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| general description t he max3668 is a complete, +3.3v laser driver with automatic power control (apc) circuitry for sdh/sonet applications up to 622mbps. it accepts differential pecl inputs, provides bias and modulation currents, and operates over a temperature range of -40? to +85?. an apc feedback loop is incorporated to maintain a constant average optical power over temperature and lifetime. the wide modulation current range of 5ma to 75ma and bias current of 1ma to 80ma are easy to program, making this product ideal for use in various sdh/sonet applications. the max3668 also provides enable control and a failure- monitor output to indicate when the apc loop is unable to maintain the average optical power. the max3668 is available in a 5mm 32-pin tqfp package as well as in dice. applications 622mbps sdh/sonet access nodes laser driver transmitters section regenerators features ? single +3.3v or +5.0v operation ? 38ma supply current at +3.3v ? programmable modulation current from 5ma to 75ma ? programmable bias current from 1ma to 80ma ? rise/fall time <200ps ? automatic average power control with failure monitor ? complies with ansi, itu, and bellcore sonet/sdh specifications ? enable control max3668 ? +3.3v, 622mbps sdh/sonet laser driver with automatic power control ________________________________________________________________ maxim integrated products 1 data+ biasmax modset apcset capc fail enable gnd md bias out+ 0.1f laser out- c md 1000pf r filt 20? c filt 5pf c d 1.0f r+ 20? r- 6.3? r d 5? ferrite bead v cc data- pecl +3.3v +3.3v 82? 82? 130? 130? 4:1 serializer with clock gen max3693 max3668 t ypical operating circuit 19-4799; rev 4; 2/05 ordering information pin configuration appears at end of data sheet. note: dice are designed to operate over a -40? to +140? junction temperature (tj) range, but are tested and guaranteed at t a = +25?. *contact factory for availability. +denotes lead-free package. ? c0 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package max3668ehj -40 c to +85 c 32 tqfp (5mm x 5mm) max3668ehj+ -40 c to +85 c 32 tqfp (5mm x 5mm) max3668e/d note dice*
max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control 2 _______________________________________________________________________________________ absolute maximum ratings dc electrical characteristics (v cc = +3.14v to +5.5v, t a = -40? to +85?, unless otherwise noted. typical values are at v cc = +3.3v, t a = +25?.) (note 1) stresses beyond those listed under ?bsolute 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. supply voltage, v cc .............................................-0.5v to +7.0v current into bias ............................................-20ma to +150ma current into out+, out- ............................... -20ma to +100ma current into md....................................................-5ma to +5ma voltage at data+, data-, enable, fail .......................................................-0.5v to (v cc + 0.5v) voltage at out+, out- .............................+1.5v to (v cc + 1.5v) voltage at modset, apcset, biasmax, capc............................................................... -0.5v to +3.0v voltage at bias .........................................+1.0v to (v cc + 0.5v) continuous power dissipation (t a = +85?) 32-pin tqfp (derate 14.3mw/? above +85?)........ 929mw operating junction temperature range ...........-55? to +150? processing temperature (die).........................................+400? storage temperature range ........................... -65? to +165? lead temperature (soldering, 10s) ................................+300? enable = low (note 3) v bias = v cc - 1.6v (note 2) sinking 100? sourcing 50? (note 4) apc open loop apc open loop figure 1 pecl compatible conditions v 0.1 0.44 v ol ttl output low voltage (fail) v 2.4 v cc - 0.3 v cc v oh ttl output high voltage (fail) v 0.8 v il ttl input low voltage v 2.0 v ih ttl input high voltage v 0.8 v md monitor diode input voltage (md pin) ? 100 bias off current ma 18 0 i bias ma 38 60 supply current bias current range ? 18 1000 i md dc monitor diode current % -15 15 monitor diode current absolute accuracy ppm/? -480 10 480 monitor diode current stability ? -1 10 i in data+, data- input current ppm/? 390 bias current stability % -15 15 bias current absolute accuracy mvp-p 200 1600 v id differential input voltage v v cc -v cc -v cc - 1.49 1.32 v id /4 v icm common-mode input voltage units min typ max symbol parameter i bias = 80ma i bias = 1ma 920 i md = 1ma i md = 18? (note 5) 70 max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control _______________________________________________________________________________________ 3 ac electrical characteristics (v cc = +3.14v to +5.5v, load as shown in figure 2, t a = -40? to +85?, unless otherwise noted. typical values are at v cc = +3.3v, t a = +25?.) (note 6) note 1: characteristics at -40? are guaranteed by design and characterization. dice are tested at t a = +25? only. note 2: tested with r modset = 5.11k ? (i mod 38ma), r biasmax = 4.56k ? (i bias 52ma), excluding i bias and i mod . note 3: both the bias and modulation currents will be disabled if any of the current set pins are shorted to ground. note 4: this assumes that the laser to monitor diode transfer function does not change with temperature. note 5: see typical operating characteristics for worst-case distributions. note 6: ac characteristics are guaranteed by design and characterization. note 7: total i mod out of out+. refer to the design procedure for information regarding current delivered to the laser. note 8: input signal is a 622mbps, 2 13 - 1 prbs with 80 inserted zeros. note 9: input signal is a 622mbps, 11110000 pattern. note 10: pwd = (wider pulse ?narrower pulse) / 2. i mod = 75ma enable = low (note 3) (note 7) open loop 20% to 80%, r l = 10? || 20 ? load conditions ppm/? -620 175 620 modulation current stability ? 