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general description the max3996 is a high-speed laser driver for small- form-factor (sff) fiber optic lan transmitters. it con- tains a bias generator, a laser modulator, and comprehensive safety features. automatic power con- trol (apc) adjusts the laser bias current to maintain average optical power, regardless of changes in tem- perature or laser properties. the driver accommodates common anode or differential laser configurations. the output current range of the max3996 is appropriate for vcsels and high-efficiency edge-emitting lasers. the max3996 operates up to 3.2gbps. it can switch up to 30ma of laser modulation current and sink up to 60ma bias current. adjustable temperature compensa- tion is provided to keep the optical extinction ratio with- in specifications over the operating temperature range. the max3996 accommodates various laser packages, including low-cost to-46 headers. low deterministic jit- ter (9ps p-p ), combined with fast edge transitions, (65ps) provides excellent margins compared to indus- try-standard transmitter eye masks. this laser driver provides extensive safety features to guarantee single-point fault tolerance. safety features include a transmit disable, redundant shutdown, and laser-bias monitoring. the safety circuit detects faults that could cause hazardous light levels and immediate- ly disables the laser output. the max3996 safety cir- cuits are compliant with sff and small-form-factor pluggable (sfp) multisource agreements (msa). the max3996 is available in a compact 4mm ? 4mm, 20-pin qfn package and a 20-pin thin qfn package. it operates over a temperature range of 0? to +70?. applications fibre channel optical transmitters vcsel transmitters gigabit ethernet optical transmitters atm lan optical transmitters 10 gigabit ethernet wwdm features 9ps p-p deterministic jitter 20-pin qfn 4mm ? 4mm package 3.0v to 5.5v supply voltage automatic power control integrated safety circuits 30ma laser modulation current temperature compensation of modulation current compliant with sff and sfp msa max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver ________________________________________________________________ maxim integrated products 1 max3996 0.01 f 0.01 f c pordly r tc r mod c comp n.c. 0.01 f 0.01 f 0.01 f l1* 25 ? optional shutdown circuitry 1.8k ? v cc v cc v cc tx_disable fault in+ in- pordly tc modset mon1 mon2 comp gnd md bias out+ out- shdndrv *ferrite bead r set typical application circuit ordering information 19-2194; rev 3; 5/04 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 package code max3996cgp 0 c to +70 c 20 qfn g2044-3 MAX3996CTP+ 0 c to +70 c 20 thin qfn t2044-3 pin configuration appears at end of data sheet. + denotes lead-free package
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v cc = 3.0v to 5.5v, t a = 0 c to +70 c, unless otherwise noted. typical values are at v cc = 3.3v, tc pin not connected, t a = +25 c.) (figure 1) 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 at v cc ...........................................-0.5v to +7.0v voltage at tx_disable, pordly, mon1, comp, in+, in-, md, bias, modset, tc..........-0.5v to (v cc + 0.5v) voltage between comp and mon2 .....................................2.3v voltage between in+ and in- ..................................................5v voltage at out+, out- .........................(v cc - 2v) to (v cc + 2v) voltage between mon1 and mon2 .....................................1.5v voltage between bias and mon2...........................................4v current into fault, shdndrv ..........................-1ma to +25ma current into out+, out- ....................................................60ma current into bias ..............................................................120ma continuous power dissipation (t a = +70 c) 20-pin qfn (derate 20mw/ c)...................................1600mw operating ambient temperature range .............