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19-0323; Rev 4; 8/97
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver
_______________General Description
The MAX3261 is a complete, easy-to-program, single +5V-powered, 1.25Gbps laser diode driver with complementary enable inputs and automatic power control (APC). The MAX3261 accepts differential PECL inputs and provides complementary output currents. A temperature-stabilized reference voltage is provided to simplify laser current programming. This allows modulation current to be programmed up to 30mA and bias current to be programmed up to 60mA with two external resistors. Complementary enable inputs allow the MAX3261 to interface with open-fiber-control architecture--a feature not found in other 1.25Gbps laser diode drivers. An APC circuit is provided to maintain constant laser power in transmitters that use a monitor photodiode. Only two external components are required to implement the APC function. The MAX3261's fully integrated feature set includes a TTL-compatible laser failure indicator and a programmable slow-start circuit to prevent laser damage. The slow-start is preset to 50ns and can be extended by adding an external capacitor.
____________________________Features
o Rise Times Less than 250ps o Differential PECL Inputs o Single +5V Supply o Automatic Power Control o Temperature-Compensated Reference Voltage o Complementary Enable Inputs
MAX3261
______________Ordering Information
PART MAX3261CCJ MAX3261ECJ MAX3261E/D TEMP. RANGE 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 32 TQFP 32 TQFP Dice*
*Dice are designed to operate over a -40C to +140C junction temperature (Tj) range. Tested and guaranteed at Tj = +25C.
________________________Applications
Laser Diode Transmitters 531Mbps and 1062Mbps Fibre Channel 622Mbps SDH/SONET Gigabit Ethernet
__________Typical Operating Circuit
+5V
+5V
VCCA VCCB VIN+ PECL INPUTS VINGND A GND B ENB+ TO OPEN FIBER CONTROL IBIASOUT IMODSET IBIASSET IBIASFB OSADJ VREF1 VCCB ENB+ OUT+
+5V
__________________Pin Configuration
VCCA VCCA VCCA GNDA OUT+ GNDA OUTGNDA
TOP VIEW
PHOTODIODE
LASER
MAX3261
IPIN
32 31 30 29 28 27 26 25
IBIASOUT ENBSLWSTRT VREF1 VREF2 OUTFAILOUT IBIASFB ZO = 25 MICROSTRIP 2.7k +5V
GNDA GNDA IPIN SLWSTRT GNDB VREF2 IPINSET FAILOUT
1 2 3 4 5 6 7 8
MAX3261
24 23 22 21 20 19 18 17
IBIASSET IMODSET OSADJ IPINSET
________________________________________________________________ Maxim Integrated Products
GNDB VIN+ GNDB VINGNDB VCCB VCCB ENB-
9 10 11 12 13 14 15 16
TQFP
1
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Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
ABSOLUTE MAXIMUM RATINGS
Terminal Voltage (with respect to GND) Supply Voltages (VCCA, VCCB) ...............................-0.3V to 6V VIN+, VIN-, FAILOUT...............................................0V to VCC_ OUT+, OUT-, IBIASOUT .......................................1.5V to VCC_ ENB+, ENB- .......................VCC_ or 5.5V, whichever is smaller Differential Input Voltage (| VIN+ - VIN- |) ...........................3.8V Input Current IBIASOUT ............................................................0mA to 75mA OUT+, OUT- ........................................................0mA to 40mA IBIASSET ........................................................0mA to 1.875mA IMODSET...............................................................0mA to 2mA IPIN, IPINSET, OSADJ...........................................0mA to 2mA FAILOUT..............................................................0mA to 10mA IBIASFB................................................................-2mA to 2mA Output Current VREF1, VREF2.....................................................0mA to 20mA SLWSTRT ..............................................................0mA to 5mA Continuous Power Dissipation (TA = +70C) TQFP (derate 10.2mW/C above +70C)......................816mW Operating Temperature Ranges MAX3261CCJ ......................................................0C to +70C MAX3261ECJ ...................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-55C to +175C Processing Temperature (die) .........................................+400C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(VCC = VCCA = VCCB = +4.75V to +5.25V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) (Note 1) PARAMETER Range of Programmable Laser Bias Current Reference Voltage Available Reference Current Supply Current PECL Input High PECL Input Low TTL Input High TTL Input Low FAILOUT Output High FAILOUT Output Low SYMBOL IBIAS VREF IREF IVCC VIH VIL VIH VIL VOH VOL Loaded with 2.7k pull-up resistor to VCC Loaded with 2.7k pull-up resistor to VCC 4.5 0.5 2 0.8 (Note 2) VCC - 1.165 VCC - 1.475 TA = +25C 3.15 3.3 12 50 CONDITIONS MIN TYP MAX 60 3.45 UNITS mA V mA mA V V V V V V
Note 1: Dice are tested at TA = +25C. Note 2: IVCC = IVCCA + IVCCB, IBIAS = 60mA, IMOD = 30mA, and IPIN = 140 A.
