The driving circuit of max3867 laser diode and its

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Max3867 laser diode driving circuit and its application

1 Overview max3867 is a single power supply, high-speed laser diode drive circuit, with a transmission rate of 2.5Gbps. It is especially suitable for sdh/sonet systems, duplexers, and 2.5Gbps optical transmission systems where the cobalt proportion in the digital hand motor pool is 60%

now, let's introduce the automatic power control (APC) closed-loop circuit in max3867 to compensate for the change of optical output power caused by temperature change and chip aging and maintain the constant output power of laser diode; In addition, there are other auxiliary functions, such as enabling control, soft start, APC failure monitoring, etc., so the application is very flexible and convenient. 2. main performance index 2.1 limit parameter

power supply voltage: - 0.5V ~ + 7.0v

bias current: - 20mA ~+ 150mA

output current: - 20mA ~ + 100mA

continuous power consumption: 1354mw (ta=+ 85 ℃)

storage temperature range: - 65 ℃ ~ + 165 ℃

operating junction temperature range: - 55 ℃ ~ + 150 ℃

pin welding temperature: +300 ℃ (10 seconds)

2.2 electrical performance parameters

max3867 main electrical performance parameters are listed in Table 1. 3. packaging form and pin function max3867 is packaged with 48 pin square patch type (TQFP) plastic, and its pin functions are listed in Table 2. 4. basic working principle max3867 laser diode driving circuit is composed of high-speed modulation driving circuit and automatic power control (APC) circuit, as shown in Figure 1. The high-speed modulation drive circuit consists of a high-speed differential stage and a preset modulation current source. When DC coupling is adopted, the parasitic inductance will produce an instantaneous high voltage impulse. Therefore, the modulation output of the max3867 is coupled to the negative pole of the laser diode LD through AC; At the same time, the external pull-up inductor ensures the DC bias of the laser diode LD, so that the forward voltage drop of the laser diode is effectively isolated from the output circuit to achieve large swing output. In the automatic power control (APC) circuit, the optical output of laser diode LD is converted into corresponding photocurrent by monitoring photocell FD, and the bias current of laser diode LD is controlled by APC loop feedback, so as to maintain the constant optical output power. The constant power value is set by the external resistance rapcset, and the time constant of the APC loop is determined by the external capacitance CAPC. 5. other auxiliary functions 5.1 APC open-loop operation

at this time, APC does not work, and the working current of laser diode LD is set by the external resistance rmodset and rbiasmax

5.2 data input locking

in order to improve the reliability and stability of data transmission, the data input locking circuit adopts different forms of clock signals, which are selected by the level of latch terminal. This terminal is at high level, and the input data is synchronized with the rising edge of clock CLK +; This terminal is at low level, and the input data is directly sent to the output stage. If the locking function is not required, the time CLK + can be connected to the power end, and CLK - can be suspended

5.3 enable control

the enable end is at low level, the bias current and modulation current are turned off, the laser diode LD has no output, the enable end is at high level, and the max3867 normally drives the laser diode LD

5.4 soft start

the function of the soft start circuit is to set the conduction delay time of the laser diode LD to avoid damage of the laser diode LD due to instantaneous impact. The delay time is determined by the external capacitance cslwstrt

5.5 APC failure monitoring

when the APC loop fails, the failure detection circuit can output a prompt signal falt

5.6 short circuit protection

when there is a short circuit in the modulation, bias and internal control current source, the short circuit protection circuit will immediately turn off the bias and modulation output circuit, so as to prevent the laser diode LD from burning due to overcurrent. 6. when using design 6.1 design basis

to design a laser transmitter, the optical output is expressed by the average power. The relationship of other relevant quantities is as follows:

average power pave= (P0 + P1)/2

extinction rate re=p1/p0

optical power amplitude pp-p=2pave (RE-1)/(re + 1)

laser efficiency η= Pp-p/imod

laser monitoring current change ρ Mon=imd/pay

where P1 and P0 are the peak and valley values of optical output power respectively, and IMOD is the modulation current. IMD is used to monitor photocell current

6.2 refer to the preset

(1) the preset

root measurement and control software of modulation current IMOD and bias current ibiasmax can expand the function according to user requirements. According to the relationship curve between IMOD and rmodset, ibiasmax and rbiasmax (as shown in Figure 2 and figure 3 respectively), find out rmodset and rbiasmax according to the required IMOD and ibiasmax values

(2) APC constant power value preset

according to the automatic constant power (reflected by the monitoring photocurrent IMD of the monitoring photocell FD) and the automatic constant work

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