-40 c to +85 c operating junction temperature range. ..........-40 c to +150 c storage temperature range.... .........................-55 c to +150 c parameter symbol conditions min typ max units v cc = 3.3v, i mod = 15ma 47 supply current i cc (figure1) (note 1) v cc = 5.5v, i mod = 30ma, r modset = 2.37k ? 52 75 ma data input voltage swing v id total differential signal (figure 2) 200 2200 mv p-p tx_disable input current 0 < v pin < v cc -100 +100 a tx_disable input high voltage v ih 2.0 v tx_disable input low voltage v il 0.8 v fault output high voltage v oh i oh = -100a, 4.7k ? < r fault < 10k ? 2.4 v fault output low voltage v ol i ol = 1ma 0.4 v bias generator minimum bias current i bias current into bias pin 1 ma maximum bias current i bias current into bias pin 60 ma apc loop is closed 1.04 1.12 fault = high v cc - 0.73 md quiescent voltage v md tx_disable = high v cc - 0.73 v monitor resistance r mon (figure 4) 9.3 11 12.7 ? power-on reset (por) por threshold measured at v cc 2.65 2.7 3.0 v pordly = open (note 3) 30 55 ? por delay t pordly c pordly = 0.001? (note 3) 1.7 2.4 ms por hysteresis 20 mv shutdown i shdndrv = 10?, fault = high v cc - 0.4 i shdndrv = 1ma, fault = low v cc - 2.4 voltage at shdndrv i shdndrv = 15ma, fault = low 0 v cc - 1.2 v laser modulator data rate < 3.2 gbps
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver _______________________________________________________________________________________ 3 electrical characteristics (continued) (v cc = 3.0v to 5.5v, t a = 0 c to +70 c, unless otherwise noted. typical values are at v cc = 3.3v, tc pin not connected, t a = +25 c.) (figure 1) parameter symbol conditions min typ max units minimum modulation current i mod 2 ma p-p maximum modulation current i mod r l 25 ? 30 40 ma p-p accuracy of modulation current (part-to-part variation) r modset = 2.37k ? (i mod 30ma p-p into 25 ? ) -10 +10 % i mod = 5ma into 25 ? , 20% to 80% (note 3) 54 100 i mod = 10ma into 25 ? , 20% to 80% (note 3) 55 125 edge transition time t r , t f i mod = 30ma into 25 ? , 20% to 80% (note 3) 65 130 ps i mod = 5ma into 25 ? (notes 2, 3) 17 35 i mod = 10ma into 25 ? (notes 2, 3) 14 22 deterministic jitter i mod = 30ma into 25 ? (notes 2, 3) 9 20 ps p-p random jitter (note 3) 2 8 ps rms modulation current during fault i mod_off 15 200 a p-p tempco = max, r mod = open 4000 modulation current tempco tempco = min, r tc = open 50 ppm/ c input resistance r in differential 85 115 ? ? safety features (see typical operating characteristics) modset and tc pin fault threshold 200 mv bias pin fault threshold a fault will be triggered if v bias is less than this voltage 300 400 mv excessive bias current fault a fault will be triggered if v mon2 exceeds this voltage 400 440 mv tx disable time t_off time from rising edge of tx_disable to i bias = i bias _off and i m od = i m od_off ( note 3) 0.06 5s tx disable negate time t_on time from falling edge of tx_disable to i bia s and i m od at 95% of stead y state ( n ote 3) 37 500 s reset initialization time t_init fr om p ow er on or neg ation of fau lt usi ng tx _d is able . ti me to set fault = l ow, i m od = 95% of stead y state and i bias = 95% of steady state ( n ote 3) 23 200 ms fault assert time t_fault time from fault to fault = high, c fault < 20pf, r fault = 4.7k ? (note 3) 14 50 s tx_disable reset t_reset time tx_disable must be held high to reset fault (note 3) 0.01 1s note 1: supply current excludes bias and modulation currents. note 2: deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a k28.5 bit pattern 00111110101100000101. note 3: ac characteristics guaranteed by design and characterization.
