Abstract: MAX3737 is one kind which MAXIM Corporation produces has the extinction ratio control laser driver. This component’s transmission speed is 155Mbps~2.7Gbps, may serve as in the optical fiber network the photoemission machine. This product has the power loss to be low, the operating current is small and extinction ratio permanent grade characteristic. In the article introduced the MAX3737 main performance, the pin function, the internal structure and the principle of work; Simultaneously unified its model to introduce the MAX3737 application design process using the electric circuit.
Key word: MAX3737; ERC; APC; Light emitting diode
1 introduction
MAX3737 is American MAXIM Corporation produces has the extinction ratio control laser driver, its working speed is 155Mbps~2.7Gbps, may serve as multi-speed OC-3 to optical fiber system’s and so on in the OC-48 FEC photoemissions machine. This product compared with formerly the similar products, not only has the transmission speed to be high, the supply current is small and output average power permanent grade characteristic, moreover maintains throughout in the laser tube’s service life and the temperature change scope internal energy the extinction ratio is constant. The MAX3737 principal characteristic is as follows:
* uses the 3.3V single power source working pattern;
* only needs 47mA the power supply electric current;
* can provide reaches as high as 85mA the modulating current and reach as high as 100mA the bias electric current;
* content automatic power control (APC), automatic modulation control (AMC) and temperature compensation electric circuit;
* has take the place as the reference point electric current monitoring establishment end;
* has the safety control and the expiration warning indicating circuit.
2 pin functions, internal structure and principle of work
2.1 pin functions
MAX3737 uses 32-pin the QFN seal form, its pin arrangement as shown in Figure 1. Various pins function is as follows:
GND (1,10,15,16): Ground terminal;
TX DISABLE (2): The laser tube output control terminal voltage input, the low level is effective;
VCC (3,6,11,18,23): 3.3V power source port;
IN (4), IN- (5): Respectively is the data signal forward and reverse input end;
PC MON (7): Monitoring 煼 stupid. 牴 erroneous manages the electric current monitoring end extremely. This end may produce through the external connection resistance with feeds back the light diode electric current to be proportional, and take the place as the reference point reference voltage;
BC MON (8): The laser tube bias electric current monitoring end, may produce through the external connection resistance with the bias electric current is proportional, and take the place as the reference point reference voltage;
MC MON (9): The modulating current monitoring end, may produce through the external connection resistance with the modulating current is proportional, and take the place as the reference point reference voltage;
TX FAULT(12): Transmission expiration instruction end;
SHUTDOWN (13): Closure driver out-port;
VBS (14): Laser tube bias voltage instruction end;
BIAS (17): Laser tube bias electric current out-port;
OUT- (19,20): Reverse out-port, when application should these two foot interconnection;
OUT (21,22): Forward out-port, when application should these two foot interconnection;
MD (24): Monitoring 煼 stupid. 牴 erroneous manages the electric current input end extremely, when the application and monitoring (feedback) light diode’s positive electrode connection;
VMD (25): Monitoring 煼 stupid. 牴 erroneous manages the voltage instruction end extremely;
APCFILT1 (26), APCFILT2 (27): APC ring circuit main pole establishment end, when application should indirect of the capacitor in these two feet (CAPC);
APCSET (28): Average luminous power establishment end;
MODSET (29): Modulating current part establishment end;
MODBCOMP (30): Bias electric current to modulating current compensation coefficient establishment end;
TH TEMP (31): Temperature compensation electric circuit threshold value establishment end;
MODTCOMP (32): Temperature compensation coefficient establishment end.
