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general description the max3737 is a 3.3v laser driver designed for multirate transceiver modules with data rates from 155mbps to 2.7gbps. lasers can be dc-coupled to the max3737 for reduced component count and ease of multirate operation. laser extinction ratio control (erc) combines the features of automatic power control (apc), modulation compensa- tion, and built-in thermal compensation. the apc loop maintains constant average optical power. modulation compensation increases the modulation current in pro- portion to the bias current. these control loops combined with thermal compensation maintain a constant optical extinction ratio over temperature and lifetime. the max3737 accepts differential data input signals. the wide 5ma to 60ma (up to 85ma ac-coupled) mod- ulation current range and up to 100ma bias current range makes the max3737 ideal for driving fp/dfb lasers in fiber-optic modules. external resistors set the required laser current levels. the max3737 provides transmit disable control (tx_disable), single-point fault tolerance, bias-current monitoring, modulation-cur- rent monitoring, and photocurrent monitoring. the device also offers a latched failure output (tx_fault) to indicate faults, such as when the apc loop is no longer able to maintain the average optical power at the required level. the max3737 is compliant with the sff- 8472 transmitter diagnostic and sfp msa timing requirements. the max3737 is offered in a 5mm x 5mm 32-pin thin qfn and qfn package and operates over the -40? to +85? extended temperature range. applications multirate oc-3 to oc-48 fec transceivers gigabit ethernet sff/sfp and gbic transceivers 1gbps/2gbps fibre channel sff/sfp and gbic transceivers features ? single 3.3v power supply ? 47ma power-supply current ? 85ma modulation current ? 100ma bias current ? automatic power control (apc) ? modulation compensation ? on-chip temperature compensation ? self-biased inputs for ac-coupling ? ground-referenced current monitors ? laser safety, shutdown, and alarm outputs max3737 multirate laser driver with extinction ratio control ________________________________________________________________ maxim integrated products 1 32 31 30 29 28 27 26 modtcomp th_temp modbcomp modset apcset apcfilt2 apcfilt1 25 vmd 9 10 11 12 13 14 15 mc_mon gnd v cc tx_fault shutdown vbs gnd 16 gnd 17 18 19 20 21 22 23 bias *the exposed paddle must be soldered to supply ground to achieve specified performance. v cc out- out- out+ out+ v cc 8 7 6 5 4 3 2 bc_mon *ep pc_mon v cc in- in+ v cc tx_disable MAX3737EGJ 1 gnd 24 md top view 5mm x 5mm qfn pin configurations ordering information 19-2818; rev 3; 6/11 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. evaluation kit available part temp range pin-package max3737etj -40c to +85c 32 thin qfn-ep* max3737etj+ -40c to +85c 32 thin qfn-ep* MAX3737EGJ -40c to +85c 32 qfn-ep* functional diagram and typical application circuit appear at end of data sheet. pin configurations continued at end of data sheet. + denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad.
max3737 multirate laser driver with extinction ratio control 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v cc = 2.97v to 3.63v, t a = -40? to +85?. typical values are at v cc = 3.3v, i bias = 60ma, i mod = 60ma, t a = +25?, unless other- wise noted.) (notes 1, 2) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. supply voltage v cc ...............................................-0.5v to +6.0v in+, in-, tx_disable, tx_fault, shutdown, mc_mon, bc_mon, pc_mon, vbs, vmd, apcfilt1, apcfilt2, md, th_temp, modtcomp, modbcomp, modset, and apcset voltage .......................................-0.5v to v cc + 0.5v out+, out-, bias current.............................-20ma to +150ma continuous power dissipation (t a = +85?) qfn/tqfn (derate 21.2mw/? above +85?) ................1.3w operating junction temperature range ...........-55? to +150? storage temperature range .............................-55? to +150? lead temperature (soldering, 10s) .................................+300? soldering temperature (reflow) lead(pb)-free...............................................................+260? containing lead(pb) .....................................................