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver 4 _______________________________________________________________________________________ typical operating characteristics (v cc = 3.3v, t a = +25 c, unless otherwise noted.) 120mv/ div 64ps/div electrical eye diagram (i mod = 30ma, 2 7 - 1 prbs, 2.5gbps) max3996 toc01 25 ? load 120mv/ div 52ps/div electrical eye diagram (i mod = 30ma, 2 7 - 1 prbs, 3.2gbps) max3996 toc02 25 ? load 57ps/div optical eye diagram (i mod = 5ma, 850nm vcsel, 2 7 - 1 prbs, 2.5gbps, 1870mhz filter) max3996 toc03 57ps/div max3996 toc04 optical eye diagram (i mod = 15ma, 1310nm laser, 2 7 - 1 prbs, 2.5gbps, 1870mhz filter) 20 40 30 60 50 70 80 transition time vs. modulation current max3996 toc05 i mod (ma) transition time (ps) 51520 10 25 30 35 fall time rise time 0 10 5 20 15 25 30 deterministic jitter vs. modulation current max3996 toc06 i mod (ma) deterministic jitter (ps p-p ) 51520 10 25 30 35 total dj pwd 30 35 40 45 50 55 60 65 70 030 15 45 60 75 supply current vs. temperature (i mod = 15ma) max3996 toc07 ambient temperature ( c) supply current (ma) excludes i bias , i mod 25 ? load 10 100 10m 1m 100m 1 por delay vs. c pordly max3996 toc08 c pordly (f) por delay (s) 10p 1n 100p 10n 100n
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver _______________________________________________________________________________________ 5 laser ouput tx_disable v cc fault 10.0ms/div hot plug with tx_disable low max3996 toc09 3.3v t_init = 23ms 0v low low typical operating characteristics (continued) (v cc = 3.3v, t a = +25 c, unless otherwise noted.) laser ouput tx_disable v cc fault 10.0ms/div startup with slow ramping supply max3996 toc10 0v low low 3.3v laser ouput tx_disable v cc fault 20.0 s/div transmitter enable max3996 toc11 low low high 3.3v t_on = 37 s laser ouput tx_disable v cc fault 20.0ns/div transmitter disable max3996 toc12 low low high 3.3v t_off = 60ns electrical ouput fault v mon2 i bias 10.0 s/div response to fault max3996 toc13 on off low high t_fault = 14 s externally forced fault laser ouput tx_disable v tc fault 10.0 s/div fault recovery time max3996 toc14 external fault removed laser ouput tx_disable v tc fault 1.00ms/div frequent assertion of tx_disable max3996 toc15 externally forced fault ov
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver 6 _______________________________________________________________________________________ pin description pin name function 1tc temperature compensation set. the resistor at tc programs the temperature-increasing component of the laser-modulation current. 2 fault fault indicator. see table 1. 3, 9 gnd ground 4 tx_disable transmit disable. laser output is disabled when tx_disable is high or left unconnected. the laser output is enabled when this pin is asserted low. 5 pordly power-on reset delay. a capacitor connected between pordly and gnd can be used to extend the delay for the power-on reset circuit. see the design procedure section. 6, 16, 19 v cc supply voltage 7 in+ noninverting data input 8 in- inverting data input 10 mon1 attaches to the emitter of the bias driving transistor through a 10 ? resistor. see the design procedure section. 11 mon2 this pin attaches to the emitter of the bias driving transistor. see the design procedure section. 12 comp a capacitor connected from this pin to ground sets the dominant pole of the apc loop. see the design procedure section. 13 md monitor diode connection. md is used for automatic power control. 14 shdndrv shutdown driver output. provides a redundant laser shutdown. 15 bias laser bias current output 17 out+ positive modulation-current output. current flows from this pin when input data is high. 18 out- negative modulation-current output. current flows to this pin when input data is high. 20 modset a resistor connected from this pin to ground sets the desired modulation current. ep exposed pad ground. this must be soldered to the circuit board ground for proper thermal and electrical performance. see the layout considerations section.