200 ma 57 5 i mod modulation current range modulation off current ns 250 enable/start-up delay ps 70 155 pulse-width distortion (peak-to-peak) % -15 15 modulation current absolute accuracy 100 200 t r/ t f output rise/fall time ps 230 375 units min typ max symbol parameter (notes 9, 10) i mod = 5ma data+ data- (data+) - (data-) i out + 100mv min 800mv max 200mvp-p min 1600mvp-p max i mod figure 1. required input signal and output polarity i mod = 5ma (note 5) 300 v cc 15? 10? 1.0f 1.0f 12.4? 20? out- out+ bias 20? 50? oscilloscope max3668 v cc figure 2. output termination for characterization (note 8) ps 100 jitter generation (peak-to-peak) i mod = 75ma i mod = 5ma bits 80 cid maximum consecutive identical digits at 622mbps i mod = 75ma 10 135 eye diagram (i mod = 35ma) max3668 toc02 200ps/div pattern = 2 13 - 1 + 80 cid i mod = 35ma 622mbps eye diagram (i mod = 75ma) max3668 toc03 200ps/div pattern = 2 13 - 1 + 80 cid i mod = 75ma 622mbps eye diagram (622mbps, 1300nm laser with 467mhz filter) max3668 toc01 193ps/div 2 23 - 1 prbs 10 0.01 11 0 100 monitor diode current vs. apc set resistor 0.1 1 max3668 toc04 r apcset (k?) i md (ma) 1000 1 0.1 10 1 100 bias current vs. maximum bias set resistor 10 100 max3668 toc05 r biasmax (k?) i bias (ma) 100 1 0.1 100 1000 modulation current vs. modulation set resistor 10 max3688 toc06 r modset (k?) i mod (ma) 10 1 16 17 19 18 20 21 040 20 60 80 random jitter vs. modulaton current max3668 toc07 i mod (ma) random jitter (ps p-p) includes random jitter due to measurement equipment 0 15 10 5 20 25 30 35 40 45 50 040 20 60 80 pulse-width distortion vs. modulation current max3668 toc08 i mod (ma) pwd (ps) 30 35 45 40 50 55 -40 10-15 35 60 85 supply current vs. temperature max3668 toc09 temperature (?) supply current (ma) v cc = +3.3v v cc = +5.0v i bias = 48ma i mod = 33ma max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control 4 _______________________________________________________________________________________ t ypical operating characteristics (v cc = +3.3v, t a = +25?, unless otherwise noted.) max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control _______________________________________________________________________________________ 5 1000 0.1 1.4 1.81.6 2.2 2.6 available bias current vs. maximum bias set resistor 1 10 100 max3668 toc10 r bias (k?) bias current (ma) 2.0 2.4 v cc = +3.3v v cc = +3.3v, + 5v closed-loop operation v cc = +5v 0 5 15 10 20 25 -25 15565 245 335 425 515 distribution of modulation current stability (worst case) max3668-11 modulation diode current stability (ppm/?) percent of units (%) t a = -40? to +85? i mod = 5ma 0 5 15 10 20 25 -52-88 20 92 160 distribution of monitor diode current stability (worst case) max3668-12 monitor diode current stability (ppm/?) percent of units (%) t a = -40? to +85? i md = 18a t ypical operating characteristics (continued) (v cc = +3.3v, t a = +25?, unless otherwise noted.) pin description name function 1, 2, 6, 15, 17, 20, 24 v cc positive supply voltage 29 apcset apc set resistor. a resistor connected from this pad to ground sets the desired average opti- cal power. the resulting current is equal to the desired dc monitor diode current. connect a 100k ? resistor from this pad to ground if apc is not used. pin 3 data+ positive pecl data input 4 data- negative pecl data input 32 biasmax maximum bias set resistor. a resistor from this pad to ground sets the maximum laser bias current. the apc function can subtract from this maximum value but cannot add to it. this resistor controls the bias-current level when the apc loop is not used. 31 modset modulation set resistor. a resistor from this pad to ground sets the laser modulation current. 5, 7, 8, 10, 14, 21, 22, 30 gnd ground 12, 13, 26, 27, 28 n.c. no connection. leave unconnected. 11 fail ttl output. indicates apc failure when low. internally pulled high through a 6k ? resistor. 9 enable ttl/cmos enable input. high for normal operation, low to disable laser bias and modulation currents. internally pulled high. 16 bias laser bias current output. isolate from laser with a ferrite bead. 19 out- negative modulation current output. i mod flows into this pad when the input signal is low. connect this pad to v cc through a 6.3? resistor. 18 out+ positive modulation current output. i mod flows into this pad when the input signal is high. connect this pad to ac coupling network. 