+240? parameter symbol conditions min typ max units power supply supply current i cc (note 3) 47 60 ma power-supply noise rejection psnr f 1mhz, 100mv p-p (notes 4, 6) 33 db i/o specifications differential input swing v id dc-coupled, figure 1 0.2 2.4 v p-p common-mode input v cm 1.7 v cc - v id /4 v laser bias bias-current setting range 1 100 ma bias off current tx_disable = high 0.1 ma bias-current monitor ratio i bias /i bc_mon 62 76 90 ma/ma laser modulation modulation-current setting range i mod (note 5) 5 85 ma 5ma i mod 10ma 71 80 output edge speed 20% to 80% (notes 6, 7) 10ma < i mod 85ma 52 80 ps output overshoot/undershoot (note 7) 6 % random jitter (notes 6, 7) 0.65 1.3 ps 5ma i mod 10ma 25.6 40 2.7gbps 10ma < i mod 85ma 16 40 5ma i mod 10ma 32 41 1.25gbps 10ma < i mod 85ma 15 41 5ma i mod 10ma 39 46 622mbps 10ma < i mod 85ma 21 46 5ma i mod 10ma 65 100 deterministic jitter (notes 6, 8) 155mbps 10ma < i mod 85ma 46 70 ps p-p modulation-current temperature stability (note 6) 150 480 ppm/c
max3737 multirate laser driver with extinction ratio control electrical characteristics (continued) (v cc = 2.97v to 3.63v, t a = -40? to +85?. typical values are at v cc = 3.3v, i bias = 60ma, i mod = 60ma, t a = +25?, unless other- wise noted.) (notes 1, 2) parameter symbol conditions min typ max units 5ma i mod 10ma 20 modulation-current setting error 15 load, t a = +25c 10ma < i mod 85ma 15 % modulation off current tx_disable = high 0.1 ma modulation-current monitor ratio i mod /i mc_mon 223 262 302 ma/ma extinction ratio controls monitor-diode input current range i md average current into the md pin 18 1500 a md pin voltage 1.4 v md-current monitor ratio i md /i pc_mon 0.85 1.0 1.15 ma/ma apc loop time constant c apc_filt = 0.01f, i md / i bias = 1/70 3.3 s apc setting stability 100 480 ppm/c apc setting accuracy t a = +25c 15 % i mod compensation setting range by bias k k = i mod / i bias 0 1.5 ma/ma i mod compensation setting range by temperature tc tc = i mod / (note 6) 0 1.0 ma/c threshold setting range for temperature compensation t th (note 6) 10 60 c laser safety and control bias and modulation turn-off delay c apc_filt = 0.01f, i md / i bias = 1/80 (note 6) 5 s bias and modulation turn-on delay c apc_filt = 0.01f, i md / i bias = 1/80 (note 6) 600 s threshold voltage at monitor pins v ref figure 5 1.14 1.3 1.39 v interface signals tx_disable input high v hi 2.0 v tx_disable input low v lo r pull = 7.5k 0.8 v v hi = v cc 15 tx_disable input current v lo = gnd -450 -800 a tx_fault output low sinking 1ma, open collector 0.4 v shutdown output high sourcing 100a v cc - 0.4 v shutdown output low sinking 100a 0.4 v note 1: ac characterization is performed using the circuit in figure 2 using a prbs 2 23 - 1 or equivalent test pattern. note 2: specifications at -40? are guaranteed by design and characterization. note 3: excluding i bias and i mod . input data is ac-coupled. tx_fault open, shutdown open. note 4: power-supply noise rejection (psnr) = 20log 10 (v noise (on vcc) / v out ). v out is the voltage across the 15 load when in+ is high. note 5: the minimum required voltage at the out+ and out- pins is +0.75v. note 6: guaranteed by design and characterization. note 7: tested with 00001111 pattern at 2.7gbps. note 8: dj includes pulse-width distortion (pwd). _______________________________________________________________________________________ 3
max3737 multirate laser driver with extinction ratio control 4 _______________________________________________________________________________________ typical operating characteristics (v cc = 3.3v, c apc = 0.01?, i bias = 20ma, i mod = 30ma, t a = +25?, unless otherwise noted.) optical eye diagram (2.7gbps, 2 7 - 1prbs, 2.3ghz filter) max3737 toc01 54ps/div 1310nm fp laser e r = 8.2db optical eye diagram (1.25gbps, 2 7 - 1prbs, 940mhz filter) max3737 toc02 116ps/div 1310nm fp laser e r = 8.2db optical eye diagram (155mbps, 2 7 - 1prbs, 117mhz filter, c apc = 0.1 f) max3737 toc03 920ps/div electrical eye diagram (i mod = 30ma, 2.7gbps, 2 7 - 1prbs) max3737 toc04 52ps/div 75mv/div supply current (i cc ) vs. temperature (excludes bias and modulation currents) max3737 toc05 temperature ( c) supply current (ma) 60 40 20 0 -20 40 45 50 55 60 65 35 -40 80 v cc = 3.63v v cc = 2.97v v cc = 3.3v i mod = 60ma i bias = 60ma bias-current monitor gain vs. temperature max3737 toc06 temperature ( c) i bias /i bc_mon (ma/ma) 60 35 10 -15 72 74 76 78 80 82 84 86 88 90 70 -40 85 photo-current monitor gain vs. temperature max3737 toc07 temperature ( c) i md /i pc_mon (ma/ma) 60 35 -15 10 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 0.80 -40 85
