M02061_17 Datasheet, PDF (32/45 Pages) M/A-COM Technology Solutions, Inc. – 3.3 or 5 Volt Laser Driver

M02061-12/-21
3.3 or 5 Volt Laser Driver
Rev V6
Table 3-2. Modulation Current Maximums (2 of 2)
Max
Modulation Current
Max Bias Current
V_CC=3.3V, Laser DC coupled
100⁽²⁾
100
V_CC=3.3V, Laser AC coupled
100
100
When differentially coupling, the maximum modulation and bias current is determined by either the AC or DC coupling of the OUT+ or OUT- output, whichever has
the minimum rating.
1. When AC coupling the output should never be allowed to swing above the absolute voltage rating of the part, which is 6V.
2. When V_CC=3.3V, the OUT+ and OUT- should not be driven below 1.6V. In most 3.3V applications, this will make DC coupling impractical.
When DC coupled, OUT+ should be connected through a series resistor to the laser such that the total impedance
seen at the output is 25 ohms. This will result in the optimum pulse response while allowing the maximum
modulation current (see Figure 4-3).
The output can also be AC coupled to the laser. This is the required operating mode when using a 3.3V supply
(unless the laser has a small forward voltage and OUT+ will not go below 1.6V). When AC coupled the dynamic
resistance seen by OUT+ should still be 25 ohms. In addition to a resistor in series with the laser, a capacitor is
added in series and a ferrite is used to pull up the collector at OUT+ to V_CC.
When the laser is AC coupled, the OUT- pin is usually tied to the laser anode through an AC coupled series resistor
which matches the impedance seen by the OUT+ pad (see Figure 4-1).
The output stage also has a separate current path to GND labelled GND₀. This isolates the output switching
currents from the rest of the system.
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