+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC
Optional Data Input Latch
To minimize input data pattern-dependent jitter, the dif-
ferential clock signal should be connected to the data
input latch, which is selected by an external LATCH
control. If LATCH is high, the input data is retimed by
the rising edge of CLK+. If LATCH is low, the input data
is directly connected to the output stage. When this
Programming the Modulation Current
For a given laser power P , slope efficiency (η), and
AVG
extinction ration (r , the modulation current can be cal-
e)
culated using Table 1. See the I
vs. R
graph
MOD
MODSET
in the Typical Operating Characteristics and select the
value of R
that corresponds to the required cur-
MODSET
rent at +25°C.
latch function is not used, connect CLK+ to V
leave CLK- unconnected.
and
CC
Programming the Bias Current
When using the MAX3869 in open-loop operation, the
Enable Control
The MAX3869 incorporates a laser driver enable func-
tion. When ENABLE is low, both the bias and modulation
currents are off. The typical laser enable time is 250ns,
and the typical disable time is 25ns.
bias current is determined by the R
resistor. To
BIASMAX
select this resistor, determine the required bias current
at +25°C. See the I vs. R graph in the
BIASMAX
BIASMAX
Typical Operating Characteristics and select the value
of R that corresponds to the required current at
BIASMAX
+25°C.
When using the MAX3869 in closed-loop operation, the
resistor sets the maximum bias current avail-
Current Monitors
The MAX3869 features bias- and modulation-current
monitor outputs. The BIASMON output sinks a current
R
BIASMAX
able to the laser diode over temperature and life. The
APC loop can subtract from this maximum value but
equal to 1/37 of the laser bias current (I
/ 37). The
BIAS
MODMON output sinks a current equal to 1/29 of the
laser modulation current (I / 29). BIASMON and
cannot add to it. See the I
vs. R
graph
BIASMAX
BIASMAX
MOD
in the Typical Operating Characteristics and select the
value of R that corresponds to the end-of-life
MODMON should be connected through a pull-up resis-
tor to V . Choose a pull-up resistor value that ensures a
BIASMAX
CC
bias current at +85°C.
voltage at BIASMON greater than V
age at MODMON greater than V
- 1.6V and a volt-
CC
- 1.0V.
CC
Programming the APC Loop
When the MAX3869’s APC feature is used, program the
average optical power by adjusting the APCSET resistor.
To select this resistor, determine the desired monitor cur-
rent to be maintained over temperature and life. See the
Slow-Start
For laser safety reasons, the MAX3869 incorporates a
slow-start circuit that provides a delay of 250ns for
enabling a laser diode.
I
vs. R
graph in the Typical Operating
MD
APCSET
Characteristics and select the value of R
responds to the required current.
that cor-
APCSET
APC Failure Monitor
The MAX3869 provides an APC failure monitor
(TTL/CMOS) to indicate an APC loop tracking failure.
FAIL is set low when the APC loop can no longer adjust
the bias current to maintain the desired monitor current.
Interfacing with Laser Diodes
To minimize optical output aberrations caused by signal
reflections at the electrical interface to the laser diode, a
series damping resistor (R ) is required (Figure 4).
D
Short-Circuit Protection
The MAX3869 provides short-circuit protection for the
modulation, bias, and monitor current sources. If either
BIASMAX, MODSET, or APCSET is shorted to ground,
the bias and modulation output will be turned off.
Additionally, the MAX3869 outputs are optimized for a
25Ω load. Therefore, the series combination of R and
D
R
(where R represents the laser-diode resistance)
L
L
Table 1. Optical Power Definition
Design Procedure
PARAMETER
SYMBOL
RELATION
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and extinc-
tion ratio. Table 1 gives the relationships that are helpful
in converting between the optical average power and the
modulation current. These relationships are valid if the
mark density and duty cycle of the optical waveform are
50%.
Average Power
P
P
AVG
= (P + P ) / 2
AVG
0
1
Extinction Ratio
r
e
r = P / P
e 1 0
Optical Power High
Optical Power Low
Optical Amplitude
P
1
P
0
P = 2PAVG · r / (r + 1)
1
e
e
P = 2P
0
/ (r + 1)
AVG
e
Pp-p
Pp-p = 2P
(r - 1) / (r + 1)
AVG e e
Laser Slope
Efficiency
η
η = Pp-p / I
MOD
Modulation Current
I
I
= Pp-p / η
MOD
MOD
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