High-speed laser driver MAX3296 and its applications

    Abstract: MAX3296 is a high-speed laser driver produced by MAXIM Company of the United States. It can be used as an optical transmitter in Gigabit Ethernet, Fiber Channel and ATM LAN in a LAN with a transmission rate of 2.5Gbps. This product has the characteristics of high transmission rate, small modulation current, safe and stable performance. The article introduces the structure, pin definitions and specific applications of MAX3296 in detail, and gives a typical application circuit.

    Keywords: MAX3296 high-speed laser driver laser modulation current APC

1 Main features

MAX3296 is a high-speed laser driver produced by MAXIM Company of the United States. Its main features are as follows:

●Adopt 3.0~5.5V working power supply;

●With optional laser tube working mode (common cathode or common anode);

●Only 30mA laser modulation current;

●Only systematic jitter as low as 7ps;

●Contains current modulation temperature compensation circuit;

●With automatic optical power control circuit (APC);

●With integrated safety circuit;

●With automatic power reset function (POR);

●Adopts 32-pin TQFP package.

2 Pin functions and internal structure

2.1 Pin function

Figure 1 shows the MAX3296 pinout. The functions of each pin of MAX3296 are described as follows:

●FUALT (pin 1), FUALT (pin 3): respectively the reverse and forward default indication terminals;

●NC (2 pins, 16 pins): no connection;

●POR (pin 4): power reset terminal;

●GND (5 feet, 14 feet, 22 feet, 30 feet): ground terminal;

●EN (pin 6), EN (pin 7): control terminal. Only when the EN terminal is connected to high level and the EN terminal is connected to low level, the MAX3296 can work normally, otherwise it will be invalid.

●PORDLY (pin 8): Power reset delay terminal. An external capacitor can extend the power reset time.

    ●FLTDLY (pin 9): Default delay input. For the delay output of FAULT terminal and FAULT terminal, an external capacitor can be used to provide a suitable delay pulse width. If the FLTDLY terminal is connected to ground, then the FAULT terminal will be low level and the FAULT terminal will be high level; but when the FLTDLY terminal is connected to ground, even if the EN terminal is connected to high level, the EN terminal is connected to low level, and the low power supply is on. Even within the working range, the safety circuit of MAX3296 cannot work properly;

●LV (pin 10): low voltage working mode terminal. If it is grounded, the operating voltage range is 4.5V~5.5V; if it is left floating, the operating voltage range is 3.0V~5.5V;

●Vcc (pin 11, pin 25, pin 26, pin 29): working power supply terminal. If the LV terminal is left floating, the value of Vcc will be in the range of 2.65~3V; if the LV terminal is connected to ground, then the Vcc value will be in the range of 3.9V~4.5V;

●IN+ (pin 12), IN- (pin 13): data forward and reverse input terminals;

●REF (pin 15): reference voltage terminal. When the APC voltage acts on the common cathode laser tube, a resistor should be connected between the REF terminal and the MD terminal to determine the power of the laser tube;

●POL (17 pins), POL (18 pins): Laser tube working mode selection terminal. When POL is connected to Vcc and POL is connected to ground, it is a common cathode working mode; when POL is connected and POL is connected to Vcc, it is a common anode working mode;

●MD (20 pins): Connect to the monitoring photodiode terminal;

●MDN (pin 21): laser deviation current monitoring terminal;

●SHDNDRV (pin 23): Turn off the driver output;

●BIASDRV (pin 24): The deviation signal controls the external transistor driver end. External PNP or NPN transistor;

●OUT+ (pin 27), OUT- (pin 28): modulation signal output terminal;

●MODSET (pin 31): modulation current setting terminal. An external resistor can be used to set the modulation circuit at normal temperature;

●TC (pin 32): Modulation current temperature compensation setting terminal. An external resistor can be used to compensate for the effect of temperature changes on the modulation current.

2.2 Internal structure

The interior of MAX3296 mainly includes power reset circuit, safety circuit, deviation signal generator circuit and laser modulation circuit. Its internal principle block diagram is shown in Figure 2.

3 Typical applications

When designing with the MAX3296, the design of the modulation current is extremely important. Its value is determined by the external resistors at the MODSET terminal and TC terminal. The external resistor (RMOD) at the MODSET terminal determines the modulation current value (IMOD) when the temperature is constant. When the laser tube is selected, the IMOD is generally determined. The external resistor (RTC) at the TC terminal can be used to determine the compensation current value when the temperature changes. Therefore, the essence of designing the modulation current is to determine the resistance values ​​of RMOD and RTC.

When the laser tube is selected, its temperature coefficient LT can be calculated according to the following formula:

LT=[(α70-α25)/α25（70℃-25℃）] ×10 6ppm/℃

Here: α is the slope of the straight line determined by the laser tube output power and laser tube current. α25 and α70 represent the α values ​​when the temperature is 25°C and 70°C respectively.

After LT is calculated, the values ​​of RTC and RMOD can be calculated according to the following two formulas:

RTC=0.21/Tempco(iMOD)-25Ω

RMOD=[52Tempco(RTC+250Ω)]/(0.19-48Tempco)-250Ω

Here: Tempco=LT

In addition, when the laser tube is selected, the values ​​of RMOD and RTC can also be determined according to Table 1.

Table 1 RMOD and RTC values ​​under different Tempco

3.2 Typical application circuit

The MAX3296 is a high-speed laser driver. It has the advantages of high transmission rate, small modulation current, safe and reliable performance, etc. Therefore, it can be widely used as a light reflector in optical fiber communications and ATM local area networks.

MAX3296 can be used in three circuit modes, namely common cathode laser tube circuit with photoelectric tube, common cathode laser tube circuit without photoelectric tube and common anode laser tube circuit with photoelectric tube. Figure 3 and Figure 4 are respectively with Common cathode and common anode laser tube circuits with phototubes. The working mode of its laser tube is determined by the connection method of the POL end and POL end of MAX3296.