detailed description the max3996 contains a bias generator with automatic power control and smooth start, a laser modulator, a power-on reset (por) circuit, and safety circuitry (figure 3). bias generator figure 4 shows the bias generator circuitry that con- tains a power-control amplifier, smooth-start circuitry, and two bias-fault sensors. the power-control amplifier combined with an internal npn transistor provides dc laser current to bias the laser in a light-emitting state. the apc circuitry adjusts the laser bias current to main- tain average power over temperature and changing laser properties. the smooth-start circuitry prevents current spikes to the laser during power-up or enable, ensuring compliance with safety requirements and extending the life of the laser. the md input is connected to the anode of a monitor diode, which is used to sense laser power. the bias output is connected to the cathode of the laser through an inductor or ferrite bead. the power-control amplifier drives a transistor to control the laser s bias current. in a fault condition (table 1), the base of the bias-driving transistor is pulled low to ensure that bias current is turned off. max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver _______________________________________________________________________________________ 7 max3996 modulation current generator v cc 3.0v to 5.5v i cc in+ in- 0.01 f 0.01 f v id r in r out r out v cc out- out+ 0.01 f 25 ? 0.01 f 25 ? i mod i out ferrite bead* r mod modset tc *murata blm11ha102sg figure 1. output load for ac specification i mod current v id = v in+ - v in- v in- v in+ volts time 100mv p-p min 1100mv p-p max 200mv p-p min 2200mv p-p max single-ended signal differential signal figure 2. required input signal and modulation-current polarity max3996 safety circuitry bias generator with smooth start modulation current generator modulation enable modulation fault 100 ? in+ in- input buffer laser modulation 50 ? 50 ? v cc out- out+ tc modset por circuit bias enable bias md comp mon1 mon2 pordly tx_disable v cc fault shdndrv figure 3. laser driver functional diagram max3996 r mon (11 ? ) 400mv 400mv bias disable 1.1v smooth start power-control amplifier bias fault 1 bias fault 2 md bias mon2 mon1 comp figure 4. bias circuitry
max3996 smooth-start during startup, the laser does not emit light, and the apc loop is not closed. the smooth-start circuit pulls the md pin to approximately 2.5v during the por delay and while tx_disable is high. this causes the power- control amplifier to shut off the bias transistor. when por delay is over and tx_disable is low, the md pin is released and pulled to gnd by r set because there is no laser power and thus no monitor diode current. the output voltage of the power-control amplifier then begins to increase. a capacitor attached to comp (c comp ) slows the slew rate and allows a controlled increase in bias current (figure 11). maxim recom- mends c comp = 0.1f. modulation circuitry the modulation circuitry consists of an input buffer, a current mirror, and a high-speed current switch (figure 5). the modulator drives up to 30ma of modulation cur- rent into a 25 ? load. many of the modulator performance specifications depend on total modulator current. to ensure good driver performance, the voltage at either out+ or out- must not be less than v cc - 1v. the amplitude of the modulation current is set with resis- tors at the modset and temperature coefficient (tc) pins. the resistor at modset (r mod ) programs the temperature-stable portion of the modulation current, and the resistor at tc (r tc ) programs the temperature- increasing portion of the modulation current. figure 6 shows modulation current as a function of temperature for two extremes: r tc is open (the modulation current has zero temperature coefficient), and r mod is open (the modulation temperature coefficient is 4000ppm/ c). intermediate temperature coefficient values of the mod- ulation current can be obtained as described in the design procedure section. table 2 is the r tc and r mod selection table. safety circuitry the safety circuitry contains a disable input, a fault latch, and fault detectors (figure 7). this circuitry moni- tors the operation of the laser driver and forces a shut- down if a single-point fault is detected. a single-point fault can be a short to v cc or gnd, or between any two 3.0v to 5.5v, 2.5gbps vcsel and laser driver 8 _______________________________________________________________________________________ pin fault condition mon2 v mon2 > 400mv bias v bias < 400mv tc, modset v modset or v tc < 200mv table 1. typical fault conditions max3996 100 ? in+ in- input buffer 50 ? 50 ? v cc out+ out- 1.2v reference 0ppm/ c modset fault 200mv modset r mod 1.2v reference 4000ppm/ c tc fault 200mv tc r tc current amplifier 96x enable modulation current generator current switch figure 5. modulation circuitry 0.6 0.8 0.7 1.0 0.9 1.2 1.1 1.3 0203040 10 50 60 70 80 90 100 110 junction temperature ( c) i mod /(i mod at +52 c) r tc 1.9k ? r mod = open tempco = 4000ppm/ c r tc = open tempco = 50ppm/ c figure 6. modulation current vs. temperature for maximum and minimum temperature coefficient
