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general description the max6397/max6398 are small, high-voltage overvolt- age protection circuits. these devices disconnect the output load or limit the output voltage during an input overvoltage condition. these devices are ideal for appli- cations that must survive high-voltage transients such as those found in automotive and industrial applications. the max6397/max6398 monitor the input or output voltages and control an external n-channel mosfet to isolate or limit the load from overvoltage transient energy. when the monitored input voltage is below the user- adjustable overvoltage threshold, the external n-channel mosfet is turned on by the gate output. in this mode, the internal charge pump fully enhances the n-channel mosfet with a 10v gate-to-source voltage. when the input voltage exceeds the overvoltage thresh- old, the protection can disconnect the load from the input by quickly forcing the gate output low. in some applications, disconnecting the output from the load is not desirable. in these cases, the protection circuit can be configured to act as a voltage limiter where the gate output sawtooths to limit the voltage to the load. the max6397 also offers an always-on linear regulator that is capable of delivering up to 100ma of output current. this high-voltage linear regulator consumes only 37? of quiescent current. the regulator is offered with output options of 5v, 3.3v, 2.5v, or 1.8v. an open-drain, power-good output (pok) asserts when the regulator output falls below 92.5% or 87.5% of its nominal voltage. the max6397/max6398 include internal thermal-shut- down protection, disabling the external mosfet and linear regulator if the chip reaches overtemperature conditions. the devices operate over a wide 5.5v to 72v supply voltage range, are available in small tdfn packages, and are fully specified from -40? to +125?. applications automotive industrial firewire notebook computers wall cube power devices features ? 5.5v to 72v wide supply voltage range ? overvoltage protection controllers allow user to size external n-channel mosfets ? internal charge-pump circuit ensures mosfet gate-to-source enhancement for low r ds(on) performance ? disconnect or limit output from input during overvoltage conditions ? adjustable overvoltage threshold ? thermal-shutdown protection ? always-on, low-current (37?) linear regulator sources up to 100ma (max6397) ? fully specified from -40? to +125? (t j ) ? small, thermally enhanced 3mm x 3mm tdfn package max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v ________________________________________________________________ maxim integrated products 1 max6397 8 reg 6 gate 7 out 5 gnd 3 shdn 2 1 in tdfn top view *ep 4 pok set *exposed pad. connect to gnd. pin configurations ordering information 19-3668; rev 3; 1/07 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. * replace ?t?with ?t?for lead-free packages. ** ep = exposed pad. the max6397 linear regulator is offered in four output voltage options and a choice of a 92.5% or 87.5% pok threshold assertions. see the selector guide. part temp range pin-package pkg code max6397 _ata-t* -40? to +125? 8 tdfn-ep** t833-2 max6398 att-t* -40? to +125? 6 tdfn-ep** t633-2 selector guide and typical operating circuit appear at end of data sheet. pin configurations continued at end of data sheet. firewire is a registered trademark of apple computer, inc.