max3737 multirate laser driver with extinction ratio control 200 230 220 210 240 250 260 270 280 290 300 -40 10 -15 35 60 85 modulation-current monitor gain vs. temperature max3737 toc08 temperature ( c) i mod /i mc_mon (ma/ma) 90 0 1 10 100 modulation current vs. r modset 20 10 max3737 toc09 r modset (k ) i mod (ma) 40 30 60 70 50 80 0 15 10 5 20 25 30 35 40 45 50 020 10 30 50 40 70 80 60 90 deterministic jitter vs. modulation current max3737 toc11 i mod (ma) dj (ps p-p ) 0 0.5 1.0 1.5 2.0 0 20406080100 random jitter vs. modulation current max3737 toc12 i mod (ma) rj (ps rms ) 10 0.01 0 0.1 100 compensation (k) vs. r modbcomp 0.1 1 max3737 toc13 r modbcomp (k ) k (ma/ma) 110 30 50 40 60 70 80 90 100 -20 20 0 40 60 80 100 temperature compensation vs. r th_temp (r modtcomp = 500 ) max3737 toc14 temperature ( c) i mod (ma) r th_temp = 12k r th_temp = 7k r th_temp = 4k r th_temp = 2k typical operating characteristics (continued) (v cc = 3.3v, c apc = 0.01?, i bias = 20ma, i mod = 30ma, t a = +25?, unless otherwise noted.) 1.6 0 0.1 10 100 photodiode current vs. r apcset 0.4 0.2 0.6 0.8 1.0 1.2 1.4 max3737 toc10 r apcset (k ) i md (ma) 1 _______________________________________________________________________________________ 5
max3737 multirate laser driver with extinction ratio control 6 _______________________________________________________________________________________ typical operating characteristics (continued) (v cc = 3.3v, c apc = 0.01?, i bias = 20ma, i mod = 30ma, t a = +25?, unless otherwise noted.) 30 32 34 36 38 40 42 44 -20 20 0 406080100 temperature compensation vs. r th_temp (r modtcomp = 10k ) max3737 toc15 temperature ( c) i mod (ma) r th_temp = 12k r th_temp = 7k r th_temp = 4k r th_temp = 2k 20ms/div hot plug with tx_disable low v cc fault max3737 toc16 tx_disable laser output t_init = 60ms 3.3v 0v low low 20 s/div transmitter enable v cc fault max3737 toc17 tx_disable laser output t_on = 75 s 3.3v low high low 40ns/div transmitter disable v cc fault max3737 toc18 tx_disable laser output 3.3v low low t_off = 134ns high 1 s/div response to fault v pc_mon fault max3737 toc19 tx_disable laser output t_fault = 0.9 s high externally forced fault low low 100ms/div fault recovery time v pc_mon fault max3737 toc20 tx_disable laser output t_init = 68ms low external fault removed low high high low
max3737 multirate laser driver with extinction ratio control pin description pin name function 1, 10, 15, 16 gnd ground 2 tx_disable transmitter disable, ttl. laser output is disabled when tx_disable is asserted high or left unconnected. the laser output is enabled when this pin is asserted low. 3, 6, 11, 18, 23 v cc 3.3v supply voltage 4 in+ noninverted data input 5 in- inverted data input 7 pc_mon photodiode-current monitor output. current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the monitor-diode current. 8 bc_mon bias-current monitor output. current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the bias current. 9 mc_mon modulation-current monitor output. current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the modulation current amplitude. 12 tx_fault open-collector transmit fault indicator (table 1) 13 shutdown shutdown driver output. voltage output to control an external transistor for optional shutdown circuitry. 14 vbs bias voltage sense. isolated tap (3k 15%) on the bias output reduces component count when a precision bias sense resistor is used. 17 bias laser bias-current output 19, 20 out- inverted modulation-current output (connect pins 19 and 20 together). i mod flows into this pin when input data is low. 21, 22 out+ noninverted modulation-current output (connect pins 21 and 22 together). i mod flows into this pin when input data is high. 24 md monitor photodiode input. connect this pin to the anode of a monitor photodiode. a capacitor to ground is required to filter the high-speed ac monitor photocurrent. 25 vmd monitor photodiode voltage sense. isolated tap (3k 15%) on the md input reduces component count when a precision photodiode current-sense resistor is used. 26 apcfilt1 connect a capacitor (c apc ) between pin 26 (apcfilt1) and pin 27 (apcfilt2) to set the dominant pole of the apc feedback loop. 27 apcfilt2 (see pin 26.) the maximum capacitance allowed on this pin is 10pf. 28 apcset a resistor connected from this pin to ground sets the desired average optical power. the maximum capacitance allowed on this pin is 10pf. 29 modset a resistor connected from this pin to ground sets the desired constant portion of the modulation current. 