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MAX3261CCJ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3261CCJ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Table 1. MAX3261 Truth Table Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver 8 _______________________________________________________________________________________ the IPIN node will drop below 2.6V. This will trigger the failout comparator, which provides a TTL signal indicat- ing laser failure. The FAILOUT output asserts only if the monitor-diode current is low, not in the reverse situation where the monitor current exceeds IPINSET. FAILOUT is an open-collector output that requires an external pull-up resistor of 2.7k Ωto VCC. The transconductance amplifier can source or sink cur- rents up to approximately 1mA. Since the laser bias generator has a gain of approximately 40, the APC function has a limit of approximately 40mA (up or down) from the initial set point. To take full advantage of this adjustment range, it may be prudent to program the laser bias current slightly higher than required for normal operation. However, do not exceed the IBIASOUT absolute maximum rating of 75mA. To maintain APC loop stability, a 1000pF bypass capaci- tor may be required across the photodiode. If the APC function is not used, leave IBIASFB unconnected. Enable Inputs The MAX3261 provides complementary enable inputs (ENB+, ENB-) for interfacing with open-fiber-control architecture. The laser is disabled by reducing the ref- erence voltage outputs (VREF1, VREF2). Only one logic state will enable laser operation (Table 1). With a 1000pF stability capacitor, the MAX3261 modula- tion and bias can be enabled and disabled within 5µs (Figure 3). This timing satisfies the requirements of the Open Fiber Control system used in Fibre Channel networks. Temperature Considerations The MAX3261 output currents are programmed by cur- rent mirrors. These mirrors each have a 2VBE temperature coefficient. The reference voltage (VREF) is adjusted 2VBE so these changes largely cancel, resulting in output currents that are very stable with respect to temperature (see Typical Operating Characteristics). Wire Bonding Die For reliable operation, the MAX3261 has gold metalliza- tion. Make connections to the die with gold wire only, using ball bonding techniques. Wedge bonding is not recommended. Pad size is 4mils. __________________Design Procedure Interfacing Suggestions Use high-frequency design techniques for the board layout of the MAX3261 laser driver. High-speed inter- faces often require fixed-impedance transmission lines (Figure 5). Adding some damping resistance in series with the laser raises the load impedance, making the transmission line more realizable, and it also helps reduce power consumption (see the section Reducing Power Consumption). Minimize any series inductance to the laser, and place a bypass capacitor as close to the laser’s anode as possible. Power connections labeled VCCA are used to supply the laser modulation and laser bias circuits. VCCB connec- tions supply the bias-generator and automatic-power control circuits. For optimum operation, isolate these sup- plies from each other by independent bypass filtering. VCCA, VCCB, GNDA, and GNDB all have multiple pins. Connect all pins to optimize the MAX3261’s high- frequency performance. Ground connections between signal lines (VIN+, VIN-, OUT+, OUT-) improve the quali- ty of the signal path by reducing the impedance of the interconnect. Multiple connections, in general, reduce inductance in the signal path and improve the high- speed signal quality. GND pins should be tied to the ground plane with short runs and multiple vias. Avoid ground loops, since they are a source of high-frequency interference. The MAX3261 data inputs accept PECL input signals, which require 50 Ωtermination to (VCC - 2V). Figure 4 shows alternative termination techniques. When a ter- mination voltage is not available, use the Thevenin- equivalent termination. When interfacing with a non-PECL signal source, use one of the other alterna- tive termination methods shown in Figure 4. Bias Network Compensation When driving the laser diode with transmission lines, it is important to maintain a constant load impedance in order to minimize aberrations due to reflections. The inductive nature of laser packages will cause the laser impedance to increase with frequency, and the parasitic capacitance of the laser driver bias output (IBIASOUT) has some loading effects at high frequency. Of these two effects, the loading due to the laser lead induc- tance dominates. Impedance variation must be com- pensated for high-frequency operation. One possible approach is to use a shunt R-C network in parallel with the laser diode to compensate for the laser impedance (Figures 5 and 6). Add an R-L circuit in series with the bias output to compensate for the IBIASOUT capaci- tance (Figures 5 and 7). ENB- 0 1 0 0 ENABLED 1 ENB+ OUTPUT CURRENTS 0 DISABLED DISABLED 1 1 DISABLED |
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