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v in = 14v; c gate = 6000pf, c reg = 4.7?, t a = t j = -40? to +125?, unless otherwise noted. typical values are at t a = t j = +25?.) (note 1) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional oper- ation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications i s not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. (all pins referenced to gnd, unless otherwise noted.) in, gate, out ............................................................-0.3v to +80v shdn ..................................................................-0.3v to (in + 0.3v) gate to out .................................................................-0.3 to +20v set, reg, pok ...........................................................-0.3v to +12v maximum current: in, reg...............................................................................350ma all remaining pins ...................................................................50ma continuous power dissipation (t a = +70?) 6-pin tdfn (derate 18.2mw/? above +70?) .............1455mw 8-pin tdfn (derate 18.2mw/? above +70?) .............1455mw operating temperature range (t a ) ......................-40? to +125? junction temperature ...........................................................+150? storage temperature range .................................-65? to +150? lead temperature ................................................................+300? parameter symbol conditions min typ max units supply voltage range v in 5.5 72 v shdn = high, no load (max6397) 118 140 shdn = high, (max6398) 104 130 shdn = low, no load (max6397) 37 45 input supply current shdn = low, (max6398) 11 20 ? in undervoltage lockout v in rising, enables gate 4.66 5 5.50 v in undervoltage lockout hysteresis v in falling, disables gate 175 mv set threshold voltage v th with respect to gnd 1.181 1.215 1.248 v set threshold hysteresis v hyst 4% set input current i set -50 +50 na startup response time t start shdn rising (note 2) 100 ? gate rise time gate rising from gnd to v out + 8v, c gate = 6000pf, out = gnd 1ms set to gate propagation delay t ov set rising from v th - 100mv to v th + 100mv 0.75 ? v out = v in = 6v, r gate to in = 1m ? v in + 3.8v v in + 4.2v v in + 4.6v gate output high voltage v oh v out = v in ; v in 14v, r gate to in = 1m ? v in + 8.5v v in + 9.2v v in + 11.5v v gate output low voltage v ol gate sinking 20ma, v out = gnd 0.38 v gate charge-pump current i gate gate = gnd 75 ? gate to out clamp voltage v clmp 13 18 v shdn logic-high input voltage v ih 1.4 shdn logic-low input voltage v il 0.4 shdn input pulldown current v shdn = 2v, shdn is internally pulled down to gnd 1a thermal shutdown (note 3) +150 ? thermal shutdown hysteresis 20 ? regulator (max6397) i reg = 1ma 40 48 ground current i gnd shdn = gnd i reg = 100ma 60 ?
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v _______________________________________________________________________________________ 3 electrical characteristics (continued) (v in = 14v; c gate = 6000pf, c reg = 4.7?, t a = t j = -40? to +125?, unless otherwise noted. typical values are at t a = t j = +25?.) (note 1) note 1: specifications to -40c are guaranteed by design and not production tested. note 2: the max6397/max6398 power up with the external fet in off mode (v gate = gnd). the external fet turns on t start after the device is powered up and all input conditions are valid. note 3: for accurate overtemperature shutdown performance, place the device in close thermal contact with the external mosfet. note 4: dropout voltage is defined as v in - v reg when v reg is 2% below the value of v reg for v in = v reg (nominal) + 2v. note 5: operations beyond the thermal dissipation limit may permanently damage the device. parameter symbol conditions min typ max units i reg = 1ma 4.925 5 5.120 max6397l/m 1ma < i reg < 100ma 4.85 5.15 i reg = 1ma 3.243 3.3 3.360 max6397s/t 1ma < i reg < 100ma 3.201 3. 