AC ELECTRICAL CHARACTERISTICS
(VCC = VCCA = VCCB = +4.75V to +5.25V, RLOAD (at OUT+ and OUT-) = 25 connected to VCC, TA = -40C to +85C, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) (Note 3) PARAMETER Range of Programmable Modulation Current Modulation-Current Rise and Fall Time Aberrations, Rising and Falling Edge Modulation-Current PulseWidth Distortion PWD SYMBOL IMOD tR, tF CONDITIONS Minimum differential input swing is 1100mVp-p (Note 4) IBIAS = 25mA, IMOD = 12mA, 4ns unit interval; measured from 10% to 90%. IMOD = 12mA, TA = +25C MAX3261E/D MAX3261ECJ MIN TYP MAX 30 250 10 15 80 UNITS mA ps % ps
IBIAS = 25mA, IMOD = 12mA, 4ns unit interval
Note 3: AC characteristics are guaranteed by design and characterization. Note 4: An 1100mVp-p differential is equivalent to complementary 550mVp-p signals on VIN+ and VIN-. 2 _______________________________________________________________________________________
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver
__________________________________________Typical Operating Characteristics
(MAX3261E/D, load at OUT+ and OUT- = 25, VCC = VCCA = VCCB = +5V, TA = +25C, unless otherwise noted.)
EYE DIAGRAM (622Mbps, LOAD = 25, NOT FILTERED)
MAX3261-1
MAX3261
EYE DIAGRAM (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)*
MAX3261-2
EYE DIAGRAM (1062Mbps, LOAD = 25, NOT FILTERED)
MAX3261-3
250.3mV
265mV
250.3mV
50mV/div
30mV/div
-250mV 38.23ns 200ps/div 40.23ns
-35mV 38.14ns 200ps/div 40.14ns
50mV/div -250mV 38.13ns 117ps/div 39.3ns
EYE DIAGRAM (1062Mbps, LOAD AT OUT- = 1300nm LASER WITH 800MHz BESSEL FILTER)*
MAX3261-4
MAX3261CCJ EYE DIAGRAM (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)*
MAX3261-5
MAX3261CCJ EYE DIAGRAM (1062Mbps, LOAD AT OUT- = 1300nm LASER WITH 800MHz BESSEL FILTER)*
MAX3261-6
450mV
265mV
513.7mV
50mV/div
30mV/div
-50mV 38.74ns 117ps/div 39.91ns
-35mV 38.15ns 200ps/div 40.15ns
58mV/div -66.2mV 37.78ns 117ps/div 38.95ns
RBIASSET vs. BIAS CURRENT
MAX3261-07
RMODSET vs. MODULATION CURRENT
MAX3261-08
RPINSET vs. MONITOR CURRENT
MAX3261-09
8 7 6 RBIASSET (k) 5 4 3 2 1 0 0 20 IBIAS (mA) * LASER = EPITAXX EDL 1300RFC TO-STYLE HEADER 40
12 10 RMODSET (k) 8 6 4
DIFFERENTIAL INPUT SWING = 1100 mVp-p
1,000,000
100,000 RPINSET () 0 5 10 15 20 25 30
10,000
1000 2 0 60 MODULATION CURRENT (mAp-p) 100 0 500 MONITOR CURRENT (A) 1000
_______________________________________________________________________________________
3
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
____________________________Typical Operating Characteristics (continued)
(MAX3261E/D, LOAD at OUT+ = OUT- = 25, VCC = VCCA = VCCB = +5V, TA = +25C, unless otherwise noted.)
