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general description the max3646 is a +3.3v laser driver designed for mul- tirate transceiver modules with data rates from 155mbps to 622mbps. lasers can be dc-coupled to the max3646 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, com- bined with thermal compensation, maintain a constant optical extinction ratio over temperature and lifetime. the max3646 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, make the max3646 ideal for driving fp/dfb lasers in fiber optic modules. external resistors set the required laser current levels. the max3646 provides transmit disable control (tx_disable), single-point fault tolerance, bias-current monitoring, and photocur- rent 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 max3646 is compliant with the sff-8472 transmitter diagnostic and sfp msa timing requirements. the max3646 is offered in a 4mm x 4mm, 24-pin thin qfn package and operates over the extended -40 c to +85 c temperature range. applications multirate oc-3 to oc-12 fec transceivers 125mbps ethernet sfp, gbic, and 1 x 9 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 shutdown and alarm outputs ? enable control and laser safety feature max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control ________________________________________________________________ maxim integrated products 1 ordering information 19-3161; rev 2; 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 bias apcfilt2 v cc v cc tx_disable modtcomp th_temp out+ apcfilt1 shutdown gnd bc_mon tx_fault gnd v cc v cc md modset apcset 1 2 3 4 5 6 7 8 9 10 11 12 24 23 22 21 20 19 13 14 15 16 17 18 top view *the exposed paddle must be soldered to supply ground on the circuit board. in+ in- pc_mon out- modbcomp max3646 *ep pin configuration typical application circuit appears at end of data sheet. + denotes a lead-free/rohs-compliant package. * ep = exposed pad. t = tape and reel. part temp range pin-package max3646etg -40c to +85c 24 thin qfn-ep* max3646etg-t -40c to +85c 24 thin qfn-ep* max3646etg+ -40c to +85c 24 thin qfn-ep* max3646etg+t -40c to +85c 24 thin qfn-ep*
max3646 155mbps to 622mbps sff/sfp 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 otherwise 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, bc_mon, pc_mon, 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 = +70?) 24-pin tqfn (derate 27.8mw/? above +70?) .......2222mw operating junction temperature range ...........-55? to +150? lead temperature (soldering, 10s) .................................+300? soldering temperature (reflow) lead (pb)-free packages..............................................+260? packages containing lead (pb).....................................+240? storage temperature range .............................-55? to +150? 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, 100ma p-p (note 4) 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 output edge speed 20% to 80% (notes 6, 7) 5ma i mod 85ma 100 200 ps output overshoot/undershoot (note 7) 6% random jitter (notes 6, 7) 1.1 2.5 ps rms 622mbps, 5ma i mod 85ma 24 46 deterministic jitter (notes 6, 8) 155mbps, 5ma i mod 85ma 45 100 ps p-p 5ma i mod 10ma ?75 ?00 modulation-current temperature stability (note 6) 10ma < i mod 85ma ?25 ?80 ppm/ c 5ma i mod 10ma 20 modulation-current-setting error 15 load, t a = +25 c 10ma < i mod 85ma 15 % modulation off current tx_disable = high 0.1 ma
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control _______________________________________________________________________________________ 3 note 1: ac characterization is performed using the circuit in figure 2 using a prbs 2 23 - 1 or equivalent 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 622mbps. note 8: dj includes pulse-width distortion (pwd). parameter symbol conditions min typ max units automatic power and extinction ratio controls monitor-diode input current range i md average current into the md pin 18 1500 ? md pin voltage 1.4 v md current monitor ratio i md / i pc_mon 0.85 0.93 1.15 ma/ma apc loop time constant c apc_filt = 0.01?, i md / i bias = 1/70 3.3 ? apc setting stability (note 6) 100 480 ppm/ c apc setting accuracy t a = +25 c 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 / t (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 = 0.01?, i md / i bias = 1/80 (note 6) 5s bias and modulation turn-on delay c apc_filt = 0.01?, i md / i bias = 1/80 (note 6) 600 ? 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 = 45k (typical) 0.8 v v hi = v cc 15 tx_disable input current v lo = gnd -70 -140 ? tx_fault output low sinking 1ma, open collector 0.4 v shutdown output high sourcing 100? v cc - 0.4 v shutdown output low sinking 100? 0.4 v 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 otherwise noted.) (notes 1, 2)
