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general description the max3740a is a high-speed vcsel driver for small- form-factor (sff) and small-form-factor pluggable (sfp) fiber optic lan transmitters. it contains a bias genera- tor, a laser modulator, and comprehensive safety fea- tures. the automatic power control (apc) adjusts the laser bias current to maintain average optical power over changes in temperature and laser properties. the driver accommodates common cathode and differential configurations. the max3740a operates up to 3.2gbps. it can switch up to 15ma of laser modulation current and source up to 15ma of bias current. adjustable temperature com- pensation is provided to keep the optical extinction ratio within specifications over the operating tempera- ture range. the max3740a interfaces with the dallas ds1858 to meet sff-8472 timing and diagnostic requirements. the max3740a accommodates various vcsel packages, including low-cost to-46 headers. the max3740a safety circuit detects faults that could cause hazardous light levels and disables the vcsel output. the safety circuits are compliant with sff and sfp multisource agreements (msa). the max3740a is available in a compact 4mm ? 4mm, 24-pin thin qfn package and operates over the -40? to +85? temperature range. applications multirate (1gbps to 3.2gbps) sfp/sff modules gigabit ethernet optical transmitters fibre channel optical transmitters infiniband optical transmitters features ? supports all sff-8472 digital diagnostics ? 2ma to 15ma modulation current ? 1ma to 15ma bias current ? optional peaking current to improve vcsel edge speed ? supports common cathode and differential configuration ? automatic power control ? safety circuits compliant with sff and sfp msas ? 4mm ? 4mm, 24-pin thin qfn package max3740a 3.2gbps sfp vcsel driver with diagnostic monitors ________________________________________________________________ maxim integrated products 1 ordering information v cc ref comp md bias out+ out- modset biasset r biasset pwrmon tc1 tc2 in+ in- tx_disable fault squelch gnd 0.01 f 0.01 f 50 l1* 0.047 f +3.3v 0.1 f 0.1 f r tc ? ? optional component *ferrite bead peakset r peakset ? c f ? r f ? biasmon r biasmon 4.7k ? r pwrset r modset max3740a 19-3118; rev 3; 1/10 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package max3740aetg -40c to +85c 24 thin qfn-ep* max3740aetg + -40c to +85c 24 thin qfn-ep* typical application circuit + denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad.
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 2 _______________________________________________________________________________________ absolute maximum ratings 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 voltage at tx_disable, in+, in-, fault, squelch, tc1, tc2, modset, peakset, biasset, bias, biasmon, comp, md, ref, pwrmon ...............................................-0.5v to (v cc + 0.5v) voltage at out+, out- .........................(v cc - 2v) to (v cc + 2v) current into fault ............................................ -1ma to +25ma current into out+, out- ....................................................60ma continuous power dissipation (t a = +85?) 24-lead thin qfn (derate 20.8mw/? above +85?)................................. 1354mw operating temperature range ...........................-40? to +85? storage temperature range .............................-55? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics (v cc = +2.97v to +3.63v, t a = -40? to +85?. typical values are at v cc = +3.3v, tc1 and tc2 are shorted, peakset open, t a = +25?, unless otherwise noted.) parameter symbol conditions min typ max units i mod = 2ma p-p 32 squelch set low, tx_disable set low, peaking is not used (note 1) i mod = 15ma p-p 55 68 additional current when peaking is used (note 2) 15 20 i cc additional current when squelch is high 5 10 supply current i cc-shdn total current when tx_disable is high 3.9 5 ma fault output output high voltage v oh r load = 10k to 2.97v 2.4 v output low voltage v ol r load = 4.7k to 3.63v 0.4 v tx_disable input input impedance 4.7 10.0 k input high voltage v ih 2.0 v input low voltage v il 0.8 v power-down time the time for i cc to reach i cc-shdn when tx_disable transitions high 50 ? squelch squelch threshold 25 85 mv p-p squelch hysteresis 10 mv p-p time to squelch data (note 3) 0.02 5.00 ? time to resume from squelch (note 3) 0.02 5.00 ? bias generator (note 4) minimum 1 bias current i bias maximum 15 ma 5ma i bias 15ma -8 +8 accuracy of programmed bias current bias 1ma i bias 5ma -12 +12 %
