Design Note HFDN-18.0 (Rev.1; 04/08)
Maxim Integrated Products
Page 9 of 9
,
)
(
)
(
)
(
)
(
k
1
k
3
set
AMC
2
1
set
APC
set
AMC
2
1
set
APC
set
APC
2
1
set
AMC
set
APC
2
1
set
AMC
diode)
(monitor
p
p
2
1
diode)
(monitor
av
diode)
(monitor
p
p
2
1
diode)
(monitor
av
−
−
−
−
−
−
−
−
−
−
−
+
=
Ω
+
−
Ω
+
+
=
−
+
=
R
I
R
I
R
I
R
I
R
R
R
R
I
I
I
I
r
e
where A
monitor
is the monitor-diode gain as defined in
Equation 4.
The MAX3865 can be DC-coupled to the laser for
operation at V
CC
= 5V, or AC-coupled for V
CC
=
3.3V. Programming resistors R
APC_set
and R
AMC_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 R
bias_max
or
R
mod_max
(Equations #8 and #9).
Alternatively, approaching end-of-life can be
detected by observing the scaled versions of I
bias
and
I
mod
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
I
mon
=
(22)
If outputs BIAS_MON and MOD_MON are not
used, these pins should be tied to V
CC
.
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)