max3646 155mbps to 622mbps sff/sfp 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 c, unless otherwise noted.) optical eye diagram (622.08mbps, 2 7 - 1 prbs, 467mhz filter) max3646 toc01 270ps/div 1310nm fp laser r e = 8.2db optical eye diagram (155mbps, 2 7 - 1 prbs, 117mhz filter) max3646 toc02 1ns/div 1310nm fp laser r e = 8.2db electrical eye diagram (i mod = 30ma, 622.08mhz, 2 7 - 1 prbs) max3646 toc03 320ps/div 75mv/div 2pf between out+ and out- supply current (i cc ) vs. temperature (excludes bias and modulation currents) max3646 toc04 temperature ( c) supply current (ma) 80 70 60 50 40 30 20 10 0 -10 -20 -30 35 40 45 50 55 60 30 -40 90 3.63v 2.97v 3.3v bias-current monitor ratio vs. temperature max3646 toc05 temperature ( c) i bias /i bc_mon (ma/ma) 80 70 50 60 -10 0 10 20 30 40 -30 -20 72 74 76 78 80 82 84 86 88 90 70 -40 90 photocurrent monitor ratio vs. temperature max3646 toc06 temperature ( c) i md /i pc_mon (ma/ma) 80 70 50 60 -100 10203040 -30 -20 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 0.80 -40 90 modulation current vs. r modset max3646 toc07 r modset (k ) i mod (ma) 10 10 20 30 40 50 60 70 80 90 0 1 100 photodiode current vs. r apcset max3646 toc08 r apcset (k ) i md (ma) 10 1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 0.1 100 deterministic jitter vs. modulation current max3646 toc09 i mod (ma) dj (ps p-p ) 80 70 50 60 20 30 40 10 30 40 50 60 70 80 90 100 20 090 155mbps
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control _______________________________________________________________________________________ 5 typical operating characteristics (continued) (v cc = +3.3v, c apc = 0.01?, i bias = 20ma, i mod = 30ma, t a = +25 c, unless otherwise noted.) random jitter vs. modulation current max3646 toc10 i mod (ma) rj (ps rms ) 80 70 50 60 20 30 40 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 090 compensation (k) vs. r modbcomp max3646 toc11 r modbcomp (k ) k (ma/ma) 10 1 0.1 0.01 0.1 1 10 0.01 0.001 100 temperature compensation vs. r th_temp (r modtcomp = 500 ) max3646 toc12 temperature ( c) i mod (ma) 80 70 60 50 40 30 20 10 0 40 50 60 70 80 90 100 30 -10 90 r th_temp = 12k r th_temp = 7k r th_temp = 4k r th_temp = 2k temperature compensation vs. r th_temp (r modtcomp = 10k ) max3646 toc13 temperature ( c) i mod (ma) 80 70 60 50 40 30 20 10 0 32 34 36 38 40 42 44 30 -10 100 90 r th_temp = 12k r th_temp = 7k r th_temp = 4k r th_temp = 2k hot plug with tx_disable low max3646 toc14 20ms/div v cc fault tx_disable laser output 0v 3.3v t_init = 59.6ms low low transmitter enable max3646 toc15 10 s/div v cc fault tx_disable laser output 3.3v t_on = 23.8 s low high low transmitter disable max37646 toc16 20ns/div v cc fault tx_disable laser output 3.3v 91.2ns low high low response to fault max3646 toc17 400ns/div v pc_mon fault tx_disable laser output t_fault = 160ns externally forced fault fault recovery time max3646 toc18 40ms/div v pc_mon fault tx_disable laser output t_init = 58ms externally forced fault low low low high high
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control 6 _______________________________________________________________________________________ pin name function 1 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. 2, 5, 14, 17 v cc +3.3v supply voltage 3 in+ noninverted data input 4 in- inverted data input 6 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. 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 shutdown shutdown driver output. voltage output to control an external transistor for optional shutdown circuitry. 10, 12 gnd ground 11 tx_fault open-collector transmit fault indicator (table 1) 13 bias laser bias-current output 15 out- inverted modulation-current output. i mod flows into this pin when input data is low. 16 out+ noninverted modulation-current output. i mod flows into this pin when input data is high. 18 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. 19 apcfilt1 connect a capacitor (c apc ) between pin 19 (apcfilt1) and pin 20 (apcfilt2) to set the dominant pole of the apc feedback loop. 20 apcfilt2 (see pin 19) 21 apcset a resistor connected from this pin to ground sets the desired average optical power. the total capacitive load at the apcset pin should be no more than 10pf. minimize metal resistance for ground connections. 22 modset a resistor connected from this pin to ground sets the desired constant portion of the modulation current. the total capacitive load at the modset pin should be no more than 10pf. minimize metal resistance for gr ound connections. 23 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. 24 th_temp threshold for temperature compensation. a resistor at this pin programs the temperature above which compensation is added to the modulation current. ep exposed pad. solder the exposed pad to the circuit board ground for specified thermal and electrical performance. pin description
detailed description the max3646 laser driver consists of three main parts: a high-speed modulation driver, biasing block with erc, and safety circuitry. the circuit design is opti- mized 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 max3646 is optimized for driving a 15 load. the minimum instantaneous voltage required at out- is 0.7v for modulation currents 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 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. any capacitive load at the cathode of a laser diode degrades optical output performance. 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 para- sitics form 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 ) max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control _______________________________________________________________________________________ 7 v in+ v in- 200mv (min) 2400mv (max) 100mv (min) 1200mv (max) i mod voltage current time single ended differential (v in+ ) - (v in- ) i out+ figure 1. required input signal and output polarity max3646 21 21 50 out+ out- oscilloscope 50 1 f bias-t bias-t 1 f figure 2. test circuit for characterization l1 1 h c1 0.1 f c3 0.1 f c2 10 f voltage supply source noise optional optional filter defined by sfp msa host board module to laser driver v cc figure 3. supply filter
