
across minute lead inductances. These can degrade
the data waveforms.
FAIL

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 7 of 9
,
dt
dV
CI =(12)
.
dt
dI
LV =(13)
Every effort should therefore be made to minimize
parasitic Cand L. Good physical layout is essential.
As shown in Figure 4, the two sides of the
modulation-output differential pair (the so-called
DUMMY and LASER outputs of the chip) are
loaded equally. The output currents are routed via
equal-length 25Ωstrip lines to the 25Ωdummy load
and laser. The laser impedance is built out to 25Ωby
a series resistor (approximately 20Ω), and an RC
snubber at least partially compensates for laser
inductance. The 25Ωdummy load, the laser and
snubber all return to the same Vcc supply point.
4.1.2 AC-Coupling and DC-Coupling
The collectors of the modulation-output transistors
must remain above 1.8V, in order to provide
headroom for these collectors and the tail transistor
which sits underneath them. In Figure 5,
Vmodulation out = Vcc - Vlaser - Vseries R - Vparasitic L. (14)
Typical lasers drop more than a volt and, at the
modulation current peaks, the series resistor drops
another volt. It is extremely difficult to achieve
parasitic inductance less than 1nH—this corresponds
to about 1mm of PCB trace unless that trace is a
properly terminated transmission line. In accordance
with Equation 13, the peak drop across this
inductance is yet another volt. Substitution of all
these values into Equation 14 shows that the Vcc
supply needs to be around 5V; the industry standard
of 3.3V is simply not enough.
However the MAX3865 provides for 3.3V operation
by AC-coupling the modulation outputs to the laser.
In Figure 5, the mean voltage at the modulation
outputs is +Vcc (the average voltage across an
inductor must be zero). The actual output voltage
swings about this mean, rising above Vcc when the
instantaneous modulation current is small, and
falling below it when the current is large. Referring
to the laser currents in Figure 1:
,chip
2
1
chiplaser modbiasmin III −= (15)
.chip
2
1
chiplaser modbiasmax III += (16)
4.2 The APC Mode
In the APC mode, the second input to subtractor #5
is enabled. What goes to the bias output stage is not
the current as programmed in #3 and #4, but the
difference between this and the output from the bias
integrator #14. The current control Ibias subtracts
from the programmed maximum Ibias to give the
actual Ibias in the laser, and this cannot exceed the
programmed current. Thus, the terminology Rbias_max
is appropriate for this mode of operation. The
arrangement provides an automatic safety feature
against overdriving and destroying the laser under
fault conditions.
When the APC loop has settled to equilibrium, the
average or DC component of the feedback current
from the monitor diode, going into #20, must be
equal to the sum of the average components of the
other three currents. If, for example, the monitor-
diode feedback current is momentarily too large, the
output of integrator #14 moves positive and, at #5,
this reduces the bias current to the laser. But the
averages of the tone reference and mark-density
compensation currents must be zero because these
originate in multipliers. (See Section 4.2.1 below.)
Therefore, the average current in the monitor diode
must equal the programmed bias reference current,
which originates in #12:
Figure 5.
AC coupling of the MAX3865 to a
laser for operation with Vcc = 3.3V. The
combination of 6 µ
H and 56nF gives critical
damping with 25 Ω
and a coupling time
constant of 600nsec, equivalent to 1500
bits at 2.5 Gbps. Both L and C should be
increased proportionately if a longer time
constant is required.

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 8 of 9
).(5
k2
V2.1
5
APC_set
APC_set
diode)(monitor
av
RI
R
I
×=