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors _______________________________________________________________________________________ 3 electrical characteristics (continued) (v cc = +2.97v to +3.63v, t a = -40? to +85?. typical values are at v cc = +3.3v, tc1 and tc2 are shorted, peakset open, t a = +25?, unless otherwise noted.) parameter symbol conditions min typ max units bias current during fault i bias_off current out of the bias pin 1.5 10 ? 1ma < i bias < 3ma 0.0875 0.105 0.1375 biasmon gain 3ma i bias 15ma 0.085 0.105 0.125 ma/ma biasmon stability (notes 5,6) -10 +10 % automatic power control (apc) md nominal voltage v md apc loop is closed 1 v ref - 0.16 2v voltage at ref v ref 1.2 1.8 2.2 v md voltage during fault 0v md input current normal operation (fault = low) -2 0.7 +2 ? apc time constant c comp = 0.047? (note 6) 5 20 s pwrmon nominal gain v pwrmon / (v ref - v md ) 1.85 2.15 2.45 v/v laser modulator (note 7) minimum 250 data input voltage swing v id maximum 2200 mv p - p single-ended resistance at out+ 80 105 output resistance single-ended resistance at out- 72 100 minimum 2 modulation current i mod maximum 15 ma p - p minimum peaking current range 0.2 ma maximum peaking current range 2ma peaking current duration 80 ps tolerance of programmed modulation current tc1 is shorted to tc2 -10 +10 % minimum programmable temperature coefficient 0 ppm/? maximum programmable temperature coefficient temperature range 0 c to +70 c +5000 ppm/? modulation transition time t r , t f 5ma i mod 15ma, 20% to 80% (note 6) 65 95 ps deterministic jitter dj 5ma i mod 15ma, 3.2gbps (notes 6, 8) 12 20 ps p-p random jitter rj (note 6) 1.3 4 ps rms laser modulation during fault or while squelch is active i mod_off 15 50 ? p-p input resistance differential resistance 85 100 115 input bias voltage v in v cc - 0.3 v
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 4 _______________________________________________________________________________________ electrical characteristics (continued) (v cc = +2.97v to +3.63v, t a = -40 c to +85 c. typical values are at v cc = +3.3v, tc1 and tc2 are shorted, peakset open, t a = +25 c, unless otherwise noted.) parameter symbol conditions min typ max units safety features (see the typical operating characteristics section) high-current fault threshold v bmth v biasmon > v bmth causes a fault 0.7 0.8 0.9 v v bias fault threshold v bth v bias referenced to v cc -0.250 -0.2 -0.150 v power-monitor fault threshold v pmth v pwrmon > v pmth causes a fault 0.7 0.8 0.9 v tx disable time t_ off time from rising edge of tx_disable to i bias = i bias_off and i mod = i mod_off (note 6) 1.8 5 s tx disable negate time t_ on time from rising edge of tx_disable to i bias and i mod at 99% of steady state (note 6) 55 500 ? fault reset time t_ init 1 time to set v fault = low after power-on or after rising edge of tx_disable (note 6) 60 200 ms power-on time t_ init 2 time after power-on to transmitter-on with tx_disable low (note 6) 60 200 ms fault assert time t_ fault time from fault occurrence to v fault = high; c fault < 20pf, r fault = 4.7k (note 6) 1.4 50 ? fault delay time t_ fltdly time from fault to i bias = i bias_off and i mod = i mod_off (note 6) 15s tx_disable reset t_ reset time tx_disable must be held high to reset fault (note 6) 1s note 1: supply current measurements exclude i bias from the total current. note 2: tested with r peak = 1.18k . note 3: measured by applying a pattern that contains 20? of k28.5, followed by 5? of zeros, then 20? of k28.5, followed by 5? of ones. data rate is equal to 2.5gbps, with inputs filtered using 1.8ghz bessel filters. note 4: v bias < v cc - 0.7v. note 5: variation of bias monitor gain for any single part over the range of v cc , temperature, 3ma < i bias < 15ma. note 6: guaranteed by design and characterization. note 7: measured electrically with a 50 load ac-coupled to out+. note 8: deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a k28.5 bit pattern at 3.2gbps (00111110101100000101).