max3646 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 laser over time and temperature: p avg = i md / mon p p-p = x i mod modulation compensation from bias increases the mod- ulation current by a user-selected proportion (k) needed to maintain peak-to-peak laser power as bias current increases with temperature. refer to application note 1119: hfan-02.2.1: maintaining the extinction ratio of optical transmitters using k-factor control for details: k = i mod / i bias this provides a first-order approximation of the current increase needed to maintain peak-to-peak power. slope efficiency decreases more rapidly as tempera- ture increases. the max3646 provides additional tem- perature compensation as temperature increases past a user-defined threshold (t th ). 155mbps to 622mbps sff/sfp laser driver with extinction ratio control 8 _______________________________________________________________________________________ max3646 in+ in- out- out+ input buffer data path i mod v bg r modset modset v bg r apcset apcset i apcset c apc apcfilt1 apcfilt2 i bias bias md v cc i bias enable x1 tx_fault tx_disable safety logic and power detector r bc_mon bc_mon v cc r pc_mon pc_mon r d v cc i md c md shutdown shutdown x268 r pull = 45k t xk xtc t > t th i bias i mod enable x1/2 modtcomp r modtcomp th_temp r th_temp modbcomp r modbcomp i bias 82 i md 1 figure 4. functional diagram
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control _______________________________________________________________________________________ 9 1 if any of the i/o pins are shorted to gnd or v cc (single-point failure; see table 2), and the bias current or the photocurrent exceeds 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 r bc_mon bc_mon v cc v cc r pc_mon pc_mon comp v ref v ref ttl open collector cmos shutdown tx_fault r s q rs latch counter 60ms delay por and counter 60ms delay 100ns delay i bias enable i mod enable v cc tx_disable comp excessive apc current setpoint excessive mod current setpoint i md 1 i bias 82 figure 5. simplified safety circuit
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control 10 ______________________________________________________________________________________ pin circuit response to overvoltatge or short to v cc circuit response to undervoltage or short to ground tx_fault does not affect laser power. does not affect 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 the bias current until a fault is detected, then a fault* state occurs. shutdown does not affect laser power. if the shutdown circuitry is used, the laser current is disabled. does not affect laser power. bias in this condition, the laser forward voltage is 0v and no light is emitted. fault state* occurs. if the shutdown circuitry is used, the 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, the 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. 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 shutdown pin.
safety circuitry the safety circuitry contains a disable input (tx_disable), a latched fault output (tx_fault), and fault detectors (figure 5). this circuitry monitors the operation 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 failure. the transmit fault condition is latched until reset by a toggle or tx_disable or v cc . the laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the typical application circuit . this shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition. safety circuitry current monitors the max3646 features monitors (bc_mon, pc_mon) for bias current (i bias ) and photocurrent (i md ). the monitors are realized by mirroring a fraction of the cur- rents and developing voltages across external resistors connected to ground. voltages greater than v ref at pc_mon or bc_mon result in a fault state. for exam- ple, connecting a 100 resistor to ground at each mon- itor output gives the following relationships: v bc_mon = (i bias / 82) x 100 v pc_mon = i md x 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 shows 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 modu- lation currents can be calculated using the equations in table 3. proper setting of these currents requires knowledge of the laser to monitor transfer ( mon ) and slope efficiency ( ). programming the monitor-diode current set point the max3646 operates in apc mode at all times. the bias current is automatically set so average laser power is determined by the apcset resistor: p avg = i md / mon 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 apcset that corresponds to the required current at +25 c: i md = 1/2 x v ref / r acpset the laser driver automatically adjusts the bias to main- tain the constant average power. for dc-coupled laser diodes: i avg = i bias + i mod / 2 programming the modulation current with compensation determine the modulation current form the laser slope efficiency: i mod = 2 x p avg / x (r e - 1)/(r e+ + 1) the modulation current of the max3646 consists of a static modulation current (i mods ), a current proportion- al 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 refer- ence voltage of v ref across the external programming resistor. see the i mod vs. r modset graph in the typical operating characteristics and select the value max3646 155mbps to 622mbps sff/sfp 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 one p 1 p 1 = 2p avg x 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.