PERCENT CHANGE IN MODULATION CURRENT vs. TEMPERATURE
MAX3261-10
PERCENT CHANGE IN BIAS CURRENT vs. TEMPERATURE
MAX3261-11
SUPPLY CURRENT vs. TEMPERATURE
48 SUPPLY CURRENT (mA) 46 44 42 40 38 36
MAX3261-12
10 8 % CHANGE (w.r.t. +25C) 6 4 2 0 -2 -4 -6 -8 -10 0 20 40 60
3 APC DISABLED % CHANGE (w.r.t. +25C) 2
50
1
0
-1
-2 80 0 10 20 30 40 50 60 70 80 TEMPERATURE (C) TEMPERATURE (C)
34 0 20 40 60 80 TEMPERATURE (C)
ALLOWABLE ROSADJ vs. MODULATION CURRENT
MAXIMUM MODULATION CURRENT (mAp-p)
MAX3261-13
MAXIMUM MODULATION CURRENT vs. MINIMUM DIFFERENTIAL INPUT SIGNAL AMPLITUDE
35 30 25 20 15 10 5 0 0 400 800 1200 1600 2000 MINIMUM DIFFERENTIAL INPUT SIGNAL AMPLITUDE (mVp-p) RMODSET = 1.2k ROSADJ = 2k
MAX3261-14
12 10 8 6 4 2 0 0 5 10 15 20 25 ALLOWABLE RANGE
40
ALLOWABLE ROSADJ (k)
30
MODULATION CURRENT (mAp-p)
4
_______________________________________________________________________________________
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver
______________________________________________________________Pin Description
PIN 1, 2, 25, 27, 29 3 4 5, 9, 11, 13 6 7 8 10 12 14, 15, 18 16 17 19 20 21 22 23 24 26 28 30, 31, 32 NAME GNDA IPIN SLWSTRT GNDB VREF2 IPINSET FAILOUT VIN+ VINVCCB ENBENB+ VREF1 OSADJ IBIASFB IBIASSET IMODSET IBIASOUT OUTOUT+ VCCA FUNCTION Ground for Bias and Modulation Current Drivers Monitor Photodiode Current Input. Connect IPIN to photodiode's anode. Slow-Start Capacitor Input. Connect capacitor to ground or leave unconnected to set start-up time, tSTARTUP = 25.4k (CSLWSTRT + 2pF). Ground for Voltage Reference and Automatic Power-Control Circuitry Temperature-Compensated Reference Output. VREF2 is internally connected to VREF1. Monitor Photodiode Programming Input. Connect IPINSET to VREF1 or VREF2 through a resistor to set the monitor current when using automatic power control (see Typical Operating Characteristics). Failout Output. Active-low, open-collector TTL output indicates if automatic power-control loop is out of regulation due to insufficient monitor-diode current (when VIPIN is below the 2.6V threshold). Connect FAILOUT to VCC_ through a 2.7k pull-up resistor. Noninverting PECL Data Input Inverting PECL Data Input +5V Supply Voltage for Voltage Reference and Automatic Power-Control Circuitry. Connect VCCB to the same potential as VCCA, but provide separate bypassing for VCCA and VCCB. Inverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low. Noninverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low. Temperature-Compensated Reference Output. VREF1 is internally connected to VREF2. Overshoot-Adjust Input. Connect to internal voltage reference through a resistor to adjust the overshoot of the modulation output signal (see Typical Operating Characteristics). Bias-Feedback Current Output. Output from automatic power-control circuit. Connect to IBIASSET when using APC. Laser Bias Current-Programming Input. Connect to internal voltage reference through a resistor to set bias current (see Typical Operating Characteristics). IIBIASOUT = 40 x (IIBIASSET + IIBIASFB). Laser Modulation Current-Programming Input. Connect to internal voltage reference through a resistor to set modulation current (see Typical Operating Characteristics). IMOD = 20 x IIMODSET. Laser Bias Current Output. Connect to laser cathode through an R-L compensation network (see the Bias Network Compensation section). Modulation Output. When VIN+ is high and VIN- is low, OUT- sinks IMOD. Modulation Output. When VIN+ is low and VIN- is high, OUT+ sinks IMOD. +5V Supply Voltage for Bias and Modulation Current Drivers. Connect VCCA to the same potential as VCCB, but provide separate bypassing for VCCA and VCCB.
