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M
3.0V to 5.5V, 1.25Gbps/2.5Gbps
LAN Laser Drivers
12
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2) EN is switched low, then high.
3)
EN
is switched to high, then low.
Power-On Reset (POR)
Figure 7 shows the POR circuit for the MAX3286/
MAX3296 series devices. A POR signal asserts low
when V
CC
is in the operating range. The voltage operat-
ing range is determined by the LV pin, as shown in
Table 2. POR contains an internal delay to reject noise
on V
CC
during power-on or hot-plugging. The delay can
be extended by adding capacitance to the PORDLY
pin. The POR comparator includes hysteresis to improve
noise rejection. The laser driver is shut down while V
CC
is out of the operating range.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
260ps or better for 1.25Gbps, or 130ps or better for
2.5Gbps applications. To obtain the MAX3286/
MAX3296’s AC specifications, the instantaneous output
voltage at OUT+ must remain above V
CC
- 1V at all
times. Select a high-efficiency laser that requires low
modulation current and generates low-voltage swing at
OUT+. Laser package inductance can be reduced by
trimming the leads. Typical package leads have induc-
tance of 25nH/in (1nH/mm); this inductance causes a
larger voltage swing across the laser. A compensation fil-
ter network also can be used to reduce ringing, edge
speed, and voltage swing.
Programming the Modulation Current
Resistors at the MODSET and TC pins set the ampli-
tude of the modulation current. The resistor R
MOD
sets
the temperature-stable portion of the modulation cur-
rent, while the resistor R
TC
sets the temperature-
increasing portion of the modulation current.
To determine the appropriate temperature coefficient
from the slope efficiency (
α
)
of the laser, use the following
equation:
where
α
is the slope of the laser output power to the
laser current.
For example, suppose a laser has a slope efficiency
α
25
of 0.021mW/mA at +25°C, which reduces to
0.018mW/mA at +70°C. Using the above equation pro-
duces a laser tempco of -3175ppm/°C.
To obtain the desired modulation current and tempco
for the device, the following two equations can be used
to determine the required values of R
MOD
and R
TC
:
where tempco = -laser tempco.
Figure 8a shows a family of curves derived from these
equations. The straight diagonal lines depict constant
tempcos. The curved lines represent constant modula-
tion currents. If no temperature compensation is
desired, Figure 8b displays a series of curves that
show laser modulation current with respect to R
MOD
for
different loads.
The following useful equations were used to derive
Figure 8a and the equations at the beginning of this
section. The first assumes R
L
= 25
.
Programming the Bias Current/APC
Three application circuits are described below: com-
mon-cathode laser with photodiode, common-cathode
laser without photodiode, and common-anode laser
I
I
I
tempco
(
C
C A
)
°
MOD
C
)(
MOD
–
25
C
MOD
C
(
)
70
(
)
(
)
+
70
25
25
°
°
°
=
°
I
R
R
T
C
A
MOD
MOD
TC
=
×
+
+
3
+
×
×
°
(
)
[ ]
51
1 15
.
250
1 06
.
250
1 4 0
10
25
.
–
R
R
(
empco
empco
t
MOD
TC
0 19
=
+
×
×
)
)
250
52
48
250
t
R
tempco I
TC
MOD
=
(
)
0 21
.
250
Laser tempco
=
C
°
°
°
[
]
×
α
α
α
70
70
(
25
25
25
6
10
)
/
C
ppm
C
V
CC
PORDLY
POR
1.2V
36k
25k
28k
LV
VARIABLE
DELAY
BANDGAP
= 0.7s/
μ
F C
PORDLY
MAX3286
MAX3296
Figure 7. Power-On Reset Circuit