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors _______________________________________________________________________________________ 5 typical operating characteristics (v cc = +3.3v, r tc = 0 , peakset open, measured electrically with a 50 load ac-coupled to out+, t a = +25 c, unless otherwise noted.) electrical eye max3740a toc01 50ps/div 73mv/div 3.2gbps, k28.5, 10ma modulation, peaking off electrical eye with peaking max3740a toc02 50ps/div 73mv/div 3.2gbps, k28.5, 10ma modulation, r peakset = 2.4k electrical eye with max peaking max3740a toc03 50ps/div 73mv/div 3.2gbps, k28.5, 10ma modulation, r peakset = 500 optical eye max3740a toc04 68ps/div e r = 8.2db, 2.125gbps, k28.5, 850nm vcsel, with 2.3ghz o-to-e converter emcore sc-tosa-8585-3420 vcsel optical eye max3740a toc05 58ps/div e r = 8.2db, 2.5gbps, k28.5, 850nm vcsel sonet mask with +20% margin emcore sc-tosa-8585-3420 vcsel i biasmon vs. bias current max3740a toc06 bias current (ma) i biasmon (ma) 12 8 4 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 016 deterministic jitter vs. modulation current max3740a toc07 i mod (ma p-p ) deterministic jitter (ps p-p ) 10 5 5 10 15 20 25 30 35 40 0 015 random jitter vs. modulation current max3740a toc08 i mod (ma p-p ) random jitter (ps rms ) 10 5 1 2 3 4 5 6 7 0 015 transition time vs. modulation current max3740a toc09 i mod (ma p-p ) transition time (ps) 14 12 10 8 6 4 50 60 70 80 90 100 40 216 rise fall
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 6 _______________________________________________________________________________________ typical operating characteristics (continued) (v cc = +3.3v, r tc = 0 , peakset open, measured electrically with a 50 load ac-coupled to out+, t a = +25 c, unless otherwise noted.) bias current vs. r biasset max3740a toc10 r biasset ( ) bias current (a) 10k 1m 10m 100m 100 1k 100k modulation current vs. r modset max3740a toc11 r modset ( ) modulation current (a p-p ) 1k 10m 100m 1m 10k 100 measured with a 50 electrical load monitor diode current vs. r pwrset max3740a toc12 r pwrset ( ) monitor diode current (a) 1k 10 100 1m 10m 1 100 10k supply current vs. temperature max3740a toc13 temperature ( c) supply current (ma) 60 35 10 -15 20 30 40 50 60 70 80 10 -40 85 i mod = 2ma i mod = 15ma input return loss max3740a toc14 frequency (hz) s 11 (db) 1g -35 -30 -25 -20 -15 -10 -5 0 -40 100m 10g differential measurement output return loss max3740a toc15 frequency (hz) s 22 (db) 1g -16 -14 -12 -10 -8 -6 -4 -2 0 -18 100m 10g single-ended measurement modulation current vs. temperature max3740a toc16 temperature ( c) modulation current (ma p-p ) 80 70 60 50 40 30 20 10 5 6 7 8 9 10 11 4 090 r mod = 1.35k r tc = 1k r tc = 500k r tc = 100k r tc = 60k r tc = 10k r tc = 5k r tc = 100 modulation current tempco vs. r tc max3740a toc17 r tc ( ) tempco (ppm/ c) 100k 10k 1k 500 1500 2500 3500 4500 5500 -500 100 1m referenced to +25 c monitor diode current vs. temperature max3740a toc18 temperature ( c) monitor diode current ( a) 60 35 -15 10 125 150 175 200 225 250 275 300 100 -40 85
hot plug with tx_disable low max3740a toc19 20ms/div tx_disable v cc laser output fault low t_init = 60ms 3.3v low ov max3740a 3.2gbps sfp vcsel driver with diagnostic monitors _______________________________________________________________________________________ 7 startup with slow ramping supply max3740a toc20 20ms/div tx_disable v cc laser output fault low t_init = 62ms 3.3v low ov tx_disable negate time max3740a toc21 20 s/div tx_disable v cc laser output fault high t_on = 54 s 3.3v low low transmitter disable max3740a toc22 1 s/div tx_disable v cc laser output fault low t_off = 1.86 s 3.3v low high response to fault max3740a toc23 200ns/div tx_disable v pwrmon laser output fault low t_fault = 245ns low high externally forced fault fault recovery time max3740a toc24 40 s/div tx_disable v pwrmon laser output fault low t_init = 54 s high external fault removed low low high frequent assertion of tx_disable max3740a toc25 200 s/div tx_disable v pwrmon laser output fault externally forced fault typical operating characteristics (continued) (v cc = +3.3v, r tc = 0 , peakset open, measured electrically with a 50 load ac-coupled to out+, t a = +25 c, unless otherwise noted.)