MAX3261
_______________________________________________________________________________________
5
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
_______________Detailed Description
The MAX3261 laser driver has three main sections: a reference generator with temperature compensation, a laser bias block with automatic power control, and a high-speed modulation driver. The reference generator provides temperature-compensated biasing and a voltage-reference output. The voltage reference is used to program the current levels of the high-speed modulation driver, laser diode, and PIN (p+, intrinsic, n-) monitor diode. The laser bias block sets the bias current in the laser diode and maintains it above the threshold current. A current-controlled current source (current mirror) programs the bias, with IBIASSET as the input. The mirror's gain is approximately 40. Keep the output voltage of the bias stage above 2.2V to prevent saturation. The modulation driver consists of a high-speed input buffer and a common-emitter differential output stage. The modulation current mirror sets the laser modulation current in the output stage. This current is switched between the OUT+ and OUT- ports of the laser driver. The modulation
VCC
MAX3261
VIN+
OUT+
LASER VINOUTVCCA 20 x IIMODSET VCCB IBIASOUT GNDA GNDB 40 x IIBIASSET +2.6V FAILOUT BIAS COMPENSATION PHOTODIODE
ENB+
COMPARATOR MAIN BIAS GENERATOR IPIN LOOPSTABILITY CAPACITOR 1000pF
ENB-
SLWSTRT BANDGAP REFERENCE
TRANSCONDUCTANCE AMPLIFIER
+3V
1 X IIPINSET
IMODSET
IBIASSET
VREF1 OR VREF2
IBIASFB
IPINSET RPINSET
IOSADJ ROSADJ
RMODSET
Figure 1. Functional Diagram
6 _______________________________________________________________________________________
RBIASSET
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
OUTPUTS VCC
MAX3261
280 280 ENB+
INPUTS
DATA OUT (LOAD = 1300nm LASER AT OUT-) 9 9 2s/div 400 2(IIOSADJ) 2(IIOSADJ) IIMODSET
Figure 3. Enable/Disable Operation
Automatic Power Control
INPUT BUFFER OUTPUT STAGE
Figure 2. MAX3261 Modulation Driver (Simplified)
The automatic power control (APC) feature allows an optical transmitter to maintain constant power, despite changes in laser efficiency with temperature or age. The APC feature requires the use of a monitor photodiode. The APC circuit incorporates the laser diode, the monitor photodiode, the PIN set current mirror, a transconductance amplifier, the bias set current mirror, and the laser fail comparator (Figure 1). Light produced by the laser diode generates an average current in the monitor photodiode. This current flows into the MAX3261's IPIN input. The PIN set current mirror draws current away from the IPIN node. When the current into the IPIN node equals the current drawn away by IPINSET, the node voltage is set by the 3/5 x VCC reference of the transconductance amplifier. When the monitor current exceeds IPINSET, the IPIN node voltage will be forced higher. If the monitor current decreases, the IPIN node voltage is decreased. In either case, the voltage change is amplified by the transconductance amplifier, and results in a feedback current at the IBIASFB node. Under normal APC operation, IBIASFB is summed with IBIASSET, and the laser bias level is adjusted to maintain constant output power. This feedback process continues until the monitor-diode current equals IPINSET. If the monitor-diode current is sufficiently less than IPINSET (i.e., the laser stops functioning), the voltage on
current mirror has a gain of approximately 20. Keep the voltages at OUT+ and OUT- above 2.2V to prevent saturation. The overshoot mirror sets the bias in the input buffer stage (Figure 2). Reducing this current slows the input stage and reduces overshoot in the modulation signal. At the same time, the peak-to-peak output swing of the input buffer stage is reduced. Careful design must be used to ensure that the buffer stage can switch the output stage completely. The input swing required to completely switch the output stage depends on both R OSADJ and the modulation current. See Allowable ROSADJ Range vs. Modulation Current and Modulation Current vs. Differential Input Signal graphs in the Typical Operating Characteristics. Failure to ensure that the output stage switches completely results in a loss of modulation current (and extinction ratio). In addition, if the modulation port does not switch completely off, the modulation current will contribute to the bias current, and may complicate module assembly.