23 md monitor photodiode connection. connect this pad to the monitor photodiode anode. a capaci- tor to ground is required to filter high-speed ac monitor photocurrent. 25 capc apc compensation capacitor. a 0.1? capacitor connected from this pad to ground controls the dominant pole of the automatic power control (apc) feedback loop. max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control 6 _______________________________________________________________________________________ max3668 data+ out+ out- data- 100k enable i md r apcset r biasmax r modset apcset capc c apc biasmax md modset fail bias i bias 165x v cc failure detector 40x 5x figure 3. functional diagram detailed description the max3668 laser driver consists of two main parts: a high-speed modulation driver and a laser-biasing block with automatic power control (apc). the circuit is opti- mized for low-voltage (+3.3v) operation. the output stage is composed of a high-speed differential pair and a programmable modulation current source. since the modulation output drives a maximum current of 75ma into the laser with a 230ps edge speed, large transient voltage spikes can be generated due to the parasitic inductance. these transients and the laser for- ward voltage leave insufficient headroom for the proper operation of the laser driver if the modulation output is dc-coupled to the laser diode. to solve this problem, the max3668? modulation output is designed to be ac-coupled to the cathode of a laser diode. a simpli- fied functional diagram is shown in figure 3. the max3668 modulation output is optimized for driv- ing a 20??? 10 ? load; the minimum required voltage at out+ is 2.0v. modulation current swings of 75ma are possible. to interface with the laser diode, a damping resistor (r d ) is required for impedance matching. an rc shunt network may be used to compensate for the laser-diode parasitic inductance, thereby improving the optical output aberrations and duty-cycle distortion. at a 622mbps data rate, any capacitive load at the cath- ode of a laser diode degrades the optical output perfor- mance. since the bias output is directly connected to the laser cathode, minimize the parasitic capacitance associ- ated with this pin by using an inductor to isolate the bias pin from the laser cathode. automatic power control to maintain constant average optical power, the max3668 incorporates an apc loop to compensate for the changes in laser threshold current over temperature and lifetime. a back-facet photodiode mounted in the laser package is used to convert the optical power into a photocurrent. the apc loop adjusts the laser bias cur- rent so the monitor current is matched to a reference cur- rent set by r apcset . the time constant of the apc loop is determined by an external capacitor (c apc ). to elimi- nate the pattern-dependent jitter associated with the apc loop-time constant and to guarantee loop stability, the recommended value for c apc is 0.1?. when the apc loop is functioning, the maximum allowable bias current is set by an external resistor, r biasmax . an apc failure flag ( fail ) is set low when the bias current can no longer be adjusted to achieve the desired average optical power. apc closed-loop operation requires the user to set three currents with external resistors connected between ground and biasmax, modset, and apcset. detailed guidelines for these resistor settings are described in the design procedure section. open-loop operation if necessary, the max3668 is fully operational without apc. in this case, the laser current is directly set by two external resistors connected from ground to biasmax and modset. connect a 100k ? resistor from apcset to ground and leave md open for open-loop operation. enable control the max3668 incorporates a laser driver enable func- tion. when enable is low, both the bias and modulation currents are off. the typical laser enable time is 250ns. apc failure monitor the max3668 provides an apc failure monitor (ttl/cmos) to indicate an apc loop tracking failure. fail is set low when the apc loop can no longer adjust the bias current to maintain the desired monitor current. this output is internally pulled up to v cc through a 6k ? resistor. short-circuit protection the max3668 provides short-circuit protection for the modulation, bias, and monitor current sources. if either biasmax, modset, or apcset is shorted to ground, the bias and modulation outputs will be turned off. design procedure when designing a laser transmitter, the optical output is usually expressed in terms of average power and extinc- tion ratio. table 1 gives the relationships that are helpful in converting between the optical average power and the modulation current. these relationships are valid if the average duty cycle of the optical waveform is 50%. programming the modulation current in addition to being a function of r modset , the modula- tion current delivered to the laser (i modl ) also depends on the values of the series damping resistor (r d ), the shunt compensation resistance (r filt ), and the laser diode? resistance (see typical operating circuit). the modulation current (assuming c filt < ratio is r e = 6.6 (8.2db), the operating temperature is -40? to +85?, and the laser diode has the