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 8 _______________________________________________________________________________________ pin description pin name function 1, 10, 13 gnd ground 2 tx_disable transmit disable. driver output is disabled when tx_disable is high or left unconnected. the driver output is enabled when the pin is asserted low. 3 in+ noninverted data input 4 in- inverted data input 5 fault fault indicator. open-drain output with esd protection. fault is asserted high during a fault condition. 6 squelch squelch enable. squelch is enabled when the pin is set high. squelch is disabled when the pin is set low or left open. 7, 16, 20 v cc +3.3v supply voltage 8 tc1 temperature compensation set pin 1. a resistor placed between tc1 and tc2 (r tc ) programs the temperature coefficient of the modulation current. 9 tc2 temperature compensation set pin 2. a resistor placed between tc1 and tc2 (r tc ) programs the temperature coefficient of the modulation current. 11 modset modulation set. a resistor connected from modset to ground (r modset ) sets the desired modulation current amplitude. 12 peakset peaking current set. a resistor connected between peakset and ground (r peakset ) programs the peaking current amplitude. to disable peaking, leave peakset open. 14 out- inverted modulation-current output 15 out+ noninverted modulation-current output 17 biasset bias current set. when a closed-loop configuration is used, connect a 1.7k resistor between ground and biasset to set the maximum bias current. when an open configuration is used, connect a resistor between biasset and ground (r biasset ) to program the vcsel bias current. 18 bias bias current output 19 biasmon bias current monitor. the output of biasmon is a sourced current proportional to the bias current. a resistor connected between biasmon and ground (r biasmon ) can be used to form a ground- referenced bias monitor. 21 comp compensation pin. a capacitor between comp and md compensates the apc. a typical value of 0.047f is recommended. for open-loop configuration, short the comp pin to gnd to deactivate the apc. 22 md monitor diode connection 23 ref reference pin. reference monitor used for apc. a resistor between ref and md (r pwrset ) sets the photo monitor current when the apc loop is closed. 24 pwrmon average power monitor. the pin is used to monitor the transmit optical power. for open-loop configuration, connect pwrmon to gnd. ep exposed pad. ground. must be soldered to the circuit board ground for proper thermal and electrical performance. see the layout considerations section.
detailed description the max3740a contains a bias generator with automat- ic power control (apc), safety circuit, and a laser mod- ulator with optional peaking compensation. bias generator figure 1 shows the bias generator circuitry that contains a power-control amplifier and smooth-start circuitry. an internal pnp transistor provides dc laser current to bias the laser in a light-emitting state. the apc circuitry adjusts the laser-bias current to maintain 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 cathode of a monitor diode, which is used to sense laser power. the bias output is connected to the anode of the laser through an inductor or ferrite bead. the power-control amplifier dri- ves a current amplifier to control the laser? bias current. during a fault condition, the bias current is disabled. the pwrmon output provides a voltage proportional to average laser power given by: v pwrmon = 2 ? i pd ? r pwrset the biasmon output provides a current proportional to the laser bias current given by: i biasmon = i bias / 9 when apc is not used (no monitor diode, open-loop configuration) connect the comp and pwrmon pins to gnd. in this mode, the bias current is set by the resistor r biasset . when a closed-loop configuration is used, connect a 1.7k resistor between ground and biasset to set the maximum bias current. safety circuit the safety circuit contains an input disable (tx_disable), a latched fault output (fault), and fault detectors (figure 2). this circuit monitors the operation of the laser driver and forces a shutdown (disables laser) if a fault is detected (table 1). table 2 contains 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 fault pin should be pulled high with a 4.7k to 10k resistor. max3740a 3.2gbps sfp vcsel driver with diagnostic monitors _______________________________________________________________________________________ 9 c comp comp biasset r pwrset md 1.8v ref current amplifier power- control amplifier enable 200 r biasset pwrmon bias generator smooth- start r biasmon biasmon ferrite bead bias 1.6v (2v be ) 2x i bias 34 i pd i bias 9 max3740a 1.2v figure 1. bias generator pin fault condition bias v bias > v cc - 0.2v biasmon v biasmon > 0.8v pwrmon v pwrmon > 0.8v table 1. fault conditions