_______________________________________________________________________________________
7
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver
the IPIN node will drop below 2.6V. This will trigger the failout comparator, which provides a TTL signal indicating laser failure. The FAILOUT output asserts only if the monitor-diode current is low, not in the reverse situation where the monitor current exceeds IPINSET. FAILOUT is an open-collector output that requires an external pull-up resistor of 2.7k VCC. to The transconductance amplifier can source or sink currents up to approximately 1mA. Since the laser bias generator has a gain of approximately 40, the APC function has a limit of approximately 40mA (up or down) from the initial set point. To take full advantage of this adjustment range, it may be prudent to program the laser bias current slightly higher than required for normal operation. However, do not exceed the IBIASOUT absolute maximum rating of 75mA. To maintain APC loop stability, a 1000pF bypass capacitor may be required across the photodiode. If the APC function is not used, leave IBIASFB unconnected.
MAX3261
__________________Design Procedure
Interfacing Suggestions
Use high-frequency design techniques for the board layout of the MAX3261 laser driver. High-speed interfaces often require fixed-impedance transmission lines (Figure 5). Adding some damping resistance in series with the laser raises the load impedance, making the transmission line more realizable, and it also helps reduce power consumption (see the section Reducing Power Consumption). Minimize any series inductance to the laser, and place a bypass capacitor as close to the laser's anode as possible. Power connections labeled VCCA are used to supply the laser modulation and laser bias circuits. VCCB connections supply the bias-generator and automatic-power control circuits. For optimum operation, isolate these supplies from each other by independent bypass filtering. VCCA, VCCB, GNDA, and GNDB all have multiple pins. Connect all pins to optimize the MAX3261's highfrequency performance. Ground connections between signal lines (VIN+, VIN-, OUT+, OUT-) improve the quality of the signal path by reducing the impedance of the interconnect. Multiple connections, in general, reduce inductance in the signal path and improve the highspeed signal quality. GND pins should be tied to the ground plane with short runs and multiple vias. Avoid ground loops, since they are a source of high-frequency interference. The MAX3261 data inputs accept PECL input signals, which require 50termination to (VCC - 2V). Figure 4 shows alternative termination techniques. When a termination voltage is not available, use the Theveninequivalent termination. When interfacing with a non-PECL signal source, use one of the other alternative termination methods shown in Figure 4.
Enable Inputs
The MAX3261 provides complementary enable inputs (ENB+, ENB-) for interfacing with open-fiber-control architecture. The laser is disabled by reducing the reference voltage outputs (VREF1, VREF2). Only one logic state will enable laser operation (Table 1). With a 1000pF stability capacitor, the MAX3261 modulation and bias can be enabled and disabled within 5s (Figure 3). This timing satisfies the requirements of the Open Fiber Control system used in Fibre Channel networks.
Temperature Considerations
The MAX3261 output currents are programmed by current mirrors. These mirrors each have a 2VBE temperature coefficient. The reference voltage (VREF) is adjusted 2VBE so these changes largely cancel, resulting in output currents that are very stable with respect to temperature (see Typical Operating Characteristics).
Bias Network Compensation
When driving the laser diode with transmission lines, it is important to maintain a constant load impedance in order to minimize aberrations due to reflections. The inductive nature of laser packages will cause the laser impedance to increase with frequency, and the parasitic capacitance of the laser driver bias output (IBIASOUT) has some loading effects at high frequency. Of these two effects, the loading due to the laser lead inductance dominates. Impedance variation must be compensated for high-frequency operation. One possible approach is to use a shunt R-C network in parallel with the laser diode to compensate for the laser impedance (Figures 5 and 6). Add an R-L circuit in series with the bias output to compensate for the IBIASOUT capacitance (Figures 5 and 7).