ic pins. see table 3 to view the circuit response to vari- ous single-point failures. the shutdown condition is latched until reset by a toggle of tx_disable or v cc. fault detection all critical nodes are monitored for safety faults, and any node voltage that differs significantly from its expected value results in a fault (table 1). when a fault condition is detected, the laser is shut down. see the applications information for more information on laser safety. shutdown the laser driver offers redundant bias shutdown. the shdndrv output drives an optional external transistor. the bias and modulation drivers have separate internal disable signals. max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver _______________________________________________________________________________________ 9 table 2. r tc and r mod selection table pin name circuit response to overvoltage or short to v cc circuit response to undervoltage or short to ground tc does not affect laser power. fault state* occurs. fault does not affect laser power. does not affect laser power. tx_disable modulation and bias current are disabled. normal condition for circuit operation. pordly does not affect laser power. modulation and bias current are disabled. in+, in- does not affect laser power. does not affect laser power. mon1 fault state* occurs. does not affect laser power. mon2 fault state* occurs. does not affect laser power. comp a fault is detected at either the collector or the emitter of the internal bias transistor, and a fault state* occurs. if the shutdown circuitry is used, bias current is shut off. disables bias current. md disables bias current. the apc circuit responds by increasing bias current until a fault is detected at the emitter or collector of the bias transistor, and then a fault* state occurs. shdndrv does not affect laser power. if the shutdown circuitry is used, bias current is shut off. 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, bias current is shut off. out+, out- does not affect laser power. does not affect laser power. modset does not affect laser power. fault* state may occur. fault state* occurs. table 3. circuit responses to various single-point faults * a fault state asserts the fault pin, disables the modulator outputs, disables the bias output, and asserts the shdndrv pin. i mod = 30ma i mod = 15ma i mod = 5ma tempco (ppm/?) r mod (k ? )r tc (k ? )r mod (k ? )r tc (k ? )r mod (k ? )r tc (k ? ) 3500 17.1 1.85 34.4 3.94 104 12.3 3000 8.04 2.19 16.3 4.64 49.5 14.4 2500 5.20 2.68 10.6 5.62 32.4 17.4 2000 3.81 3.42 7.86 7.08 24.1 21.8 1500 2.98 4.64 6.21 9.53 19.1 29.1 1000 2.44 7.08 5.12 14.4 15.9 43.8 500 2.05 14.4 4.34 29.1 13.5 87.8
max3996 latched fault output an open-collector fault output is provided with the max3996. this output is latched until the power is switched off, then on, or until tx_disable is switched to high and then low. power-on reset the max3996 contains an internal power-on reset delay to reject noise on v cc during power-on or hot- plugging. adding capacitance to the pordly pin can extend the delay. the por comparator includes hys- teresis to improve noise rejection. design procedure select laser select a communications-grade laser with a rise time of 260ps or better for 1.25gbps or 130ps or better for 2.5gbps applications. to meet the max3996 s ac specifications, the voltage at both out+ and out- must remain above v cc - 1v at all times. use a high-efficiency laser that requires low modulation current and generates a low voltage swing. trimming the leads can reduce laser package inductance. typical package leads have inductance of 25nh per inch (1nh/mm); this inductance causes a large voltage swing across the laser. a compensation filter network also can be used to reduce ringing, edge speed, and voltage swing. programming modulation current resistors at the modset and tc pins set the ampli- tude of the modulation current. the resistor r mod sets the temperature-stable portion of the modulation cur- rent, and the resistor (r tc) sets the temperature- increasing portion of the modulation current. to determine the appropriate temperature coefficient from the slope efficiency ( ) of the laser, use the following equation: for example, if a laser has a slope efficiency 25 = 0.021mw/ma, which reduces to 70 = 0.018mw/ma. using the above equation will produce a laser tempco of -3175ppm/ c. to obtain the desired modulation current and tempco for the device, the following equations can be used to determine the required values of r mod and r tc : where tempco = -laser tempco, 0 < tempco < 4000ppm/ c, and 2ma < i mod < 30ma. figure 8 shows a family of curves derived from these equations. the straight diagonal lines depict constant tempcos. the curved lines represent constant modula- tion currents. if no temperature compensation is desired, leave tc open, and the equation for i mod - simplifies considerably. the following equations were used to derive figure 8 and the equations at the beginning of this section. i rr r t c amps mod l mod tc = ++? + ? ? ? +? + ? ? ? ? 