M
2.7Gbps, Low-Power SFP Laser Drivers
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11
Design Procedure
When designing a laser transmitter, the optical output
usually is expressed in terms of average power and
extinction ratio. Table 3 shows relationships 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%.
Programming the Modulation Current
For a given laser power (P
AVG
), slope efficiency (
η
), and
extinction ratio (r
e
), the modulation current can be calcu-
lated using Table 3. See the Modulation Current vs.
R
MODSET
graph in the
Typical Operating Characteristics
section, and select the value of R
MODSET
that corre-
sponds to the required current at +25
°
C.
Programming the APC Loop
Program the average optical power by adjusting -R
APC-
SET
. To select the resistance, determine the desired moni-
tor current to be maintained over temperature and
lifetime. See the Monitor Diode Current vs. R
APCSET
graph in the
Typical Operating Characteristics
section,
and select the value of R
APCSET
that corresponds to the
required current.
Interfacing with Laser Diodes
To minimize optical output aberrations caused by sig-
nal reflections at the electrical interface to the laser
diode, a series-damping resistor (R
D
) is required
(Figure 4). Additionally, the MAX3735/MAX3735A out-
puts are optimized for a 15
load. Therefore, the series
combination of R
D
and R
L
, where R
L
represents the
laser-diode resistance, should equal 15
. Typical val-
ues for R
D
are 8
to 13
. For best performance, place
a bypass capacitor (0.01μF typ) as close as possible to
the anode of the laser diode. An RC shunt network
between the laser cathode and ground minimizes opti-
cal output aberrations. Starting values for most coaxial
lasers are R
COMP
= 50
in series with C
COMP
= 8pF.
Adjust these values experimentally until the optical out-
put waveform is optimized. Refer to Maxim Application
Note: HFAN 02.0:
Interfacing Maxim
’
s Laser Drivers to
Laser Diodes
for more information.
POR AND COUNTER
FOR t_init
COUNTER FOR
t_onfault
MODSET
SHORT-
CIRCUIT
DETECTOR
100ns DELAY
MAX3735
MAX3735A
V
CC
V
CC
V
CC
V
BG
COMP
V
BG
COMP
76
I
BIAS
1
I
MD
RS
LATCH
R
Q
S
PC_MON
BC_MON
I
ENABLE
I
ENABLE
TX_FAULT
CMOS
TTL
OPEN COLLECTOR
SHUTDOWN
R
PC_MON
R
BC_MON
TX_DISABLE
Figure 5. Safety Circuitry
Table 1. Typical Fault Conditions
1.
If any of the I/O pins is shorted to GND or V
CC
(single-
point failure, see Table 2), and the bias current or the
photocurrent exceed the programmed threshold.
2.
End-of-life (EOL) condition of the laser diode. The bias
current and/or the photocurrent exceed the
programmed threshold.
3.
Laser cathode is grounded and the photocurrent
exceeds the programmed thresholds.
4.
N o feed b ack for the AP C l oop ( b oken i nter connecti on,
d efecti ve m oni or p hotod od e) and the b as cur ent
exceed s the p og am m ed thr eshol d