max3646 of r modset that corresponds to the required current at +25 c: i mod = i mods + k x i bias + i modt i mods = 268 x v ref / r modset i modt = tc x (t - t th ) | t > t th i modt = 0 | t < t th 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 : k = 1700 / (1000 + r modbcomp ) + 10% if i mod must be increased from i mod1 to i mod2 to maintain the extinction ratio at elevated temperatures, the required compensation factor is: k = (i mod2 - i mod1 ) / (i bias2 - i bias1 ) a threshold for additional temperature compensation can be set with a programming resistor at the th_temp pin: t th = -70 c + 1.45m / (9.2k + r th_temp ) c + 10% the temperature coefficient of thermal compensation above t th is set by r modtcomp . leaving the modtcomp pin open disables additional thermal compensation: tc = 1 / (0.5 + r modtcomp (k )) ma/ c + 10% current compliance (i mod 60ma), dc-coupled the minimum voltage at the out+ and out- pins is 0.7v. for: v diode = diode bias point voltage (1.2v typ) r l = diode bias point resistance (5 typ) r d = series matching resistor (20 typ) for compliance: v out+ = v cc - v diode - i mod x (r d + r l ) - i bias x r l 0.7v current compliance (i mod > 60ma), ac-coupled for applications requiring modulation current greater than 60ma, headroom is insufficient from proper opera- tion of the laser driver if the laser is dc-coupled. to avoid this problem, the max3646? 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 sup- ply voltage (v cc ). when ac-coupled, the max3646 modulation current can be programmed up to 85ma. refer to application note 274: hfan-02.0: interfacing 155mbps to 622mbps sff/sfp laser driver with extinction ratio control 12 ______________________________________________________________________________________ v cc v cc v cc 0.11pf 0.7nh in+ 0.11pf 0.7nh in- package 5k 5k 24k 16k max3646 figure 6. simplified input structure max3646 package 0.7nh out- 0.7nh out+ 0.11pf 0.11pf v cc figure 7. simplified output structure
maxim laser drivers with laser diodes for more informa- tion on ac-coupling laser drivers to laser diodes. for compliance: v out+ = v cc - i mod / 2 x (r d + r l ) 0.75v determine c apc the apc loop filter capacitor (c apc ) must be selected to balance the requirements for fast turn-on and mini- mal interaction with low frequencies in the data pattern. the low-frequency cutoff is: c apc (?) ? 68 / (f 3db (khz) x ( x mon ) 1.1 high-frequency noise can be filtered with an additional cap, c md , from the md pin to ground: c md ? c apc / 4 the max3646 is designed so turn-on time is faster than 1ms for most laser gain values ( x mon ). choosing a smaller value of c apc reduces turn-on time. careful balance between turn-on time and low-frequency 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 max3646 laser driver. if dice are used, replace package parasitic elements with bondwire par- asitic elements. layout considerations to minimize loss and crosstalk, keep the connections between the max3646 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 max3646 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, recogniz- ing 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. exposed-pad (ep) package the exposed pad on the 24-pin qfn provides a very low thermal resistance path for heat removal from the ic. the pad is also electrical ground on the max3646 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 consideration of qfn and other exposed-paddle packages at www.maxim-ic.com for additional information. chip information processs: sige/bipolar max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control ______________________________________________________________________________________ 13
max3646 155mbps to 622mbps sff/sfp laser driver with extinction ratio control 14 ______________________________________________________________________________________ max3646 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 apcfilt1 apcfilt2 gnd apcset modset tx_disable tx_fault +3.3v cdr c apc r pc_mon c md 0.01 f ferrite bead pc_mon r bc_mon r modset r apcset modbcomp modtcomp th_temp r th_temp r modtcomp r modbcomp 0.1 f 0.1 f typical application 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. 24 tqfn-ep t2444-3 21-0139 90-0021
max3646 155mbps to 622mbps sff/sfp 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. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 15 ? 2011 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. revision history revision number revision date description pages changed 0 1/04 initial release 1 7/04 added the lead(pb)-free package option to the ordering information 1 2 6/11 updated ordering information ; changed absolute maximum ratings to reflect lead and soldering specs; changed continuous power dissipation specs; updated bias- current monitor ratio in electrical characteristics table; changed the apcset and modset function description in pin description table; replaced figure 2 1, 2, 6, 7

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