360 i reg = 1ma 2.456 2.5 2.542 max6397y/z 1ma < i reg < 100ma 2.41 2.55 v i reg = 1ma 1.760 1.8 1.837 reg output voltage (v in v reg + 1.8v) v reg max6397v/w 1ma < i reg < 100ma 1.715 1.837 5.5v v in 72v, i reg = 1ma, v reg = 5v 0.12 dropout voltage (note 4) ? v do 5.5v v in 72v, i reg = 100ma, v reg = 5v 1.2 mv/v current limit v in = 14v 150 300 ma overvoltage-protection threshold v ovp 105 % of v reg o ver vol tag e- p r otecti on s i nk c ur r ent i ovp v reg = 1.1 x v reg (nominal) 15 ma 6.5v v in 72v, i reg = 10ma, v reg = 5v 0.22 5.5v v in 72v, i reg = 1ma, v reg = 5v 0.05 line regulation (note 5) ? v reg / ? v reg 5.5v v in 72v, i reg = 100ma, v reg = 5v 1.5 mv/ma load regulation ? vreg / ? ireg 1ma i reg 100ma, v reg = 5v 0.8 mv/ma power-supply rejection ratio i reg = 10ma, f = 100hz, 0.5v p-p 55 db startup response time t start r reg = 500 ? , v reg = 5v, c reg = 4.7? 180 ? l 4.500 4.67 4.780 m 4.230 4.375 4.500 t 2.966 3.053 3.140 s 2.805 2.892 2.970 z 2.250 2.304 2.375 y 2.125 2.188 2.250 w 1.590 1.653 1.696 pok assertion threshold (max6397 only) v pok_th v 1.524 1.575 1.625 v reg to pok delay v reg rising or falling 35 ? pok leakage current v pok = 5v 100 na pok output low voltage v ol v in 1.5v, i sink = 1.6ma, pok asserted 0.3 v
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v 4 _______________________________________________________________________________________ t ypical operating characteristics (v in = 14v, c reg = 4.7?, i reg = 0, unless otherwise noted.) 40 60 80 100 120 140 160 020 10 30 40 50 60 70 80 supply current vs. input voltage max6397-98 toc01 input voltage (v) supply current ( a) max6397 gate on supply current vs. temperature max6397-98 toc02 temperature ( c) supply current ( a) 100 75 25 50 0 -25 90 100 110 120 130 140 150 160 170 180 80 -50 125 v in = 14v v in = 72v max6397 40 50 60 70 80 90 100 110 120 0204 06080 supply current vs. input voltage max6397-98 toc03 input voltage (v) supply current ( a) max6398 gate on 80 100 90 120 110 130 140 -50 25 50 -25 0 75 100 125 supply current vs. temperature max6397-98 toc04 temperature ( c) supply current ( a) v in = 72v v in = 14v max6398 gate on 20 30 25 40 35 45 50 040 20 60 80 shutdown supply current vs. input voltage (max6397) max6397-98 toc05 input voltage (v) supply current ( a) 10 30 50 70 regulator on gate off 0 6 4 2 8 10 12 14 16 18 20 020406 080 shutdown supply current vs. input voltage max6397-98 toc06 input voltage (v) supply current ( a) max6398 gate off 0 6 4 2 8 10 12 412 10 68 14 16 18 20 22 24 gate-drive voltage vs. input voltage max6397-98 toc07 input voltage (v) v gate - v out (v) v out = v in 4.0 4.6 4.4 4.2 5.0 4.8 5.8 5.6 5.4 5.2 6.0 -50 -25 0 25 50 75 100 125 uvlo threshold vs. temperature max6397-98 toc08 temperature ( c) v uvlo (v) set threshold vs. temperature max6397-98 toc09 temperature ( c) set threshold (v) 100 75 25 50 0 -25 1.204 1.208 1.212 1.216 1.220 1.224 1.228 1.232 1.236 1.240 1.200 -50 125
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v _______________________________________________________________________________________ 5 16.0 16.3 16.2 16.1 16.5 16.4 16.9 16.8 16.7 16.6 17.0 -50 -25 0 25 50 75 100 125 gate-to-out clamp voltage vs. temperature max6397-98 toc10 temperature ( c) gate-to-out clamp voltage (v) 0 0.4 0.2 0.8 0.6 1.2 1.0 1.4 1.8 1.6 2.0 0406 080 20 100 120 140 160 180 dropout voltage vs. reg load current max6397-98 toc11 reg load current (ma) dropout voltage (v) max6397l t a = +125 c t a = +25 c t a = -40 c 4.90 5.00 4.95 5.10 5.05 5.15 5.20 -40 -10 5 20 -25 35 50 65 80 95 110 125 reg output voltage vs. load current and temperature max6397-98 toc12 temperature ( c) reg output voltage (v) i load = 100ma i load = 10ma i load = 50ma max6397l 4.0 4.6 4.4 4.2 4.8 5.0 5.2 0 160 120 40 80 200 240 280 320 360 400 maximum reg output voltage vs. load current and temperature max6397-98 toc13 load current (ma) reg output voltage (v) thermal shutdown t a = +25 c t a = +125 c t a = -40 c power-supply rejection ratio vs. frequency max6397-98 toc15 frequency (hz) psrr (db) 1m 100k 10k 1k 100 -60 -50 -40 -30 -20 -10 0 -70 10 10m c reg = 10 f i reg = 10ma 4ms/div startup waveform (r load = 100 ? , c in = 10 f, c out = 10 f) v in 10v/div max6397-98 toc16 v gate 10v/div v out 10v/div i out 200ma/div 400 s/div startup waveform from shutdown (c in = 10 f, c out = 10 f) v shdn 2v/div max6397-98 toc17 v gate 10v/div v out 10v/div i out 200ma/div r load = 100 ? t ypical operating characteristics (continued) (v in = 14v, c reg = 4.7?, i reg = 0, unless otherwise noted.) gate-drive voltage vs. temperature max6397-98 toc14 temperature ( c) gate-drive voltage (v) 100 75 25 50 0 -25 10.455 10.460 10.465 10.470 10.475 10.480 10.485 10.490 10.495 10.500 10.450 -50 125
