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MAX3667 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX3667 Datasheet(HTML) 6 Page - Maxim Integrated Products |
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6 / 12 page 
_______________Detailed Description Low-voltage operation of laser diodes and optical transmitters produces stringent headroom conditions for laser drivers. Fast changes in modulation current produce large inductive voltage spikes, creating device saturation problems. Therefore, for +3.3V operation, the MAX3667’s modulation current should be AC coupled to the cathode of a laser diode. The recommended DC blocking capacitor value is 1µF. A simplified block dia- gram of the modulation driver is shown in Figure 1. The IMOD pin is internally biased through a 31 Ω pull-up resistor. This design decouples the headroom associat- ed with the modulation driver from the forward voltage drop of the laser diode, allowing the circuit to tolerate greater di/dt voltage transients. The design of the MAX3667 assumes a maximum DC forward-voltage drop of 1.6V across the laser diode. Bias current is DC coupled to the laser diode separately at the IBIAS out- put. In most applications, some small amount of resis- tance should be added in series with the DC blocking capacitor to help damp out the aberrations created by parasitic elements. Automatic Power Control The automatic power control (APC) feature allows an optical transmitter to maintain constant power, despite changes in laser efficiency due to temperature and aging. The APC loop requires the use of a PIN monitor photodiode, which generates a current proportional to the laser diode output power. A scaled version of the current flowing into the MD pin is compared to a scaled version of the current flowing out of the APCSET pin. When these currents are of equal value, the inputs of the operational transconductance amplifier (OTA) are balanced, and COMP is forced to approximately 1V. When the average value of the monitor diode current exceeds the value established by the APCSET current, the COMP voltage is forced lower. If the average value of the monitor diode current is less than the value established by the APCSET current, the COMP node voltage is forced higher. The output of the OTA (the APC pin), when connected directly to BIASSET (closed- loop condition), is used as an error signal to adjust the bias current flowing into BIASSET. The maximum OTA output current is approximately ±250µA. +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control 6 _______________________________________________________________________________________ MAX3667 INPUT (MODULATION) 31 Ω VCC VCC VCC INPUT (BIAS) LASER DIODE RFILT 22 Ω RDAMP 4.7 Ω 0.01 µF 1 µF MODMON IBIAS IMOD QMOD IQMOD BIASMON Figure 1. Simplified Modulation Driver Block Diagram |
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