Wire Bonding Die
For reliable operation, the MAX3261 has gold metallization. Make connections to the die with gold wire only, using ball bonding techniques. Wedge bonding is not recommended. Pad size is 4mils.
Table 1. MAX3261 Truth Table
ENB0 0 1 1 ENB+ 0 1 0 1 OUTPUT CURRENTS DISABLED ENABLED DISABLED DISABLED
8
_______________________________________________________________________________________
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
5V PECL SIGNAL SOURCE 5V 82 82 VIN+ a) THEVENIN-EQUIVALENT TERMINATION 120
MAX3261
VIN120
NON-PECL SIGNAL SOURCE 50 50 b) DIFFERENTIAL NON-PECL TERMINATION 50 50 1.8k 5V 680
VIN+
MAX3261
VIN-
NON-PECL SIGNAL SOURCE VIN+ 50 68 c) SINGLE-ENDED NON-PECL TERMINATION 5V 680 VIN1.8k 5V ECL SIGNAL SOURCE OV 1.3k 1.3k VIN+ d) ECL TERMINATION -2V THIS SYMBOL REPRESENTS A TRANSMISSION LINE WITH CHARACTERISTIC IMPEDANCE Zo = 50. VIN50 -2V 3.6k 50 3.6k 5V 180
MAX3261
MAX3261
Figure 4. Alternative PECL Data-Input Terminations
_______________________________________________________________________________________ 9
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
Reducing Power Consumption
The laser driver typically consumes 40mA of current for internal functions. Typical load currents, such as 12mA of modulation current and 20mA of bias current, bring the total current requirement to 72mA. If this were dissipated entirely in the laser driver, it would generate 360mW of heat. Fortunately, a substantial portion of this power is dissipated across the laser diode. A typical laser diode will drop approximately 1.6V when forward biased. This leaves 3.4V at the MAX3261's OUT- terminal. It is safe to reduce the output terminal voltage even further with a series damping resistor. Terminal voltage levels down to 2.2V can be used without degrading the laser driver's high-frequency performance. Power dissipation can be further reduced by adding a series resistor on the laser driver's OUT+ side. Select the series resistor so the OUT+ terminal voltage does not drop below 2.2V with the maximum modulation current. equations results in P1 = (2 x PAVE x Er) / (Er + 1) and P0 = (2 x PAVE) / (Er + 1). In this example, P1 = 1.6mW and P0 = 0.4mW. The optical modulation is 1.2mW. The modulation current required to produce this output is 1.2mW / = (1.2mW) / (0.1mA/mW) = 12mA. The Typical Operating Characteristics show that RMODSET = 3.9k yields the desired modulation current. 3) Determine the value of ROSADJ: Using the Allowable ROSADJ vs. Modulation Current graph in the Typical Operating Characteristics, a 5.6k resistor is chosen for 12mA of modulation current. The maximum ROSADJ values given in the graph minimize aberrations in the waveform and ensure that the driver stage operates fully limited. 4) Determine the value of RBIASSET: The automatic power control circuit can adjust the bias current 40mA from the initial setpoint. This feature makes the laser driver circuit reasonably insensitive to variations of laser threshold from lot to lot. The bias setting can be determined using one of two methods: a) Set the bias at the laser threshold. b) Set the bias at the midpoint of the highest and lowest expected threshold values. Method A is straightforward. In the second method, it is assumed that the laser threshold will increase with age. The lowest threshold current occurs at 0C, when the laser is new. The highest threshold current occurs at +70C, at the end of the product's life. Assume the laser is near the end of life when its threshold reaches two-times its original value. Lowest Bias Current: ITH + TH = 20mA + (0.35mA/C)(-25C) = 11.25mA I Highest Bias Current: 2 x ITH + TH = 40mA + (0.35mA/C)(+45C) = 55.8mA I In this case, set the initial bias value to 34mA (which is the midpoint of the two extremes). The adjustment range of the MAX3261 maintains the average laser power at either extreme. The Typical Operating Characteristics show that RBIASSET = 1.8k delivers the required bias current.