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v 6 _______________________________________________________________________________________ t ypical operating characteristics (continued) (v in = 14v, c reg = 4.7?, i reg = 0, unless otherwise noted.) 200 s/div overvoltage switch fault v in 20v/div max6397-98 toc18 v gate 20v/div v out 20v/div v reg 5v/div v ov = 30v 1ms/div voltage limit fault v in 20v/div max6397-98 toc19 v gate 20v/div v out 20v/div v reg 5v/div v ov = 30v 400 s/div transient response max6397-98 toc20 v in 10v/div v reg 100mv/div c reg = 10 f i reg = 10ma 1ms/div reg load-transient response v reg ac-coupled 500mv/div max6397-98 toc21 i reg 100ma/div c reg = 10 f 1ms/div regulator startup waveform v in 10v/div max6397-98 toc22 v pok 2v/div v reg 2v/div i reg = 10ma 100 s/div regulator pok assertion v reg 2v/div max6397-98 toc23 i reg 200ma/div v pok 2v/div i reg = 0 0v 0v 0a
detailed description the max6397/max6398 are ultra-small, low-current, high-voltage protection circuits for automotive applica- tions that must survive load dump and high-voltage transient conditions. these devices monitor the input/ output voltages and control an external n-channel mosfet to isolate the load or to regulate the output voltage from overvoltage transient energy. the con- troller allows system designers to size the external mosfet to their load current and board size. the max6397/max6398 drive the mosfet? gate high when the monitored input voltage is below the adjustable overvoltage threshold. an internal charge-pump circuit provides a 5v to 10v gate-to-source drive (see the typical operating characteristics ) to ensure low input-to- load voltage drops in normal operating modes. when the input voltage rises above the user-adjusted overvoltage threshold, gate pulls to out, turning off the mosfet. the max6397/max6398 are configurable to operate as overvoltage protection switches or as closed-looped volt- age limiters. in overvoltage protection switch mode, the input voltage is monitored. when an overvoltage condi- tion occurs at in, gate pulls low, disconnecting the load from the power source, and then slowly enhances upon removal of the overvoltage condition. in overvoltage limit mode, the output voltage is monitored and the max6397/ max6398 regulate the source of the external mosfet at the adjusted overvoltage threshold, allowing devices within the system to continue operating during an overvoltage condition. the max6397/max6398 undervoltage lockout (uvlo) function disables the devices as long as the input remains below the 5v (typ) uvlo turn-on threshold. the max6397/max6398 have an active-low shdn input to turn off the external mosfet, disconnecting the load and reducing power consumption. after power is applied and shdn is driven above its logic-high voltage, there is a 100? delay before gate enhancement commences. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v _______________________________________________________________________________________ 7 pin description pin max6397 max6398 name function 11 in supply voltage input. bypass with a minimum 10? capacitor to gnd. 22 shdn shutdown input. drive shdn low to force gate low, turning off the external n-channel mosfet. reg remains active when in shutdown mode. shdn is internally pulled down to gnd with a 1? source. connect to in for normal operation. 33 set overvoltage threshold adjustment input. connect set to an external resistor voltage- divider network to out (overvoltage limiter) or in (overvoltage switch) to adjust the desired overvoltage limit threshold. use set to monitor a system input or output voltage. 4 pok open-drain output. pok remains low until reg exceeds 92.5% or 87.5% of reg nominal output voltage. connect to an external pullup resistor. 54 gnd ground 65 gate gate-drive output. connect gate to the gate of an external n-channel mosfet. gate is a charge pump with a 75? pullup current to 10v (typ) above in during normal operation. gate is quickly shorted to out during an overvoltage condition. gate pulls low when shdn is low. 76 out output-voltage-sense input. connect to the source of the external n-channel mosfet. 8 reg regulator output. fixed 5.0v, 3.3v, 2.5v, or 1.8v output. reg sources up to 100ma. bypass with a minimum 4.7? capacitor to gnd. ep ep exposed pad. connect to ground plane.