77 50 50 115 250 106 250 1 0 004 25 . . (.( )) r tempco i r tempco r tempco tc mod mod tc = ? = +? () ? ? ? ? ? ? ? ? 022 10 250 10 250 52 019 48 10 250 6 6 6 . / / ./ laser tempco ppm c cc _ [/] = () ? ? ? 70 25 25 6 70 25 10 3.0v to 5.5v, 2.5gbps vcsel and laser driver 10 ______________________________________________________________________________________ rq s fault latch bias fault 1 bias fault 2 tc fault modset fault bias enable modulator enable delay startup v cc pordly shdndrv fault tx_disable v bg figure 7. safety circuitry functional diagram
determine modulator configuration the max3996 can be used in several configurations. for modulation currents less than 20ma, maxim recom- mends the configuration shown in the typical application circuit . outputs greater than 20ma could cause the voltage at the modulator output to be less than v cc - 1v, which might degrade laser output. for large currents, maxim recommends the configuration in figure 9. a differential configuration is in figure 10. designing the bias filter and output pullup beads to reduce deterministic jitter, add a ferrite bead induc- tor (l1) between the bias pin and the cathode of the laser. select l1 to have an impedance >100 ? between f = 10mhz and f = 2ghz, and a dc resistance < 3 ? ; maxim recommends the murata blm11ha102sg. these inductors are also desirable for connecting the out+ and out- pins to v cc . programming laser power and bias fault threshold the ic is designed to drive a common anode laser with a photodiode. a servo-control loop is formed by the internal npn bias-driving transistor, the laser diode, the monitor diode (r set ), and the power-control amplifier (figure 11). the voltage at md is stabilized to 1.1v. the max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver ______________________________________________________________________________________ 11 1000 1 1 100 1000 10 r mod (k ? ) r tc (k ? ) 10 500ppm 1000ppm 1500ppm 2000ppm 3000ppm 2500ppm 3500ppm 5ma 10ma 25ma 20ma 30ma 15ma r l = 25 ? figure 8. r tc vs. r mod for various conditions max3996 0.01 f 0.01 f c pordly r tc r mod c comp n.c. 0.01 f 0.01 f l1* 25 ? optional shutdown circuitry 1.8k ? v cc v cc v cc tx_disable fault in+ in- pordly tc modset mon1 mon2 comp gnd md bias out+ out- shdndrv *ferrite bead r set v cc l2* v cc l3* 0.01 f figure 9. large modulation current max3996 0.01 f 0.01 f c pordly r tc r mod c comp n.c. 0.01 f 0.01 f l1* v cc v cc v cc tx_disable fault in+ in- pordly tc modset mon1 mon2 comp gnd md bias out- out+ shdndrv *ferrite bead r set l2* figure 10. differential configuration
max3996 monitor photodiode current is set by i d = v md /r set . determine the desired monitor current (i d ), and then select r set = 1.1v/i d . a bias stabilizing capacitor (c comp ) must be connect- ed between the comp pin and ground to obtain the desired apc loop time constant. this improves power- supply and ground noise rejection. a capacitance of 0.1f usually is sufficient to obtain time constants of up to 35s. the degeneration resistance between mon2 and ground determines the bias current that causes a fault and affects the apc time constant. select r mon (the total resistance between mon2 and ground) = 400mv/(maximum bias current). a degeneration resis- tance of 10 ? can be obtained by grounding mon1. increasing r mon increases the apc time constant. the discrete components for use with the common anode with photodiode configuration are: r set = 1.1v/i d c comp = 0.1f (typ) l1 = ferrite bead, see the bias filter section r mon = 400mv/(maximum bias current) programming por delay a capacitor can be added to pordly to increase the delay when powering up the part. the delay will be approximately: see the typical operating characteristics section. designing the laser-compensation filter network laser package inductance causes the laser impedance to increase at high frequencies, leading to ringing, overshoot, and degradation of the laser output. a laser- compensation filter network can be used to reduce the laser impedance at high frequencies, thereby reducing output ringing and overshoot. t c onds pordly = ? 14 10 6 . sec 3.0v to 5.5v, 2.5gbps vcsel and laser driver 12 ______________________________________________________________________________________ max3996 v cc i d r set monitor diode 1.1v power-control amplifier v cc shutdown circuit optional shutdown circuitry shdndrv laser l1* 11 ? smooth start bias disable md bias mon2 mon1 i bias comp c comp 0.1 f *ferrite bead figure 11. apc loop
