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MAX3646 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX3646 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 14 page Detailed Description The MAX3646 laser driver consists of three main parts: a high-speed modulation driver, biasing block with ERC, and safety circuitry. The circuit design is opti- mized for high-speed, low-voltage (+3.3V) operation (Figure 4). High-Speed Modulation Driver The output stage is composed of a high-speed differ- ential pair and a programmable modulation current source. The MAX3646 is optimized for driving a 15Ω load. The minimum instantaneous voltage required at OUT- is 0.7V for modulation currents up to 60mA and 0.75V for currents from 60mA to 85mA. Operation above 60mA can be accomplished by AC-coupling or with sufficient voltage at the laser to meet the driver output voltage requirement. To interface with the laser diode, a damping resistor (RD) is required. The combined resistance damping resistor and the equivalent series resistance (ESR) of the laser diode should equal 15Ω. To further damp aberrations caused by laser diode parasitic induc- tance, an RC shunt network may be necessary. Refer to Maxim Application Note HFAN 0.0: Interface Maxim’s Laser Driver to Laser Diode for more information. Any capacitive load at the cathode of a laser diode degrades optical output performance. Because the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the BIAS pin para- sitics form the laser cathode. Extinction Ratio Control The extinction ratio (re) is the laser on-state power divided by the off-state power. Extinction ratio remains constant if peak-to-peak and average power are held constant: re = (2PAVG + PP-P) / (2PAVG - PP-P) 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control _______________________________________________________________________________________ 7 VIN+ VIN- 200mV (min) 2400mV (max) 100mV (min) 1200mV (max) IMOD VOLTAGE CURRENT TIME SINGLE ENDED DIFFERENTIAL (VIN+) - (VIN-) IOUT+ Figure 1. Required Input Signal and Output Polarity MAX3646 30Ω 30Ω 30Ω 75Ω 50Ω OUT+ OUT- VCC VCC OSCILLOSCOPE Z0 = 30Ω Z0 = 30Ω Z0 = 50Ω 0.5pF IOUT+ Figure 2. Test Circuit for Characterization L1 1µH C1 0.1µF C3 0.1µF C2 10µF VOLTAGE SUPPLY SOURCE NOISE OPTIONAL OPTIONAL FILTER DEFINED BY SFP MSA HOST BOARD MODULE TO LASER DRIVER VCC Figure 3. Supply Filter |
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