Multi-rate laser driver with extinction ratio control MAX3737
1 Introduction
The MAX3737 is a laser driver with extinction ratio control from MAXIM, USA. It operates at 155 Mbps to 2.7 Gbps and is used as an optical transmitter in fiber systems such as multi-rate OC-3 to OC-48 FEC. Compared with the previous similar products, this product not only has the characteristics of high transmission rate, small supply current and constant output average power, but also keeps the extinction ratio constant throughout the service life and temperature variation range of the laser tube. The main features of the MAX3737 are as follows:
â— Adopt +3.3V single power supply mode;
â— Only 47mA power supply is required to supply current;
â— Can provide up to 85mA modulation current and up to 100mA bias current;
â— Built-in automatic power control (APC), automatic modulation control (AMC) and temperature compensation circuit;
â— Current monitoring set with ground reference point;
â— With safety control and failure warning indication circuit. 
2-pin function, internal structure and working principle
2.1 pin function
The MAX3737 is available in a 32-pin QFN package with pinouts as shown in Figure 1. The function of each pin is as follows:
GND (1, 10, 15, 16): ground terminal;
TX DISABLE (2): laser tube output control terminal voltage input, active low;
VCC (3, 6, 11, 18, 23): +3.3V power port;
IN+(4), IN-(5): the positive and negative inputs of the data signal;
PC MON (7): Monitor? Feedback? Photodiode current monitoring terminal. The terminal can generate a reference voltage proportional to the feedback photodiode current and ground 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 ground reference point through an external resistor;
MC MON (9): modulation current monitoring terminal, which can generate a reference voltage proportional to the modulation current and ground reference point through an external resistor;
TX FAULT (12): transmission failure indication end;
SHUTDOWN (13): Turn off the output of the drive;
VBS (14): laser tube bias voltage indicating end;
BIAS (17): laser tube bias current output;
OUT-(19,20): Inverting output, the two legs should be interconnected when applied;
OUT+ (21, 22): forward output, these two pins should be interconnected when applied;
MD (24): monitoring? feedback? The photodiode current input terminal is connected to the positive terminal of the monitoring (feedback) photodiode when applied;
VMD (25): monitoring? feedback? photodiode voltage indicating end;
APCFILT1 (26), APCFILT2 (27): APC loop main pole setting terminal, a capacitor (CAPC) should be connected between these two pins when applying;
APCSET (28): average optical power setting end;
MODSET (29): modulation current part setting end;
MODBCOMP (30): bias current to the modulation current compensation coefficient setting terminal;
TH TEMP (31): temperature compensation circuit threshold setting terminal;
MODTCOMP (32): Temperature compensation coefficient setting terminal.
figure 2
2.2 Internal structure and working process
The internal structure of the 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. The high-speed modulation circuit comprises an input stage and an output stage, and is mainly composed of an input buffer, a data channel and a high-speed differential pair circuit. The 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 monitor photodiode, while passing the bias current and the modulation current. The dynamic control adjusts to maintain the extinction ratio constant; the safety logic control and indication circuit mainly provides safety for the normal operation of the drive, monitors the working state of the drive, and provides various working states and failure information of the drive.
The MAX3737 uses the APC mode of operation. When the MAX3737 is operating normally, data is input from the IN- and IN+ terminals, processed through the input buffer circuit and data channel, and then modulated by the differential pair modulator output. The modulated signal is output from OUT- The output of the terminal and the OUT+ terminal drive the external laser tube; when the output power changes, the feedback signal will be input from the MD terminal, and then the extinction ratio control circuit is used to adjust the modulation current and the bias current to automatically maintain the output power stability; When the temperature changes beyond the threshold, the temperature compensation circuit will automatically adjust the modulation current to maintain power stability; when the circuit fails and other unexpected conditions occur, the safety logic control and indication circuit will output a control signal through the SHUTDOWN terminal to turn off the laser tube output. The warning signal is output by the TX FAULT terminal.
3 Application design
The MAX3737 requires very few circuits designed by the user when it is used. The user's main design work 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 ​​of the components in the figure are typical values. The unlabeled components need to be determined in the specific design. The laser tube is DC-coupled. The application design process of the MAX3737 is described below in conjunction with a typical application circuit.
3.1 Laser tube selection
When designing an optical transmitter with the MAX3737, the first choice is to select the appropriate laser tube according to actual needs. In general, 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 desired laser tube according to the optical output power, and should make the extinction ratio as much as possible while satisfying the output power. Bigger. After the output power and the extinction ratio are determined, the laser tube that satisfies the condition is selected based on these parameters.