following characteristics: wavelength: = 1.3? threshold current: th = 22ma at +25? threshold temperature coefficient: th = 1.3%/? laser to monitor transfer: mon = 0.2a/w laser slope efficiency: = 0.05mw/ma at +25? determine r apcset the desired monitor diode current is estimated by i md = p ave mon = 200?. the monitor diode current vs. apc set resistor graph in the typical operating characteristics shows that r apcset should be 6k ? . determine r modset to achieve a minimum extinction ratio (r e ) of 6.6 over temperature and lifetime, calculate the required extinc- tion ratio at +25?. assuming r e = 20, the peak-to-peak optical power p p-p = 1.81mw according to table 1. the required modulation current is 1.81(mw) / 0.05(mw/ma) = 36.2ma. the modulation current vs. modulation set resistor graph in the (see typical operating characteri- stics) shows that r modset should be 5k ? . determine r biasmax calculate the maximum threshold current (i th(max) ) at t a = +85? and end of life. assuming i th(max) = 50ma, the maximum bias current should be: i bias = i th(max) + i mod / 2 in this example, i bias = 68.1ma. the bias current vs. maximum bias set resistor graph in the typical operating characteristics shows that r biasmax should be 3k ? . modulation current more than 50ma to drive modulation currents greater than 50ma at 3.3v, external pull-up inductors (figure 5) should be used to dc-bias the modulation output at v cc . such a configuration isolates the laser forward voltage from the output circuitry and allows the output at out+ to swing above and below the supply voltage v cc . at +5v power supply, the headroom voltage for the max3668 is significantly improved. in this case, it is possible to achieve a modulation current of more than 50ma (using resistor pull-ups as shown in the typical operating circuit ). the max3668 can also be dc-coupled to a laser diode when operating at +5v supply; the volt- age at out+ should be 2.0v for proper operation. wire bonding die for high current density and reliable operation, the max3668 uses gold metalization. make connections to the die with gold wire only, using ball-bonding tech- niques. wedge bonding is not recommended. die-pad size is 4 mils (100?) square, and die thickness is 12 mils (300?) mils. layout considerations to minimize inductance, keep the connections between the max3668 output pins and ld as close as possible. optimize the laser diode performance by placing a bypass capacitor as close as possible to the laser anode. use good high-frequency layout techniques and multilayer boards with uninterrupted ground planes to minimize emi and crosstalk. laser safety and iec 825 using the max3668 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. customers 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 sup- port 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. max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control _______________________________________________________________________________________ 9 ld c d r filt c filt 1000pf 1.0f5 ? 10? r d v cc ferrite beads ferrite bead md bias out+ out- max3668 figure 5. output termination for maximum modulation current max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control 10 ______________________________________________________________________________________ v cc n.c. v cc v cc v cc v cc 0.083" (2.10mm) 0.070" (1.78mm) v cc biasmax modset gnd gnd v cc capc n.c. gnd n.c. apcset n.c. gnd gnd enable gnd gnd v cc bias gnd n.c. n.c. gnd n.c. v cc fail gnd gnd md gnd n.c. out- n.c. gnd data- data+ v cc v cc gnd out+ gnd n.c. chip topography max3668 tqfp top view 32 28 293031 25 26 27 modset gnd apcset n.c. biasmax n.c. n.c. capc 10 13 15 14 16 11 12 9 enable fail gnd n.c. n.c. v cc gnd bias 17 18 19 20 21 22 23 md 24 v cc gnd gnd v cc out- out+ v cc 2 3 4 5 6 7 8 gnd gnd v cc gnd data- data+ v cc 1 v cc pin configuration max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control ______________________________________________________________________________________ 11 32l,tqfp.eps f 1 2 21-0079 package outline, 32l tqfp, 5x5x1.0mm, ep option package information (package information continues on next page.) max3668 +3.3v, 622mbps sdh/sonet laser driver with automatic power control maxim makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does maxim assume any lia- bility arising out of the application or use of any product or circuit and specifically disclaims any and all liability, including without limitation consequential or incidental damages. ?ypical?parameters can and do vary in different applications. all operating parameters, including ?ypicals?must be validated for each customer application by customer? technical experts. maxim products are not designed, intended or authorized for use as components in systems intended for surgical implant into the body, or 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. 12 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2005 maxim integrated products printed usa is a registered trademark of maxim integrated products, inc. package information (continued) f 2 2 21-0079 package outline, 32l tqfp, 5x5x1.0mm, ep option |
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