__________Applications Information
Programming the MAX3261 Laser Driver
Programming the MAX3261 is best explained by an example. Assume the following laser diode characteristics: Wavelength 780nm Threshold Current ITH 20mA at +25C (+0.35mA/C temperature variation)
Monitor Responsivity 0.1A/W (monitor current / mon average optical power into the fiber) Modulation Efficiency 0.1mW/mA (worst case) Now assume the communications system has the following requirements: Average Power PAVE 0dBm (1mW) Extinction Ratio Er 6dB (Er = 4) Temperature Range Tr 0C to +70C 1) Determine the value of IPINSET: The desired monitor-diode current is (PAVE)( ) = mon (1mW)(0.1A/W) = 100A. The R PINSET vs. Monitor Current graph in the Typical Operating Characteristics shows that RPINSET should be 18k. 2) Determine RMODSET: The average power is defined as (P1 + P0) / 2, where P1 is the average amplitude of a transmitted "one" and P0 is the average amplitude of a transmitted "zero." The extinction ratio is P1/P0. Combining these
10
______________________________________________________________________________________
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
25 OUT+ 0.01F AS CLOSE TO THE LASER ANODE AS POSSIBLE +5V
EYE DIAGRAM WITH R-C COMPENSATION (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)*
MAX3261-FG06
265mV
1000pF
LASER PHOTODIODE SERIES R-L 47H
MAX3261
IPIN
0.01F SHUNT RC
100pF
IBIASOUT 200
18 OUTZO = 25 MICROSTRIP
AS CLOSE TO THE LASER CATHODE AS POSSIBLE
-35mV 200ps/div *EPITAXX EDL 1300 RFC, TO-STYLE HEADER
Figure 5. Typical Laser Interface with Bias Compensation
Figure 6. Eye Diagram with R-C Compensation (LOAD at OUT- = 1300nm Laser)
30mV/div
25
Laser Safety and IEC 825
EYE DIAGRAM WITH R-C AND R-L COMPENSATION (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)*
MAX3261-FG07
265mV
Using the MAX3261 laser driver alone does not ensure that a transmitter design is compliant with IEC 825. The entire transmitter circuit and component selections must be considered. Each customer must determine the level of fault tolerance required by their application, recognizing that Maxim products are not designed or authorized for use as components in systems intended for surgical implant into the body, for applications intended to support or sustain life, or for any other application where the failure of a Maxim product could create a situation where personal injury or death may occur.
30mV/div -35mV 200ps/div *EPITAXX EDL 1300 RFC, TO-STYLE HEADER
Figure 7. Eye Diagram with R-C and R-L Compensation (LOAD at OUT- = 1300nm Laser)
______________________________________________________________________________________
11
Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261
____________________________________________________________Chip Topography
GNDA GNDA GNDA V CC A V CC A V CC A OUT+ OUT+ OUTOUT-
GNDA GNDA IPIN SLWSTRT GNDB GNDB VREF2 IPINSET FAILOUT N.C.
IBIASOUT IMODSET IBIASSET IBIASFB OSADJ VREF1 V CC B V CC B ENB+ ENB0.080" (2.032mm)
GNDB
GNDB
GNDB
N.C.
VIN-
VIN-
V CC B
0.080" (2.032mm)
TRANSISTOR COUNT: 197 SUBSTRATE CONNECTED TO GNDA AND GNDB
Maxim makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Maxim assume any liability arising out of the application or use of any product or circuit and specifically disclaims any and all liability, without limitation, consequential or incidental damages. "Typical" parameters can and do vary in different applications. All operating parameters, including "typicals" must be validated for each customer application by customer's technical experts. Maxim products are not designed, intended or authorized for use as components in systems intended for surgical implant into the body, or for other applications intended to support or sustain life, or for any other application in which the failure of the Maxim product could create a situation where personal injury or death may occur. Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
V CC B
VIN+
VIN+

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