Figure 2
2.2 internal structures and work process
MAX3737 internal structure as shown in Figure 2, the internal circuit mainly includes the high speed tone circuit, the extinction ratio control circuit and the security logical control and the indicating circuit. And, the high speed tone circuit including the input level and the output stage two parts, mainly by the input block, the data channel and the high speed difference is composed to the electric circuit, its function is carries on the modulation to the input signal, and provides the drive signal which for the exterior laser tube needs. Extinction ratio control circuit including three parts: Automatic power control 煟 grain of school vast 牭 pian two⒆The far modulation control 煟 grain stops the vast 牭 pian pan-buttocks dew alkyne hook サ pian to punish its leading role is forms the reaction control electric circuit with the monitoring light diode, simultaneously through maintains the extinction ratio to the bias electric current and modulating current’s dynamic control adjustment to be constant; The security logical control and the indicating circuit are mainly provide the safety control for the driver normal work, carries on the monitoring to the driver active status, simultaneously supplies driver’s each kind of active status and the expiration information.
MAX3737 uses the APC working pattern, when MAX3737 normal work, the data from in- end and in end input, after the input block electric circuit and data channel processing, then outputs through the control difference exchange regulator realizes the modulation, after the modulation signal actuates the external connection laser tube from the OUT- end and the OUT end output; When output change, the feedback signal from the MD end input, will then adjust the modulating current and the bias electric current change through the extinction ratio control circuit by the automatic maintenance output stability; When the temperature change surpasses the threshold value, the temperature compensation electric circuit automatic control modulating current holding power will be stable; When the electric circuit occurs the breakdown and other contingencies occurs, the security logical control and the indicating circuit through the SHUTDOWN end output control signal will close the laser tube to output, simultaneously end outputs the warning signal by TX the FAULT.
3 using design
MAX3737 when application requests the electric circuit which the user independently designs to be few, user’s main project work is the choice appropriate laser tube as well as each kind of related electric current design. MAX3737 model application electric circuit as shown in Figure 3, in the chart the sign primary device parameter value is the typical value, the sign primary device has not needed to determine in the concrete design, the laser tube selects the d-c coupling method. The following union model introduces MAX3737 using the electric circuit the application design process.
3.1 laser tube’s choice
The user is using the MAX3737 design photoemission machine-hour, the first choice is the basis physical demand choice appropriate laser tube. In the ordinary circumstances, the luminous efficiency power describes with the average luminous power and the extinction ratio, the user may according to the luminous efficiency power determine that needs the laser tube’s output average power and the extinction ratio, and should in satisfy the output under the premise, causes extinction ratio big somewhat as far as possible. After output and extinction ratio determination, simultaneously acts according to these parameters to choose satisfies the condition the laser tube.
3.2 modulating current IMOD design
After laser tube designation, the user may act according to Table 1 the relationship to infer modulating current IMOD the formula. Specifically as follows:
IMOD=2PAVG (re-1)/η (re 1);
In the formula, various parameters’ physics significance see Table 1 to arrange in order. In fact, the modulating current is by the fixed modulating current (IMODS), the bias compensation modulating current (KIBIAS) and the temperature compensation modulating current (IMODT) three parts is composed.
(1) fixed modulating current (IMODS)
The fixed modulating current is in the ideal working condition (temperature is invariable and output constant) under the driver needs modulating current. This electric current may determined by the MAX3737 internal circuit and the MODSET end external connection resistance. Therefore, should first act according to the fixed modulating current which the actual request determination needs (IMODS), then determined again the MODSET end the external connection resistance (RMODSET) the value 熅 warns li:
IMODS=268VREF/RMODSET
And, VREF is the MAX3737 internal reference voltage, in ordinary circumstances’ typical value is 1.3V.
(2) bias compensation modulating current (K IBIAS)
The bias compensation modulating current is, its function size which causes by the bias electric current change by compensation factor K determined, but the K value size determined by the MODBCOMP end’s external connection resistance. In the application, may according to the bias electric current and the modulating current change determined that appropriate compensation factor K, then determines the MODBCOMP end again according to the K value the external connection resistance (RMODBCOMP) the value.