30 modbcomp modulation-current compensation from bias. couples the bias current to the modulation current. mirrors i bias through an external resistor. leave open for zero coupling. 31 th_temp threshold for temperature compensation. a resistor at this pin programs the temperature, above which compensation is added to the modulation current. 32 modtcomp modulation-current compensation from temperature. a resistor at this pin sets the temperature coefficient of the modulation current when above the threshold temperature. leave open for zero temperature compensation. ep exposed pad. solder the exposed pad to the circuit board ground for specified thermal and electrical performance. _______________________________________________________________________________________ 7
max3737 detailed description the max3737 laser driver consists of three main parts: a high-speed modulation driver, biasing block with erc, and safety circuitry. the circuit design is optimized for high-speed, low-voltage (3.3v) operation (figure 4). high-speed modulation driver the output stage is composed of a high-speed differ- ential pair and a programmable modulation current source. the max3737 is optimized for driving a 15 load. the minimum instantaneous voltage required at out+ is 0.7v for modulation current up to 60ma and 0.75v for currents from 60ma to 85ma. operation above 60ma can be accomplished by ac-coupling or with sufficient voltage at the laser to meet the driver output voltage requirement. to interface with the laser diode, a damping resistor (r d ) is required. the combined resistance due to the series damping resistor and the equivalent series resistance (esr) of the laser diode should equal 15 . to further damp aberrations caused by laser diode parasitic induc- tance, an rc shunt network may be necessary. refer to application note 274: hfan-02.0: interfacing maxim laser drivers with laser diodes for more information. at data rates of 2.7gbps, any capacitive load at the cathode of a laser diode degrades optical output perfor- mance. because the bias output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the bias pin parasitics from the laser cathode. extinction ratio control the extinction ratio (r e ) is the laser on-state power divided by the off-state power. extinction ratio remains constant if peak-to-peak and average power are held constant: r e = (2p avg + p p-p ) / (2p avg - p p-p ) average power is regulated using apc, which keeps constant current from a photodiode coupled to the laser. peak-to-peak power is maintained by compen- sating the modulation current for reduced slope effi- ciency ( ) of the laser over time and temperature: p p-p = x i mod p i avg md mon = multirate laser driver with extinction ratio control 8 _______________________________________________________________________________________ 100mv (min) 1200mv (max) 200mv p-p (min), 2400mv (max) i mod time single ended differential voltage current v in + v in - i out + (v in +) - (v in -) figure 1. required input signal and output polarity z 0 = 30 30 0.5pf 30 30 oscilloscope out- out- v cc i out + v cc max3737 z 0 = 30 75 z 0 = 50 50 out+ out+ figure 2. test circuit for characterization voltage supply c1 0.1 f c3 0.1 f c2 10 f l1 1 h optional to laser driver v cc optional source noise host board filter defined by sfp msa module figure 3. supply filter
modulation compensation from bias increases the mod- ulation current by a user-selected proportion (k) need- ed to maintain peak-to-peak laser power as bias current increases with temperature. refer to maxim application note 1119: hfan-02.2.1: maximizing the extinction ratio of optical transmitters using k-factor control for details: this provides a first-order approximation of the current increase needed to maintain peak-to-peak power. slope efficiency decreases more rapidly as temperature increases. the max3737 provides additional tempera- ture compensation as temperature increases past a user-defined threshold (t th ). safety circuitry the safety circuitry contains a disable, input (tx_dis- able), a latched fault output (tx_fault), and fault detectors (figure 5). this circuitry monitors the opera- tion of the laser driver and forces a shutdown if a fault is detected (table 1). the tx_fault pin should be pulled high with a 4.7k to 