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MAX3643ETG+ Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX3643ETG+ Datasheet(HTML) 10 Page - Maxim Integrated Products |
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10 / 15 page  155Mbps to 2.5Gbps Burst-Mode Laser Driver 10 Maxim Integrated MAX3643 Detailed Description The MAX3643 laser driver includes a bias current gen- erator, bias current monitor, modulation current genera- tor, laser drive outputs, and monitor diode sample and hold. LVPECL-compatible inputs are provided for both high-speed data and burst enable. The high-speed burst-enable input signal is replicated on an LVCMOS output for use by the controller. Laser Diode Modulation and Bias Current Generators Laser diode modulation current amplitude is controlled by the current out of the MODSET pin, and bias current by the current out of the BIASSET pin, according to: IMOD = IMODSET x 88 IBIAS = IBIASSET x 88 A voltage source and two op amps are provided to enable IMODSET and IBIASSET to be set using either a resistor to ground or a current digital-to-analog convert- er (DAC). The high-impedance op amp reference input can be externally controlled, so that the modulation and bias currents can also be set using voltage DACs. Laser Diode Modulation and Bias Current Limiter Typical laser diodes have an absolute maximum rating of 150mA. To reduce the possibility of laser damage, the modulation current and bias current are shut off if the sum IMOD + IBIAS attempts to exceed the limit set by RIMAX; see the Typical Operating Characteristics. Bias Current Monitor The laser diode bias current can be monitored by measur- ing the voltage across an external load resistor connected from BCMON to ground. For example, a 1k Ω resistor from BCMON to ground gives the following relationship: The voltage at BCMON must be below 1.4V for proper operation. Output Drivers The modulation current ranges from 10mA to 85mA, as set by the current through MODSET. The laser modula- tion current output OUT+ is optimized to drive a 15 Ω load, and must be DC-coupled. A series damping resis- tor, RD, provides impedance matching to the laser diode. The combined value of the series damping resis- tor and the laser diode equivalent series resistance should be close to 15 Ω. An RC shunt compensation network, RCOMP/CCOMP, connected between the laser diode cathode and ground should also be provided to reduce optical output aberrations and duty-cycle dis- tortion caused by laser diode parasitic inductance. The values of RCOMP and CCOMP can be adjusted to match the laser diode and PCB layout characteristics for opti- mal optical eye performance (refer to Application Note 274: HFAN-02.0: Interfacing Maxim Laser Drivers with Laser Diodes). The OUT- pin is connected through a 15 Ω resistor and switching diode to the laser diode anode. The switching diode at OUT- improves the opti- cal output eye and burst-enable delay by better match- ing the laser diode characteristics. For data rates greater than 1Gbps, a parallel RL peak- ing network, RP/LP, connected between the laser diode anode and VCC is recommended. This network creates a differential drive for the laser diode to improve rise/fall times and reduce jitter. The values of RP and LP are also adjusted to match the laser diode and PCB layout characteristics for optimal optical eye performance. The bias current ranges from 1mA to 70mA, as set by the current through BIASSET. Current in the BIAS out- put switches at high speed when bursting; therefore, the BIAS+ pin should be connected directly through a resistor, equal to RD as determined above, to the laser cathode. The BIAS- pin is connected through a 10 Ω resistor and switching diode to VCC. When the BEN input is high, the laser driver sinks bias and modulation current according to the settings at MODSET and BIASSET. When the BEN input is low, the BIAS+ and OUT+ currents both shut off within 2ns. Note that when BEN is low, the bias current is shunted through the BIAS- output and the modulation current through the OUT- output. Monitor Diode Sample and Hold Laser monitor diode current is only generated when there is an optical output (BEN is active). When BEN is inactive, the monitor current is zero, reflecting the fact that the laser is off. A sample-and-hold circuit, trig- gered by the state of the BEN input, is provided in the MAX3643. During the burst-enable active period, the voltage present at MDIN is stored on an internal sam- ple-and-hold capacitor; and during the burst-enable inactive period, that voltage is output on MDOUT; see the timing diagram in Figure 2. While the internal sample-and-hold is sampling (BEN active), MDOUT voltage takes a 1.2V reference level. VI G k BCMON BIAS BSM =× ×1 Ω |
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