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 10 ______________________________________________________________________________________ pin name circuit response to v cc short circuit response to gnd short fault does not affect laser power. does not affect laser power. tx_disable modulation and bias current are disabled. normal condition for circuit operation. in+ does not affect laser power. does not affect laser power. in- does not affect laser power. does not affect laser power. squelch does not affect laser power. does not affect laser power. tc1 does not affect laser power. does not affect laser power. tc2 the laser modulation is increased, but average power is not affected. modulation current is disabled. modset modulation current is disabled. the laser modulation is increased, but average power is not affected. peakset does not affect laser power. does not affect laser power. out+ modulation current is disabled. modulation current is disabled. out- does not affect laser power. does not affect laser power. biasset laser bias is disabled. fault state* occurs. bias fault state* occurs. note that vcsel emissions may continue; care must be taken to prevent this condition. disables vcsel. biasmon fault state* occurs. does not affect laser power. comp the bias current is reduced, and the average power of the laser output is reduced. i bias increases to the value determined by r biasset ; if the bias monitor fault threshold is exceeded, a fault is signaled. md i bias increases to the value determined by r biasset ; if the bias-monitor fault threshold is exceeded, a fault is signaled. the bias current is reduced, and the average power of the laser output is reduced. ref i bias increases to the value determined by r biasset ; if the bias-monitor fault threshold is exceeded, a fault is signaled. the bias current is reduced, and the average power of the laser output is reduced. pwrmon fault state* occurs. does not affect laser power. table 2. circuit response to various single-point faults (closed-loop apc configuration) * a fault state asserts the fault pin, disables the modulator output, and disables the bias output. modulation circuit the modulation circuitry consists of an input buffer, a current mirror, and a high-speed current switch (figure 3). the modulator drives up to 15ma of modulation into a 50 vcsel load. the amplitude of the modulation current is set with resistors at modset and temperature coefficient (tc1, tc2) pins. the resistor at modset (r modset ) pro- grams the temperature-stable portion of the modulation current, and the resistor between tc1 and tc2 (r tc ) programs the temperature coefficient of the modulation current. for appropriate r tc and r modset values, see the typical operating characteristics section. 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. use a high-efficiency laser that requires low modulation current and generates a low- voltage swing. trim the leads to reduce laser package inductance. the typical package leads have induc- tance of 25nh per inch (1nh/mm). this inductance causes a large voltage swing across the laser. a com- pensation filter network can also be used to reduce ringing, edge speed, and voltage swing (see the designing the compensation filter network section).
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors ______________________________________________________________________________________ 11 s r q r-s latch high-power fault high-current fault v bias fault bias biasmon pwrmon por tx_disable tx_disable 0.8v 0.8v v cc - 0.2v fault enable safety circuit fault output max3740a figure 2. safety circuit v cc r out+ 100 current amplifier 30x enable in+ in- out+ out- tc1 modset r tc r modset modulation current generator input buffer current switch temperature compensation tc2 signal detect squelch peaking control peakset 200 r out- 1v r peakset max3740a figure 3. modulation circuit
programming modulation current the modulation current output of the max3740a is con- trolled by a resistor (r modset ) placed between modset and ground. the r modset resistor controls the amount of current being sourced to the vcsel. the modulation current is given by the following: it is important to note that the modulation current being sourced by the max3740a is affected by the load impedance of the vcsel. the modulation current vs. r modset graph in the typical operating characteristics shows the current into a 50 electrical load. programming bias current the bias current output of the max3740a is controlled by a resistor (r biasset ) placed between biasset and ground. in open-loop operation the r biasset controls the bias current level of the vcsel. in closed-loop operation the r biasset controls the maximum bias cur- rent provided by the apc. the bias current is given by the following: the bias current vs. r biasset graph is also shown in the typical operating characteristics . photodiode selection to ensure stable operation of the apc circuit, the time constant of the md node should be shorter than the apc time constant. (t apc = 5? if c apc = 0.047?). for typical i pd = 400?, r pwrset = 500 , select a photodiode with capacitance less than 500pf. programming modulation-current tempco compute the required modulation tempco from the slope efficiency of the laser at t a = +25 c and at a higher temperature. then select the value of r tc from the typical operating characteristics . for example, suppose a laser has a slope efficiency (se) of 0.021mw/ma at +25 c, which reduces to 0.018mw/ma at +85 c. the temperature coefficient is given by the following: from the typical operating characteristics , the value of r tc , which offsets the tempco of the laser, is 9k . if modulation temperature compensation is not desired, short tc1 and tc2. programming the apc loop program the average optical power by adjusting r pwrset . to select the resistance, determine the desired monitor current to be maintained over tempera- ture and lifetime. see the monitor diode current vs. r pwrset graph in the typical operating characteristics section, and select the value of r pwrset that corre- sponds to the required current. input termination requirements the max3740a data inputs are sfp msa compatible. on-chip 100 differential input impedance is provided for optimal termination (figure 4). because of the on-chip biasing network, the max3740a inputs self-bias to the proper operating point to accommodate ac-coupling. laser tempco se se se e ppm c = ? ? =? () () / 85 25 25 85 25 16 2380 t md t apc md md rc s ns ? = 20 5 20 250 , ii i r bias biasset bias biasset = () = + ? ? ? ? ? ? 