10k resistor to v cc as required by the sfp msa. a single-point fault can be a short to v cc or gnd. see table 2 to view the circuit response to various single-point failures. the transmit fault condition is latched until reset by a toggle of tx_disable or v cc . the laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the typical operating circuit . this shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition. k i i mod bias = max3737 multirate laser driver with extinction ratio control max3737 data path x1/2 x1 r pull = 7.5k pc_mon in- shutdown tx_fault tx_disable in+ i md 1 v cc v cc input buffer i mod enable i mod out- shutdown out+ bias r d i bias enable safety logic and power detector bc_mon i bias 82 mc_mon i mod 268 i bias i md c md i bias i apcset apcset md r apcset v cc v bg r th_temp th_temp modtcomp modbcomp apcfilt2 modset apcfilt1 t > t h v bg xtc r modtcomp r modset r mdmon r bc_mon r mc_mon r modbcomp x268 xk c apc t figure 4. functional diagram _______________________________________________________________________________________ 9
max3737 safety circuitry current monitors the max3737 features monitors (mc_mon, bc_mon, pc_mon) for modulation current (i mod ), bias current (i bias ), and photocurrent (i md ). the monitors are realized by mirroring a fraction of the currents and developing volt- ages across external resistors connected to ground. voltages greater than v ref at mc_mon, pc_mon, or bc_mon result in a fault state. for example, connecting a multirate laser driver with extinction ratio control 10 ______________________________________________________________________________________ 1 if any of the i/o pins is shorted to gnd or v cc (single-point failure; see table 2), and the bias current or the photocurrent exceed the programmed threshold. 2 end-of-life (eol) condition of the laser diode. the bias current and/or the photocurrent exceed the programmed threshold. 3 laser cathode is grounded and photocurrent exceeds the programming threshold. 4 no feedback for the apc loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the programmed threshold. table 1. typical fault conditions pin circuit response to overvoltage or short to v cc circuit response to undervoltage or short to ground tx_fault does not affect laser power. does not effect laser power. tx_disable modulation and bias currents are disabled. normal condition for circuit operation. in+ the optical average power increases and a fault occurs if v pc_mon exceeds the threshold. the apc loop responds by decreasing the bias current. the optical average power decreases and the apc loop responds by increasing the bias current. a fault state occurs if v bc_mon exceeds the threshold voltage. in- the optical average power decreases and the apc loop responds by increasing the bias current. a fault state occurs if v bc_mon exceeds the threshold voltage. the optical average power increases and a fault occurs if v pc_mon exceeds the threshold. the apc loop responds by decreasing the bias current. md this disables bias current. a fault state occurs. the apc circuit responds by increasing bias current until a fault is detected, then a fault* state occurs. shutdown does not affect laser power. if the shutdown circuitry is used, laser current is disabled. does not affect laser power. bias in this condition, laser forward voltage is 0v and no light is emitted. fault state* occurs. if the shutdown circuitry is used, laser current is disabled. out+ the apc circuit responds by increasing the bias current until a fault is detected, then a fault state* occurs. fault state* occurs. if the shutdown circuitry is used, laser current is disabled. out- does not affect laser power. does not affect laser power. pc_mon fault state* occurs. does not affect laser power. bc_mon fault state* occurs. does not affect laser power. mc_mon fault state* occurs. does not affect laser power. apcfilt1 i bias increases until v bc_mon exceeds the threshold voltage. i bias increases until v bc_mon exceeds the threshold voltage. apcfilt2 i bias increases until v bc_mon exceeds the threshold voltage. i bias increases until v bc_mon exceeds the threshold voltage. modset does not affect laser power. fault state* occurs. apcset does not affect laser power. fault state* occurs. table 2. circuit responses to various single-point faults a fault state asserts the tx_fault pin, disables the modulation and bias currents, and asserts the shutdon pin.