max6397/max6398 the max6397 integrates a high-input-voltage, low-qui- escent-current linear regulator in addition to an over- voltage protector circuit. the linear regulator remains enabled at all times to power low-current ?lways-on applications (independent of the state of the external mosfet). the regulator is offered with several stan- dard output voltage options (5v, 3.3v, 2.5v, or 1.8v). an open-drain power-good output notifies the system if the regulator output falls to 92.5% or 87.5% of its nomi- nal voltage. the max6397? reg output operates inde- pendently of the shdn logic input. the max6397/max6398 include internal thermal-shut- down protection, disabling the external mosfet and linear regulator if the chip reaches overtemperature conditions. linear regulator (max6397 only) the max6397 is available with 5.0v, 3.3v, 2.5v, and 1.8v factory-set output voltages. each regulator sources up to 100ma and includes a current limit of 230ma. the linear regulator operates in an always-on condition regardless of the shdn logic. for fully specified operation, v in must be greater than 6.5v for the max6397l/m (5v regulator output). the actual output current may be limited by the operating condition and package power dissipation. power-ok output pok is an open-drain output that goes low when reg falls to 92.5% or 87.5% (see the selector guide ) of its nominal output voltage. to obtain a logic-level output, connect a pullup resistor from pok to reg or another system voltage. use a resistor in the 100k ? range to minimize current consumption. pok provides a valid logic-output level down to v in = 1.5v. gate voltage the max6397/max6398 use a high-efficiency charge pump to generate the gate voltage. upon v in exceed- ing the 5v (typ) uvlo threshold, gate enhances 10v above in (for v in 14v) with a 75? pullup current. an overvoltage condition occurs when the voltage at set pulls above its 1.215v threshold. when the threshold is crossed, gate falls to out within 100ns with a 100ma (typ) pulldown current. the max6397/max6398 include an internal clamp to out that ensures gate is limited to 18v (max) above out to prevent gate-to-source damage to the external fet. the gate cycle during overvoltage limit and overvolt- age switch modes are quite similar but have distinct characteristics. in overvoltage switch mode (figure 2a), gate is enhanced to v in + 10v while the monitored in voltage remains below the overvoltage fault threshold (set < 1.215v). when an overvoltage fault occurs (set 1.215v), gate is pulled one diode below out, turn- ing off the external fet and disconnecting the load from the input. gate remains low (fet off) as long as v in is above the overvoltage fault threshold. as v in falls back below the overvoltage fault threshold (-5% hys- teresis) gate is again enhanced to v in + 10v. in overvoltage limit mode (figure 2b), gate is enhanced to v in + 10v. while the monitored out voltage remains below the overvoltage fault threshold (set < 1.215v). when an overvoltage fault occurs (set 1.215v), gate is pulled low one diode drop below out until out drops 5% below the overvoltage fault threshold. gate is then turned back on until out again reaches the overvoltage fault threshold and gate is again turned off. overvoltage protection switch/limiter controllers operate up to 72v 8 _______________________________________________________________________________________ max6397 max6398 10v charge pump linear regulator v pok_th max6397 only thermal protection 5v 1.23v uvlo gate out shdn reg pok gnd in set figure 1. functional diagram
