M02069 Datasheet, PDF (28/44 Pages) M/A-COM Technology Solutions, Inc. – 3.3 or 5 Volt VCSEL/FP Laser Driver IC for Applications to 4.3 Gbps

M02069 Data Sheet
Figure 3-3. Selecting TC_SLOPE resistance in common cathode configuration
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
-40
-20
-10.0
-20.0
-30.0
0
20
40
60
Ambient Temperature in degrees C
RTCslope:
27k
33k
39k
47k
51k
62k
75k
82k
100k
120k
150k
180k
220k
390k
510k
open
80
100
3.3.8
Modulator (Figure 3-4)
The output stages OUT+ and OUT- are designed to drive 25Ω output loads over a wide range of currents and cir-
cuit architectures. The VCSEL may be a common anode or common cathode device. The output can be AC, DC,
or Differentially coupled depending on the supply voltage and laser configuration.
In a common anode configuration with a VCSEL (Figure 4-1), OUT+ should be connected through a capacitor to
the VCSEL. A pull-up resistor should be added in parallel to the VCSEL from SVCC to the OUT+ output. The
dynamic impedance of the parallel combination of the VCSEL and pull-up resistor should be roughly 25 ohms. A 24
ohm pull-up resistor should also be added from SVCC to OUT- so the currents and voltage swings in the two out-
puts are balanced.
In a common anode configuration with a Fabry-Perot laser (Figure 4-4), OUT+ may be AC, DC, or Differentially
coupled to the laser cathode. A resistor should be added in series with the laser such that the dynamic impedance
of the series combination of the laser and resistor should be roughly 25 ohms. A 24 ohm pull-up resistor to SV_CC is
needed on the OUT- output.
For common cathode operation with a VCSEL (Figure 4-2), internal 50 ohm terminations are switched in between
the OUT+ and OUT- outputs and V_CC3. VCSELs with impedances from 25-75Ω can be simply AC coupled to the
OUT- output with no additional load matching resistors. In this case OUT+ should be AC coupled to ground through
50Ω.
The VCSEL driver output stage is separately grounded from the rest of the circuitry (through GND₀). At higher data
rates (above 2Gb/s) GND₀ may be connected to ground through a minimum of 2 nH of inductance to improve the
transient response. A ferrite can also provide the extra isolation (Murata BLM18HG471SN1 or equivalent recom-
mended).
28
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02069-DSH-001-D
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