100 resistor to ground at each monitor output gives the following relationships: v mc_mon = (i mod / 268) ? 100 v bc_mon = (i bias / 82) ? 100 v pc_mon = i md ? 100 external sense resistors can be used for high-accuracy measurement of bias and photodiode currents. on-chip isolation resistors are included to reduce the number of components needed to implement this function. design procedure when designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. table 3 gives relationships that are helpful in converting between the optical average power and the modulation current. these relationships are valid if the mark density and duty cycle of the opti- cal waveform are 50%. for a desired laser average optical power (p avg ) and optical extinction ratio (r e ), the required bias and modula- tion currents can be calculated using the equations in table 3. proper setting of these currents requires knowl- edge of the laser to monitor transfer ( mon ) and slope efficiency ( ). programming the monitor diode current set point the max3737 operates in apc mode at all times. the bias current is automatically set so average laser power is determined by the apcset resistor: the apcset pin controls the set point for the monitor- diode current. an internal current regulator establishes the apcset current in the same manner as the modset pin. see the i md vs. r apcset graph in the typical operating characteristics and select the value of r apc- set that corresponds to the required current at +25?: the laser driver automatically adjusts the bias to maintain the constant average power. for dc-coupled laser diodes: programming the modulation current with compensation determine the modulation current from the laser slope efficiency: the modulation current of the max3737 consists of a static modulation current (i mods ), a current proportional to i bias, and a current proportional to temperature. the portion of i mod set by modset is established by an internal current regulator, which maintains the reference voltage of v ref across the external programming resis- tor. see to the i mod vs. r modset graph in the typical operating characteristics and select the value of r mod- set that corresponds to the required current at +25?: i pr r mod avg e e = + 2 1 1 - ii i avg bias mod =+ 2 i v r md ref apcset = 1 2 p i avg md mon = max3737 multirate laser driver with extinction ratio control ______________________________________________________________________________________ 11 parameter symbol relation average power p avg p avg = (p 0 + p 1 ) / 2 extinction ratio r e r e = p 1 / p 0 optical power of a 1 p 1 p 1 = 2p avg r e / (r e + 1) optical power of a zero p 0 p 0 = 2p avg / (r e + 1) optical amplitude p p-p p p-p = p 1 - p 0 laser slope efficiency ? = p p-p / i mod modulation current i mod i mod = p p-p / threshold current i th p 0 at i i th bias current (ac-coupled) i bias i bias i th + i mod / 2 laser to monitor transfer mon i md / p avg table 3. optical power relations note: assuming a 50% average input duty cycle and mark density.
max3737 an external resistor at the modbcomp pin sets current proportional to i bias . open circuiting the modbcomp pin can turn off the interaction between i bias and i mod : if i mod must be increased from i mod1 to i mod2 to maintain the extinction ratio at elevated temperature, the required compensation factor is: a threshold for additional temperature compensation can be set with a programming resistor at the th_temp pin: the temperature coefficient of thermal compensation above t th is set by r modtcomp . leaving the modt- comp pin open disables additional thermal compensation: current compliance (i mod 60ma), dc-coupled the minimum voltage at the out+ and out- pins is 0.7v. for: v diode ?iode bias point voltage (1.2v typ) r l ?iode bias point resistance (5 typ) r d ?eries matching resistor (20 typ) for compliance: current compliance (i mod > 60ma), ac-coupled for applications requiring modulation current greater than 60ma, headroom is insufficient for proper opera- tion of the laser driver if the laser is dc-coupled. to avoid this problem, the max3737? modulation output can be ac-coupled to the cathode of a laser diode. an external pullup inductor is necessary to dc-bias the modulation output at v cc . such a configuration isolates laser forward voltage from the output circuitry and allows the output at out+ to swing above and below the supply voltage (v cc ). when ac-coupled, the max3737 modulation current can be programmed up to 85ma. refer to application note 274: hfan-02.0: interfacing maxim laser drivers with laser diodes for more information on ac-coupling laser drivers to laser diodes. for compliance: determine c apc the apc loop filter capacitor c apc must be selected to balance the requirements for fast turn-on and minimal interaction with low frequencies in the data pattern. the low-frequency cutoff is: high-frequency noise can be filtered with an additional cap c md from the md pin to ground: the max3737 is designed so that turn-on time is faster than 1ms for most laser gain values ( ? mon ). choosing a smaller value of c apc reduces turn-on time. careful balance between turn-on time and low-fre- quency cutoff may be needed at low data rates for some values of laser gain. interface models figures 6 and 7 show simplified input and output cir- cuits for the max3737 laser driver. if dice are used, replace package parasitic elements with bondwire par- asitic elements. c c md apc 4 cf f khz apc db mon () () ( ) . ?? ? 