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          Multi-rate laser driver MAX3737 with extinction ratio control

          1 Introduction

          MAX3737 is a laser driver with extinction ratio control produced by American MAXIM company. Its working rate is 155Mbps ~ 2.7Gbps. It can be used as an optical transmitter in multi-rate optical systems such as OC-3 to OC-48 FEC. Compared with similar products in the past, this product not only has the characteristics of high transmission rate, small supply current and constant average output power, but also can keep the extinction ratio constant throughout the life of the laser tube and the temperature change range. The main features of MAX3737 are as follows:

          a—? Adopt + 3.3V single power supply working mode;

          a—? Only need 47mA power supply current;

          a—? Can provide modulation current up to 85mA and bias current up to 100mA;

          a—? Contains automatic power control (APC), automatic modulation control (AMC) and temperature compensation circuit;

          a—? With current monitoring setting terminal with ground as reference point;

          a—? With safety control and failure warning indication circuit.

          2 Pin function, internal structure and working principle

          2.1 Pin function

          MAX3737 adopts 32-pin QFN package, its pin arrangement is shown in Figure 1. The functions of each pin are as follows:

          GND (1, 10, 15, 16): ground terminal;

          XX DISABLE (2): laser tube output control terminal voltage input, low level effective;

          VCC (3, 6, 11, 18, 23): + 3.3V power port;

          IN + (4), IN- (5): respectively the positive and negative input terminals of the data signal;

          PC MON (7): monitoring? Feedback? Photodiode current monitoring terminal. This terminal can generate a reference voltage proportional to the current of the feedback photodiode and connected to ground as a reference point through an external resistor;

          BC MON (8): The laser tube bias current monitoring terminal can generate a reference voltage proportional to the bias current and referenced to ground through an external resistor;

          MC MON (9): Modulation current monitoring terminal can generate a reference voltage proportional to the modulation current and referenced to ground through an external resistor;

          TX FAULT (12): Transmission failure indication terminal;

          SHUDTOWN (13): Turn off the driver output;

          VBS (14): Laser tube bias voltage indication terminal;

          BIAS (17): Laser tube bias current output terminal;

          OUT- (19, 20): Reverse output terminal, the two pins should be interconnected in application;

          OUT + (21, 22): positive output terminal, these two pins should be interconnected in application;

          MD (24): monitoring? Feedback? Photodiode current input terminal, connected to the anode of the monitoring (feedback) photodiode during application;

          VMD (25): monitoring? Feedback? Photodiode voltage indication terminal;

          APCFILT1 (26), APCFILT2 (27): APC loop main pole setting end, a capacitor (CAPC) should be connected between these two pins during application;

          APCSET (28): average optical power setting end;

          MODSET (29): Modulation current part setting terminal;

          MODBCOMP (30): the setting terminal of the bias current to the modulation current compensation coefficient;

          TH TEMP (31): Threshold setting terminal of temperature compensation circuit;

          MODCOMP (32): temperature compensation coefficient setting terminal.

          figure 2

          2.2 Internal structure and working process

          The internal structure of MAX3737 is shown in Figure 2. The internal circuit mainly includes a high-speed modulation circuit, an extinction ratio control circuit, and a safety logic control and indication circuit. Among them, the high-speed modulation circuit includes two parts of the input stage and the output stage, mainly composed of input buffer, data channel and high-speed differential pair circuit, its function is to modulate the input signal and provide the required excitation signal for the external laser tube. The extinction ratio control circuit consists of three parts: automatic power control? APC? Circuit, automatic modulation control? AMC? Circuit and temperature compensation circuit. Its main function is to form a feedback control circuit with the monitoring photodiode, and at the same time through the bias current and modulation current The dynamic control and adjustment of the system keeps the extinction ratio constant; the safety logic control and indication circuit mainly provide safety guarantee for the normal operation of the driver, monitor the working state of the driver, and provide various working states and failure information of the driver.

          MAX3737 adopts APC working mode. When MAX3737 works normally, data is input from IN- and IN +, processed by input buffer circuit and data channel, and then modulated by controlling the differential pair modulator to achieve modulation. The modulated signal is output from OUT- End and OUT + end output to drive the external laser tube; when the output power changes, the feedback signal will be input from the MD terminal, and then adjust the modulation current and bias current changes through the extinction ratio control circuit to automatically maintain the stability of the output power; when When the temperature change exceeds the threshold, the temperature compensation circuit will automatically adjust the modulation current to maintain power stability; when the circuit fails and other unexpected situations occur, the safety logic control and indication circuit will output a control signal through the SHUTDOWN terminal to turn off the laser tube output, while The warning signal is output by the TX FAULT terminal.

          3 Application Design

          The application of MAX3737 requires users to design very few circuits. The main design work of the user is to select the appropriate laser tube and various related current designs. The typical application circuit of MAX3737 is shown in Figure 3. The parameter values a€?a€?of the marked components in the figure are typical values, and the unlabeled components need to be determined in the specific design. The laser tube adopts the DC coupling mode. The following describes the application design process of MAX3737 with typical application circuits.

          3.1 The choice of laser tube

          When users use MAX3737 to design optical transmitters, the first choice is to choose the appropriate laser tube according to actual needs. Generally, the optical output power is described by the average optical power and the extinction ratio. The user can determine the average output power and extinction ratio of the required laser tube according to the optical output power, and should try to make the extinction ratio as long as the output power is met Bigger. After the output power and extinction ratio are determined, the laser tubes that meet the conditions are selected according to these parameters.