Ω+
=(17)
where I(RAPC_set) is the current flowing through
RAPC_set.Average optical power output from the laser
is controlled.
The stability of the APC feedback loop depends on a
combination of the bias-integrator time constant and
the other gains and poles around the loop. In
particular, it depends on the laser-to-monitor current
gain. Current shunt #23 is provided to reduce the
loop gain when the laser-to-monitor current gain is
too large. For laser gains less than 0.005, either
connect MD_X to ground or leave it unconnected;
for laser gains greater than 0.02, connect MD_X to
MD; for laser gains between 0.005 and 0.02, use
either arrangement.
4.2.1 Mark-Density Compensation
In the very long term, the data input contains equal
numbers of 0s and 1s, and the average optical power
output is truly the average of the powers that
correspond to data_0 and data_1. However, in the
shorter term there may be a local excess of either 0s
or 1s. Said differently, the very-long-term mark
density is 50%, but the short-term mark density is
not 50%.
Any APC loop attempts to hold the average power
constant, and therefore adjusts the powers that
correspond to data_0 and data_1, up or down
depending on the local mark density. The rate at
which this adjustment takes place is set by the APC
loop time constant. One common approach to
reducing this undesirable effect is to make the APC
time constant very long, but this has the
disadvantage of slowing the response to other
changes. The MAX3865 uses a different technique,
mark-density compensation.
When the local mark-density of the data is 50%, the
average output from the mark-density multiplier is
zero. However, when the data consists locally of an
excess of 1s, the average output from the mark-
density multiplier goes positive and adds to the bias
reference current. The numerical details in Figure 4
are such that this increase in effective reference
current compensates exactly for the local increase in
feedback current from the monitor diode, provided
the current, monitor-diode IAMC, which originates in
#15, is programmed equal to the difference between
the data_0 and data_1 currents in the monitor diode.
(This requires monitor-diode IAMC to be programmed
equal to the monitor-diode modulation current.)
There is then no error output applied to the input to
integrator #14, and no shift in the laser bias current.
4.3 AMC Mode
In the AMC mode, the second input to subtractor #9
is enabled, also the second input to multiplier #19.
The current maximum Imod programmed in #7 takes
on the significance of an upper bound to the peak-
peak modulation current output—exactly like the
arrangement for bias current described in Section
4.2. Current control Imod subtracts away from
maximum Imod to give the actual Imod in the laser.
Multiplier #19 generates a pilot-tone current and
adds this to both the bias and modulation output
currents. There is, therefore, a pilot-tone component
of current in the monitor diode.
When the AMC loop has settled to equilibrium, the
pilot-tone component of feedback current from the
monitor diode must be equal to the pilot-tone
reference current that originates in #15, #16 and #20.
If, for example the monitor-diode feedback current is
momentarily too large, the resulting difference
reduces the modulation current (and hence pilot-tone
current) in the laser. The pilot-tone current in the
monitor diode, and hence peak-peak modulation
current, is controlled:
).(5
k2
V2.1
5
AMC_set
AMC_set
diode)(monitorpp
RI
R
I
×=








Ω+
=
−(18)
where I(RAMC_set) is the current flowing through
RAMC_set.Peak-peak optical power output from the
laser is controlled.
In summary, the average optical power output and
extinction ration in the AMC mode are given by:
,
)(5
)k2( V6
monitor
APC_set
monitorAPC_set
monitor
diode)(monitor
av
av
A
RI
AR
A
I
P
×
=
Ω+
=
=
and
(19)

Design Note HFDN-18.0 (Rev.1; 04/08) Maxim Integrated Products
Page 9 of 9
,
)()(
)()(
k1
k3
setAMC
2
1
setAPC
setAMC
2
1
setAPC
setAPC
2
1
setAMC
setAPC
2
1
setAMC
diode)(monitor
pp
2
1
diode)(monitor
av
diode)(monitor
pp
2
1
diode)(monitor
av
−−
−−
−−
−−
−
−
−
+
=
Ω+−
Ω++
=
−
+
=
RIRI
RIRI
RR
RR
II
II
re
where Amonitor is the monitor-diode gain as defined in
Equation 4.
The MAX3865 can be DC-coupled to the laser for
operation at VCC = 5V, or AC-coupled for VCC =
3.3V. Programming resistors RAPC_set and RAMC_set
are not affected, mark-density compensation is
automatic in either case.
4.3.1 Laser End-of-Life
As a laser nears the end of its useful life, the bias
and modulation currents required to maintain its
optical output become larger. The MAX3865
automatically increases its output currents, as
required. The warning flag, , is set when either
of the chip-output currents attempts to exceed the
upper-bound values programmed by Rbias_max or
Rmod_max (Equations #8 and #9).
Alternatively, approaching end-of-life can be
detected by observing the scaled versions of Ibias and
Imod at the open-collector-output pins BIAS_MON
and MOD_MON, and noting when these approach
limit values:
,
48
_bias mon
bias I
I=(21)
.
32
mod
mod_ I
Imon =(22)
If outputs BIAS_MON and MOD_MON are not
used, these pins should be tied to VCC.
4.3.2 Feedback Via a Monitor Diode
Recognize that, for any feedback system, the overall
closed-loop transfer function approaches the inverse
of the transfer function of the feedback network.
Therefore, if (for example) the feedback resistors
around an operational amplifier change in value, the
overall gain must change.
The MAX3865 is no different. In any system which
uses feedback via a monitor diode to control the
optical power output from a laser, the monitor diode
constitutes the feedback network. Therefore, if the
characteristic of the monitor changes, the optical
output must change. What the MAX3865 controls
and holds constant, despite changes in the
laser/monitor-diode combination, are the average
and peak-peak currents in the monitor diode, IAPC
and IAMC.
If you change the laser characteristic by changing its
temperature, you are also likely to change the
monitor diode. Any observed change in the optical
output from the system may be associated with the
latter, not with a failure of the MAX3865 to regulate
correctly.
FAIL
(20)