gate cycles on-off-on-off-on in a sawtooth waveform until out remains below the overvoltage fault threshold and gate remains constantly on (v in + 10v). the over- voltage limiter? sawtooth gate output operates the mosfet in a switched-linear mode while the input volt- age remains above the overvoltage fault threshold. the sawtooth frequency depends on the load capacitance, load current, and mosfet turn-on time (gate charge current and gate capacitance). gate goes high when the following startup conditions are met: v in is above the uvlo threshold, shdn is high, an overvoltage fault is not present and the device is not in thermal shutdown. overvoltage monitoring when operating in overvoltage mode, the max6397/ max6398 feedback path (figure 3) consists of in, set? internal comparator, the internal gate charge pump, and the external n-channel mosfet resulting in a switch-on/off function. when the programmed over- voltage threshold is tripped, the internal fast compara- tor turns off the external mosfet, pulling gate to out within t ov and disconnecting the power source from the load. when in decreases below the adjusted over- voltage threshold, the max6397/max6398 slowly enhance gate above out, reconnecting the load to the power source. overvoltage limiter when operating in overvoltage limiter mode, the max6397/max6398 feedback path (figure 4) consists of out, set? internal comparator, the internal gate charge pump and the external n-channel mosfet, which results in the external mosfet operating as a voltage regulator. during normal operation, gate is enhanced 10v above out. the external mosfet source voltage is monitored through a resistor-divider between out and set. when out rises above the adjusted overvoltage threshold, an internal comparator sinks the charge-pump current, dis- charging the external gate, regulating out at the set overvoltage threshold. out remains active during the overvoltage transients and the mosfet continues to con- duct during the overvoltage event, operating in switched- linear mode. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v _______________________________________________________________________________________ 9 v gate 10v/div v out 10v/div v in 10v/div 10ms/div figure 2a. max6397/max6398 gate waveform during over- voltage switch mode v gate 10v/div v out 10v/div v in 10v/div 4ms/div figure 2b. max6397/max6398 gate waveform during over- voltage limit mode max6397 max6398 in set gate out gnd v batt r1 r2 figure 3. overvoltage switch protection configuration
max6397/max6398 as the transient begins decreasing, out fall time will depend on the mosfet? gate charge, the internal charge-pump current, the output load, and the tank capacitor at out. for fast-rising transients and very large-sized mosfets, add an additional external bypass capacitor from gate to gnd to reduce the effect of the fast-rising voltages at in. the external capacitor acts as a voltage-divider working against the mosfets?drain-to-gate capaci- tance. for a 6000pf c gd , a 0.1? capacitor at gate will reduce the impact of the fast-rising v in input. caution must be exercised when operating the max6397/max6398 in voltage-limiting mode for long durations. if the v in is a dc voltage greater than the mosfet? maximum gate voltage, the fet will dissipate power continuously. to prevent damage to the external mosfet, proper heatsinking should be implemented. applications information load dump most automotive applications run off a multicell, 12v lead-acid battery with a nominal voltage that swings between 9v and 16v (depending on load current, charging status, temperature, battery age, etc.). the battery voltage is distributed throughout the automobile and is locally regulated down to voltages required by the different system modules. load dump occurs when the alternator is charging the battery and the battery becomes disconnected. power in the alternator (essen- tially an inductor) flows into the distributed power sys- tem and elevates the voltage seen at each module. the voltage spikes have rise times typically greater than 5ms and decays within several hundred milliseconds but can extend out to 1s or more depending on the characteristics of the charging system (figure 5). these transients are capable of destroying semicon- ductors on the first ?ault event. setting overvoltage thresholds set provides an accurate means to set the overvoltage level for the max6397/max6398. use a resistor-divider to set the desired overvoltage condition (figure 6). set has a rising 1.215v threshold with a 5% falling hysteresis. begin by