68 3 11 vv i rr v out c mod dl + = + () c - 2 075 . vvv i rri r v out cc diode mod d l bias l + = + () - - - . 07 tc rk ma c modtcomp = + 1 05 10 . () % tc m kr c th th temp =+ + . . % _ -70 145 92 10 k ii ii mod mod bias bias = 21 21 - - k r modbcomp = + 1700 1000 10% itctttt itt modt th th modt th = () > = | | - 0 i v r mods ref modset = 268 ii kii mod mods bias modt =++ multirate laser driver with extinction ratio control 12 ______________________________________________________________________________________
layout considerations to minimize loss and crosstalk, keep the connections between the max3737 output and the laser diode as short as possible. use good high-frequency layout techniques and multilayer boards with uninterrupted ground plane to minimize emi and crosstalk. circuit boards should be made using low-loss dielectrics. use controlled-impedance lines for data inputs, as well as the module output. laser safety and iec 825 using the max3737 laser driver alone does not ensure that a transmitter design is iec 825 compliant. 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 fail- ure of a maxim product could create a situation where personal injury or death may occur. max3737 multirate laser driver with extinction ratio control ______________________________________________________________________________________ 13 pc_mon bc_mon mc_mon r pc_mon r bc_mon r mc_mon v cc v cc max3737 comp rq s rs latch cmos shutdown tx_fault tx_disable ttl open collector comp comp excessive modulation current v cc i md 1 v cc v ref v ref v ref i bias 82 i mod 268 i mod enable i bias enable counter 60ms delay 100ns delay por and counter 60ms delay figure 5. simplified safety circuit max3737 5k 16k 5k 24k package v cc v cc 0.83nh 0.83nh 0.11pf 0.11pf v cc figure 6. simplified input structure
max3737 exposed-pad (ep) package the exposed-pad on the 32-pin qfn provides a very low thermal resistance path for heat removal from the ic. the pad is also electrical ground on the max3737 and should be soldered to the circuit board ground for proper ther- mal and electrical performance. refer to application note 862: hfan-08.1: thermal considerations of qfn and other exposed-paddle packages at www.maxim-ic.com for additional information. multirate laser driver with extinction ratio control 14 ______________________________________________________________________________________ max3737 v cc package 0.82nh 0.82nh out- out+ 0.11pf 0.11pf figure 7. simplified output structure 32 31 30 29 28 27 26 modtcomp th_temp modbcomp modset apcset apcfilt2 apcfilt1 25 vmd 9 10 11 12 13 14 15 mc_mon gnd v cc tx_fault shutdown vbs gnd 16 gnd 17 18 19 20 21 22 23 bias *the exposed paddle must be soldered to supply ground to achieve specified performance. v cc out- out- out+ out+ v cc 8 7 6 5 4 3 2 bc_mon pc_mon v cc in- in+ v cc tx_disable max3737etj 1 gnd 24 md top view 5mm x 5mm thin qfn *ep pin configurations (continued) chip information process: sige/bipolar
max3737 multirate laser driver with extinction ratio control ______________________________________________________________________________________ 15 in+ in- represents a controlled-impedance transmission line v cc shutdown 3.3v optional shutdown circuitry +3.3v 15 10 out- out+ bias md bc_mon mc_mon apcfilt1 apcfilt2 gnd apcset modset tx_disable tx_fault 3.3v c md cdr c apc 0.01 f 0.1 f 0.1 f ferrite bead pc_mon r modset r apcset r mc_mon r bc_mon r pc_mon modbcomp modtcomp th_temp r modbcomp r modtcomp r th_temp max3737 typical operating circuit package information for the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages . note that a ?? ?? or ??in the package code indicates rohs status only. package drawings may show a different suffix character, but the drawing per tains to the package regardless of rohs status. package type package code outline no. land pattern no. 32 tqfn-ep t3255-3 21-0140 90-0001 32 qfn-ep g3255-1 21-0091 90-0279
max3737 multirate laser driver with extinction ratio control maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 16 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2011 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. revision history revision number revision date description pages changed 3 6/11 added lead and soldering temperature information to the absolute maximum ratings ; changed the bias-current monitor ratio parameter specs from 68ma/ma (min), 82ma/ma (typ), 95ma/ma (max) to 62ma/ma (min), 76ma/ma (typ), 90ma/ma (max) in the electrical characteristics table; updated the apcfilt2 and apcset pin functions in the pin description table; added the package information table 2, 7, 15

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