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MAX3740 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX3740 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 14 page Detailed Description The MAX3740 contains a bias generator with automatic power control (APC), safety circuit, and a laser modula- tor with optional peaking compensation. Bias Generator Figure 1 shows the bias generator circuitry that contains a power-control amplifier and smooth-start circuitry. An internal PNP transistor provides DC laser current to bias the laser in a light-emitting state. The APC circuitry adjusts the laser-bias current to maintain average power over temperature and changing laser properties. The smooth-start circuitry prevents current spikes to the laser during power-up or enable, ensuring compliance with safety requirements and extending the life of the laser. The MD input is connected to the cathode of a monitor diode, which is used to sense laser power. The BIAS output is connected to the anode of the laser through an inductor or ferrite bead. The power-control amplifier dri- ves a current amplifier to control the laser’s bias current. During a fault condition, the bias current is disabled. The PWRMON output provides a voltage proportional to average laser power given by: VPWRMON = 2 ✕ IPD ✕ RPWRSET The BIASMON output provides a current proportional to the laser bias current given by: IBIASMON = IBIAS / 9 When APC is not used (no monitor diode, open-loop configuration) connect the COMP and PWRMON pins to GND. In this mode, the bias current is set by the resistor RBIASSET. When a closed-loop configuration is used, connect a 1.7k Ω resistor between ground and BIASSET to set the maximum bias current. Safety Circuit The safety circuit contains an input disable (TX_DISABLE), a latched fault output (FAULT), and fault detectors (Figure 2). This circuit monitors the operation of the laser driver and forces a shutdown (disables laser) if a fault is detected (Table 1). Table 2 contains the circuit’s response to various single-point failures. The transmit fault condition is latched until reset by a toggle of TX_DISABLE or VCC. The FAULT pin should be pulled high with a 4.7k Ω to 10kΩ resistor. 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors _______________________________________________________________________________________ 9 CCOMP COMP BIASSET RPWRSET MD 1.8V REF CURRENT AMPLIFIER POWER- CONTROL AMPLIFIER ENABLE 200 Ω RBIASSET PWRMON BIAS GENERATOR SMOOTH- START RBIASMON BIASMON FERRITE BEAD BIAS 1.6V (2VBE) 2X IBIAS 40 IPD IBIAS 9 MAX3740 0.8V Figure 1. Bias Generator PIN FAULT CONDITION BIAS VBIAS > VCC - 0.2V BIASMON VBIASMON > 0.8V PWRMON VPWRMON > 0.8V Table 1. Fault Conditions |
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