3.2 Modulation current IMOD design
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=2PAVG(re-1)/η(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), offset 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 operating conditions (temperature constant and constant output power). This current can be determined by the MAX3737 internal circuit and the external resistor at the MODSET terminal. Therefore, you should first determine the required fixed modulation current (IMODS) according to the actual requirements, and then determine the external resistance (RMODSET) value of the MODSET terminal? Specifically:
IMODS=268VREF/RMODSET
Among them, VREF is the reference voltage inside the MAX3737, and the typical value is 1.3V.
(2) Offset compensation modulation current (K IBIAS)
The offset compensation modulation current is caused by the change of the bias current. The magnitude of the action is determined by the compensation factor K, and the value of K is determined by the external resistor of the MODBCOMP terminal. In the application, the appropriate compensation factor K can be determined according to the bias current and the modulation current change, and then the external resistance (RMODBCOMP) value of the MODBCOMP terminal is determined according to the K value.
The formula for determining the compensation factor K is:
K=â–³IMOD/â–³IBIAS=(IMOD2-IMOD1)/(IBIAS2-IBIAS1);
The relationship between K value and RMODBCOMP is:
K=[1700/(1000+RMODBCOMP)] ±10%
(3) Temperature compensated modulation current (IMODT)
The temperature compensated modulation current is generally caused by the temperature exceeding the threshold temperature. Its function is to compensate the influence of the temperature change on the modulation current. When T>TTH, the temperature compensation modulation current (IMODT) is calculated as:
IMODT=TC(T-TTH);
Where TTH is the temperature threshold, its value can be determined by the external resistor (RTH_TEMP) at the TH_TEMP terminal; TC is the temperature compensation coefficient, and its value is determined by the external resistor (RMODTCOMP) of the MODTCOMP terminal.
When applying, determine the appropriate temperature threshold and temperature compensation coefficient according to the actual situation, and then determine RTH_TEMP and RMODTCOMP: according to the following formula.
TTH=-70°C+[1.45MΩ/(9.2kΩ+RTH-TEMP)] °C±10%;
3.3 Monitoring the design of photodiode feedback current IMD
When the laser tube is selected, the transfer coefficient PMON can be determined. Refer to the parameter setting formula in Table 1 for design. After the user determines the average optical power, the theoretical value of the IMD can be determined according to the formula PAVG=IMD/PMON.
In the MAX3737, the feedback current IMD can be set by the APCSET external resistor, so the essence of the IMD design is to determine the external resistor RAPCSET at the APCSET terminal. The user can determine the RAPCSET value according to the following formula.
IMD=VREF/(2RAPCSET)
After the RAPCSET is determined, the actually provided feedback current IMD is determined, so that the APC circuit automatically adjusts the bias current IBIAS according to the change of the IMD, thereby maintaining the stability of the average optical power.
image 3
3.4 APC loop filter capacitor design
In the APC circuit, the function of the filter capacitor CAPC is to delay the action time of the APC circuit and reduce the 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(μF)≈η68ÏMON/f3DB(kHz)
In order to filter out high frequency noise, the pull capacitor CMD needs to be connected to the ground at the MD end. In general, the value of the pull-down capacitor CMD is about one quarter of the CAPC value of the filter capacitor.
3.5 Precautions
In the design process, in order to make the circuit work normally, there are certain conditions for various currents. If the required modulation current is not more than 60mA, the MAX3737 and the external laser tube can be DC-coupled; if the modulation current is greater than 60mA, AC coupling should be used. Regardless of the coupling method used, at the output OUT+, the various currents should meet the following requirements:
(1) For DC coupling
VOUT+=VCC-VDIODE-IMOD(RD+RL)-IBIASRL≥0.7V
Where, VDIODE is the bias voltage of the laser diode, typically 1.2V; RL is the bias terminal resistance of the laser diode, typically 5Ω; RD is the serial matching resistor, typically 20Ω.
(2) For AC coupling
VOUT+=VCC-IMOD(RD+RL)/2≥0.75V
In addition, since the 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 a multi-layer circuit board with a common ground plane to reduce electromagnetic Interference and intermodulation distortion; the board should use low-loss dielectric material to reduce energy loss; the data input terminal lead and the modulation output lead should use impedance-controlled transmission line, which can facilitate circuit adjustment, reduce energy loss and reduce interference.
4 Conclusion
The MAX3737 laser driver has unmatched advantages in its class. It has three control circuits (APC circuit, AMC circuit, and temperature compensation circuit), so it always maintains a constant extinction ratio and is suitable for a variety of transmission rates. This product has broad application prospects in optical fiber communication.
Linear Power Adapter,Audio Adapter ,Power Adapter ,Network Adapter
Power Adaptor Co., Ltd. , http://www.nbadapter.com