          3.2 Design of modulation current IMOD

          After the laser tube is selected, the user can derive the calculation formula of the modulation current IMOD according to the relationship in Table 1. details as follows:

          IMOD = 2PAV (re-1) / ?· (re + 1);

          In the formula, the physical meaning of each parameter is listed in Table 1. In fact, the modulation current is composed of three parts: fixed modulation current (IMODS), bias compensation modulation current (KIBIAS) and temperature compensation modulation current (IMODT).

          (1) Fixed modulation current (IMODS)

          The fixed modulation current is the modulation current required by the driver under ideal working conditions (constant temperature and constant output power). The current can be determined by the MAX3737 internal circuit and the external resistance of the MODSET terminal. Therefore, you should first determine the required fixed modulation current (IMODS) according to the actual requirements, and then determine the value of the external resistance (RMODSET) at the MODSET terminal? Specifically:

          IMODS = 268VREF / RMODSET

          Among them, VREF is the reference voltage inside MAX3737, the typical value under normal circumstances is 1.3V.

          (2) Offset compensation modulation current (K IBIAS)

          The offset compensation modulation current is caused by the change of the bias current, its effect is determined by the compensation factor K, and the value of K is determined by the external resistance of the MODCOMP terminal. In application, the appropriate compensation factor K can be determined according to the changes in bias current and modulation current, and then the external resistance (RMODBCOMP) value of the MODBCOMP terminal can be determined according to the value of K.

          The calculation formula for determining the compensation factor K is:

          K = a–3 IMOD / a–3 IBIAS = (IMOD2-IMOD1) / (IBIAS2-IBIAS1);

          The relationship between K value and RMODBCCOM is:

          K = [1700 / (1000 + RMOBDBCOMP)] ?± 10%

          (3) Temperature compensation modulation current (IMODT)

          The temperature-compensated modulation current is generally caused by the temperature exceeding the threshold temperature. Its function is to compensate the effect of temperature changes on the modulation current. When T> TTH, the calculation formula of the temperature-compensated modulation current (IMODT) is

          IMODT = TC (T-TTH);

          Among them, TTH is the temperature threshold, its value can be determined by the external resistance of the TH_TEMP terminal (RTH_TEMP); TC is the temperature compensation coefficient, and its value is determined by the external resistance of the MODCOMP terminal (RMODDCOMP).

          In application, the appropriate temperature threshold and temperature compensation coefficient should be determined according to the actual situation, and then the RTH_TEMP and RMODCCOMP should be determined according to the following formula:

          TTH = ???70 a?? + [1.45M?? / (9.2k?? + RTH-TEMP)] a?? ?± 10%;

          3.3 Design of monitoring photodiode feedback current IMD

          After the laser tube is selected, the transfer coefficient PMON can be determined, and the parameter setting formula in Table 1 can be referred to when designing. After the user determines the average optical power, the theoretical value of IMD can be determined according to the formula PAV = IMD / PMON.

          In MAX3737, the feedback current IMD can be set by an external resistor at the APCSET terminal. Therefore, the essence of the IMD design is to determine the external resistor RAPSET at the APCSET terminal. The user can determine the RAPSET value according to the following formula.

          IMD = VREF / (2RAPSET)

          After RAPSET is determined, the actual feedback current IMD is determined. In this way, the APC circuit will automatically adjust the bias current IBIAS according to the change in IMD, thereby maintaining the stability of the average optical power.

          image 3

          3.4 Design of APC loop filter capacitor

          In the APC circuit, the function of the filter capacitor CAPC is to delay the action time of the APC circuit and reduce low-frequency signal interference. The value of the filter capacitor CAPC can be determined by the low-frequency cutoff frequency f3DB. The user can first determine the low-frequency cutoff frequency f3DB according to the requirements, and then determine the value of the CAPC according to the following formula.

          CAPC (??F) a‰??·68??MON / f3DB (khZ)

          In order to filter out high-frequency noise, a pull-up capacitor CMD needs to be connected to the ground at the MD terminal. In general, the value of the pull-down capacitor CMD is about a quarter of the filter capacitor CAPC value.

          3.5 Matters needing attention

          In the design process, in order to make the circuit work properly, there must be certain conditions for various currents. If the required modulation current is not greater than 60 mA, the MAX 3737 and external laser tube can use DC coupling; if the modulation current is greater than 60 mA, the AC coupling should be used. Regardless of the coupling method used, at the output OUT +, various currents should meet the following requirements:

          (1) For DC coupling

          VOUT + = VCC-VDIODEE-IMOD (RD + RL) -IBIASRRLa‰¥0.7V

          In the formula, VDDIO is the bias voltage of the laser diode, with a typical value of 1.2V; RL is the bias voltage of the laser diode, with a typical value of 5??; RD is the serial matching resistance, with a typical value of 20??.

          (2) For AC coupling

          VOUT + = VCC-IMOD (RD + RL) / 2 a‰¥ 0.75V

          In addition, because MAX3737 is a high-frequency product, the circuit layout has a great influence on it. In the circuit design, high-frequency layout technology with superior performance should be used. At the same time, users should use multi-layer circuit boards with a common ground layer to reduce electromagnetic Interference and intermodulation distortion; the circuit board should use low-loss dielectric materials to reduce energy loss; the data input lead and modulation output lead should use impedance-controlled transmission lines, which can facilitate circuit adjustment, reduce energy loss and reduce interference.

          4 Conclusion

          The MAX3737 laser driver has advantages that cannot be matched by similar products, mainly because it has three control circuits (APC circuit, AMC circuit and temperature compensation circuit), so it can always maintain a constant extinction ratio? It is also suitable for multiple transmission rates, so? This product has broad application prospects in optical fiber communication.

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          亚洲 欧美 图片 自拍 视频
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