The determination compensation factor K formula is:
K=△IMOD/△IBIAS= (IMOD2-IMOD1)/(IBIAS2-IBIAS1);
But the K value and the RMODBCOMP relations are:
K= [1700/(1000 RMODBCOMP)] ±10%
(3) temperature compensation modulating current (IMODT)
The temperature compensation modulating current is generally surpasses by the temperature, its function which the threshold value temperature causes is the compensation temperature change to the modulating current influence, when T>TTH, temperature compensation modulating current (IMODT) the formula is:
IMODT=TC (T-TTH);
And, TTH is the temperature threshold value, its value may (RTH_TEMP) determine by the TH_TEMP end external connection resistance; TC is the temperature compensation coefficient, its value (RMODTCOMP) determined by the MODTCOMP end’s external connection resistance.
When application, should act according to the actual situation determination appropriate temperature threshold value and the temperature compensation coefficient, then acts according to the following formula to determine RTH_TEMP and RMODTCOMP: .
TTH=-70℃ [1.45MΩ/(9.2kΩ RTH-TEMP)] ℃±10%;
3.3 monitoring light diode feedback electric current IMD design
After laser tube designation, transfer ratio PMON then determined, when design may see also Table 1 the parameter establishment formula. After user determination average luminous power, then determines IMD according to formula PAVG=IMD/PMON the theoretical value.
In MAX3737, feedback electric current IMD may by the APCSET end external connection resistance hypothesis, therefore the IMD design’s essence is determines APCSET the end external connection resistance RAPCSET. The user may determine the RAPCSET value according to the equation below.
IMD=VREF/(2RAPCSET)
After RAPCSET determines, provides actually feedback electric current IMD has determined, like this, the APC electric circuit from will move adjustment bias electric current IBIAS according to the IMD change, will thus maintain the average luminous power the stability.
Figure 3
3.4 APC ring circuit filter electric capacity’s design
In the APC electric circuit, the filter electric capacity CAPC function retards the APC electric circuit’s response time, reduces the low-frequency signal disturbance. The filter electric capacity CAPC value may determine by low frequency cut-off frequency f3DB that the user may first according to request determination low frequency cut-off frequency f3DB, then determine CAPC according to the equation below the value.
CAPC (μF) ≈η68ρMON/f3DB (kHz)
For the filtration high frequency noise, must meet in the MD end pulls electric capacity CMD to arrive, in the ordinary circumstances, gets down pulls electric capacity CMD the value approximately for the filter electric capacity CAPC value 1/4.
3.5 matters needing attention
In the design process, to cause the electric circuit normal work, must have certain condition limit to each kind of electric current. If must the modulating current not to be bigger than 60mA, MAX3737 and the external connection laser tube may select the d-c coupling method; If the modulating current is bigger than 60mA, should select the alternating current coupling method. Which coupled mode no matter uses, in out-port OUT , each kind of electric current should satisfy the following request:
(1) regarding d-c coupling
VOUT =VCC-VDIODE-IMOD (RD RL) - IBIASRL≥0.7V
In the formula, VDIODE is light emitting diode’s bias terminal voltage, the typical value is 1.2V; RL is light emitting diode’s bias end resistance, the typical value is 5Ω; RD is the serial matched resistance, typical value for 20Ω.
(2) regarding alternating current coupling
VOUT =VCC-IMOD (RD RL) /2≥0.75V
In addition, because MAX3737 is the high frequency product, the electric circuit layout is very big to its influence, when circuit design, must use the performance quite superior high frequency layout technology, simultaneously, the user should use has the public earth level multi-layered circuit wafer to reduce the electromagnetic interference and the intermodulation distortion; The circuit wafer should use the low loss the medium material, reduces the energy loss; The data input end lead wire and the modulation out-port lead wire should use the impedance controllable transmission line, like this 熆 the tree top park prize surprised pian returns kun, reduces the energy loss and reduces the disturbance.
4 concluding remark
The MAX3737 laser driver has the merit which the similar products are unable to compare, mainly lies in it to have three control circuits (the APC electric circuit, the AMC electric circuit and temperature compensation electric circuit), can thus maintain throughout the extinction ratio constant 熗 time also suits many kinds of transmission speeds, therefore 煾 the appearance has the widespread application prospect in the fiber optic communications.