34 12 200 34 . ii r rr i r r rr mod modset out out load mod modset out out load = () [] + ? ? ? ? ? ? = + ? ? ? ? ? ? ? ? ? ? ? ? ? ? + ? ? ? ? ? ? + + + + 30 1 200 30 max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 12 ___________________________________________________ max3740a in+ in- 1nh 1nh 0.5pf 0.5pf v cc 50 50 16k 24k v cc v cc package figure 4. simplified input structure
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors ______________________________________________________________________________________ 13 applications information interface models figures 4 and 5 show simplified input and output circuits for the max3740a laser driver. figure 6 shows the fault circuit interface. layout considerations to minimize inductance, keep the connections between the max3740a output pins and laser diode as short as possible. use good high-frequency layout techniques and multilayer boards with uninterrupted ground planes to minimize emi and crosstalk. designing the compensation filter network laser package inductance causes the laser impedance to increase at high frequencies, leading to ringing, over- shoot, and degradation of the laser output. a laser com- pensation filter network can be used to reduce the laser impedance at high frequencies, thereby reducing output ringing and overshoot. the compensation components (r f and c f ) are most easily determined by experimentation. begin with r f = 50 and c f = 1pf. increase c f until the desired trans- mitter response is obtained (figure 7). refer to application note hfan-2-0: interfacing maxim laser drives with laser diodes for more information. exposed-pad (ep) package the exposed pad on the 24-pin thin qfn provides a very low thermal resistance path for heat removal from the ic. the pad is also electrical ground on the max3740a and must be soldered to the circuit board ground for proper thermal and electrical performance. refer to maxim application note hfan-08.1: thermal considerations for qfn and other exposed-pad packages for additional information. laser safety and iec 825 the international electrotechnical commission (iec) determines standards for hazardous light emissions from fiber optic transmitters. iec 825 defines the maximum light output for various hazard levels. the max3740a provides features that facilitate compliance with iec 825. a common safety precaution is single-point fault toler- ance, whereby one unplanned short, open, or resistive connection does not cause excess light output. using this laser driver alone does not ensure that a transmitter design is compliant with iec 825. the entire transmitter circuit and component selections must be considered. customers must determine the level of fault tolerance required by their applications, recognizing that maxim products are not designed or authorized for use as com- ponents in systems intended for surgical implant into the body, for applications intended to support or sustain life, or for any other application where the failure of a maxim product could create a situation where personal injury or death may occur. v cc max3740a package 1nh 1nh 0.5pf 0.5pf out- out+ r out+ r out- figure 5. simplified output structure time power uncompensated correctly compensated overcompensated figure 7. laser compensation max3740a v cc fault figure 6. fault circuit interface
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 14 ______________________________________________________________________________________ 24 23 22 21 20 19 pwrmon ref md comp v cc biasmon 7 8 9 10 11 12 v cc tc1 tc2 gnd modset peakset 13 14 15 16 17 18 *exposed pad is connected to gnd *ep gnd out- out+ v cc biasset bias 6 5 4 3 2 1 squelch fault in- in+ tx_disable gnd max3740a thin qfn (4mm x 4mm) top view pin configuration chip information transistor count: 3806 process: sige bipolar package information for the latest package outline information and land patterns, 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 pertains to the package regardless of rohs status. safety circuitry tx_disable fault v cc in+ in- out- out+ modulation current generator bias generator with apc enable enable 100 laser modulator peaking control comp md ref pwrmon bias biasset biasmon peakset tc1 tc2 modset squelch signal detect max3740a functional diagram package type package code document no. 24 tqfn-ep (4mm x 4mm x 0.75mm) t2444-4 21-0139
max3740a 3.2gbps sfp vcsel driver with diagnostic monitors 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 2010 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. revision history revision number revision date description pages changed 0 12/03 initial release. 1 6/04 added a lead-free package to the ordering information table. 1 in the electrical characteristics table, modified the md nominal voltage parameter of v ref - 0.2v (typ) to v ref - 0.16v (typ). 3 2 5/06 modified figure 1 to clarify the meaning of the arrow labeled i pd . 9 3 1/10 updated the package information section to correct the package code. 14

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