the compensation components (r f and c f ) are most easily determined by experimentation. for interfacing with edge-emitting lasers, refer to application note hfan-2.0, interfacing maxim laser drivers with laser diodes . begin with r f = 50 ? and c f = 2pf. increase c f until the desired transmitter response is obtained (figure 12). using external shutdown to achieve single-point fault tolerance, maxim recom- mends an external shutdown transistor (figure 11). in the event of a fault, shdndrv asserts high, placing the shutdown transistor in cutoff mode and thereby shutting off the bias current. applications information laser safety and iec825 the international electrotechnical commission (iec) determines standards for hazardous light emissions from fiber optic transmitters. iec 825 defines the maxi- mum light output for various hazard levels. the max3996 provides features that facilitate compliance with iec825. a common safety precaution is single- point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. when this laser driver is used, as shown in the typical application circuit , the circuits respond to faults as listed in table 3. using this laser driver alone does not ensure that a transmitter design is compliant with iec825. the entire transmitter circuit and compo- nent selections must be considered. customers must determine the level of fault tolerance required by their applications, recognizing that maxim products are not designed or authorized for use as components in sys- tems 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. layout considerations the max3996 is a high-frequency product whose per- formance largely depends upon the circuit board layout. use a multilayer circuit board with a dedicated ground plane. use short laser-package leads placed close to the modulator outputs. power supplies must be capaci- tively bypassed to the ground plane, with surface-mount capacitors placed near the power-supply pins. the dominant pole of the apc circuit normally is at comp. to prevent a second pole in the apc that can lead to oscillations, ensure that parasitic capacitance at md is minimized (10pf). common questions laser output is ringing or contains overshoot. induc- tive laser packaging often causes this. try reducing the length of the laser leads. modify the filter components to reduce the driver s output edge speed (see the design procedure section). extreme ringing can be caused by low voltage at the out pins. this might indicate that pullup beads or a lower modulation current are needed. low-frequency oscillation on the laser output. this is more prevalent at low temperatures. the apc might be oscillating. try increasing the value of c comp or add additional degeneration by placing some resis- tance from mon1 to gnd. ensure that the parasitic capacitance at the md node is kept very small (<10pf). the apc is not needed. connect bias to v cc , leave md open, and connect mon2 and comp to ground. the modulator is not needed. leave tc and modset open. connect in+ to v cc , in- to ground through 750 ? , and leave out+ and out- open. interface models figures 13 17 show typical models for the inputs and outputs of the max3996, including package parasitics. max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver ______________________________________________________________________________________ 13 time power uncompensated correctly compensated overcompensated figure 12. laser compensation max3996 4k ? fault note: the fault pin is an open-collector output figure 13. fault output
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver 14 ______________________________________________________________________________________ max3996 550 ? 60 ? 10k ? v cc shdndrv figure 14. shdndrv output max3996 package out- 1.1nh 0.15pf 1pf 50 ? 50 ? v cc v cc 1pf out+ package 1.1nh 0.15pf figure 15. modulator outputs
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver ______________________________________________________________________________________ 15 max3996 package in+ 1.1nh 0.15pf 1pf v cc v cc in- 1.1nh 0.15pf 1pf v cc 100 ? figure 16. data inputs v cc v cc v cc max3996 bias mon2 mon1 11 ? figure 17. bias output 20 qfn (4mm x 4mm) top view 1 2 3 4 5 678910 11 12 13 14 15 16 17 18 19 20 tc fault gnd v cc v cc v cc in+ in- gnd mon1 tx_disable pordly mon2 comp md shdndrv bias modset out- out+ max3996 exposed pad is connected to gnd 20 thin qfn (4mm x 4mm) top view 1 2 3 4 5 678910 11 12 13 14 15 16 17 18 19 20 tc fault gnd v cc v cc v cc in+ in- gnd mon1 tx_disable pordly mon2 comp md shdndrv bias modset out- out+ max3996 exposed pad is connected to gnd pin configurations chip information transistor count: 1061 process: silicon bipolar
max3996 3.0v to 5.5v, 2.5gbps vcsel and laser driver 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 ? 2004 maxim integrated products printed usa is a registered trademark of maxim integrated products. part package type package code max3996cgp 20 qfn 4mm x 4mm x 0.9mm g2044-3 MAX3996CTP + 20 thin qfn 4mm x 4mm x 0.8mm t2044-3 package information for the latest package outline information, go to www.maxim-ic.com/packages .

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