and MODSET. See the
Design Procedure
section for
more details on open-loop operation.
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
latch function is not used, connect CLK+ to V
CC
and
leave CLK- unconnected.
Enable Control
The MAX3867 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.
Slow-Start
For laser safety reasons, the MAX3867 incorporates a
slow-start circuit which provides a programmable delay
time for enabling a laser diode. An external capacitor
(C
SLWSTRT
) connected from this pad to ground pro-
grams the delay by the equation:
t
ENABLE
≅ 100kΩ · (C
SLWSTRT
+ 2.5pF)
APC Failure Monitor
The MAX3867 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.
Short-Circuit Protection
The MAX3867 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.
Design Procedure
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
average duty cycle of optical waveform is 50%
Programming the Modulation Current
For a given laser power P
AVE
, slope efficiency η, and
extinction ration r
e
, the modulation current can be calcu-
lated by Table 1. Refer to the I
MOD
vs. R
MODSET
graph
in the
Typical Operating Characteristics
and select the
value of R
MODSET
that corresponds to the required cur-
rent at +25°C.
Programming the Bias Current
When using the MAX3867 in open-loop operation, the
bias current is determined by the R
BIASMAX
resistor. To
select this resistor, determine the required bias current
at +25°C. Refer to the I
BIASMAX
vs. R
BIASMAX
graph in
the
Typical Operating Characteristics
and select the
value of R
BIASMAX
that corresponds to the required
current at +25°C.
When using the MAX3867 in closed-loop operation, the
R
BIASMAX
resistor sets the maximum bias current avail-
able to the laser diode over temperature and life. The
APC loop can subtract from this maximum value but
cannot add to it. Refer to the I
BIASMAX
vs. R
BIASMAX
graph in the
Typical Operating Characteristics
and
select the value of R
BIASMAX
that corresponds to the
end-of-life bias current at +85°C.
Programming the APC Loop
When the MAX3867’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. Refer to
the I
MD
vs. R
APCSET
graph in the
Typical Operating
Characteristics
and select the value of R
APCSET
that cor-
responds to the required current.
Interfacing with the Laser Diode
To minimize optical output aberrations due to the laser
parasitic inductance, an RC shunt network is required
(Figure 4). If R
L
represents the laser diode resistance,
the recommended total resistance for R
D
+ R
L
is 25Ω.
Starting values for coaxial lasers are R
F
= 75Ω and
C
F
= 3.3pF. R
F
and C
F
should be experimentally
adjusted until the optical output waveform is optimized.
A bypass capacitor should also be placed as close to
the laser anode as possible, for the best performance.
Pattern-Dependent Jitter (PDJ)
When transmitting NRZ data with long strings of con-
secutive identical digits (CID), LF droop can occur and
contribute to pattern-dependent jitter. To minimize this
MAX3867
+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Automatic Power Control
_______________________________________________________________________________________ 9
PARAMETER SYMBOL RELATION
Average Power P
AVE
P
AVE
= (P
0
+ P
1
) / 2
Extinction Ratio r
e
r
e
= P
1
/ P
0
Optical Power High P
1
P
1
= 2P
AVE
· r
e
/ (r
e
+ 1)
Optical Power Low P
0
P
0
= 2P
AVE
/ (r
e
+ 1)
Optical Amplitude Pp-p Pp-p = 2P
AVE
(r
e
- 1) / (r
e
+ 1)
Laser Slope
Efficiency
η
η = Pp-p / I
MOD
Modulation Current I
MOD
I
MOD
= Pp-p /η
Table 1. Optical Power Definition