selecting the total end-to-end resistance, r total = r1 + r2. choose r total to yield a total cur- rent equivalent to a minimum 100 x i set (set? input bias current) at the desired overvoltage threshold. for example: with an overvoltage threshold set to 20v: r total < 20v/(100 x i set ) where i set is set? 50na input bias current. r total < 4m ? use the following formula to calculate r2: where v th is the 1.215v set rising threshold and v ov is the overvoltage threshold. r2 = 243k ? , use a 240k ? standard resistor. r total = r2 + r1, where r1 = 3.76m ? . use a 3.79m ? standard resistor. a lower value for total resistance dissipates more power but provides slightly better accuracy. rv r v th total ov 2 = overvoltage protection switch/limiter controllers operate up to 72v 10 ______________________________________________________________________________________ max6397 max6398 in set gate out gnd v batt r1 r2 c out figure 4. overvoltage limiter protection switch configuration 100ms 200ms 300ms 400ms t rise > 5ms v peak v batt figure 5. load dump voltage profile
reverse-battery protection use a diode or p-channel mosfet to protect the max6397/max6398 during a reverse-battery insertion (figures 7a, 7b). low p-channel mosfet on-resistance of 30m ? or less yields a forward-voltage drop of only a few millivolts (versus hundreds of millivolts for a diode, figure 7a) thus improving efficiency. connecting a positive battery voltage to the drain of q1 (figure 7b) produces forward bias in its body diode, which clamps the source voltage one diode drop below the drain voltage. when the source voltage exceeds q1? threshold voltage, q1 turns on. once the fet is on, the battery is fully connected to the system and can deliver power to the device and the load. an incorrectly inserted battery reverse-biases the fet? body diode. the gate remains at the ground potential. the fet remains off and disconnects the reversed bat- tery from the system. the zener diode and resistor com- bination prevent damage to the p-channel mosfet during an overvoltage condition. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v ______________________________________________________________________________________ 11 max6397 max6398 in set gate out gnd in r1 r2 max6397 max6398 in set gate out gnd r1 r2 in figure 6. setting the max6397/max6398 overvoltage threshold max6397 max6398 load (b) v batt q1 gnd in out gate max6397 max6398 load (a) v batt gnd in out gate figure 7. reverse battery protection using a diode or p-channel mosfet
max6397/max6398 reg capacitor selection for stability for stable operation over the full temperature range and with load currents up to 100ma, use ceramic capacitor values greater than 4.7?. large output capacitors help reduce noise, improve load-transient response, and power-supply rejection at reg. note that some ceramic dielectrics exhibit large capacitance and esr variation with temperature. at lower temperatures, it may be nec- essary to increase capacitance. under normal conditions, use a 10? capacitor at in. larger input capacitor values and lower esr provide bet- ter supply-noise rejection and line-transient response. inrush/slew-rate control inrush current control can be implemented by placing a capacitor at gate (figure 8) to slowly ramp up the gate, thus limiting the inrush current and controlling gate? slew rate during initial turn-on. the inrush cur- rent can be approximated using the following formula: where i gate is gate? 75? sourcing current, i load is the load current at startup, and c out is the output capacitor. input transients clamping when the external mosfet is turned off during an over- voltage occurrence, stray inductance in the power path may cause voltage ringing exceeding the max6397/ max6398 absolute maximum input (in) supply rating. the following techniques are recommended to reduce the effect of transients: minimize stray inductance in the power path using wide traces, and minimize loop area including the power traces and the return ground path. add a zener diode or transient voltage suppressor (tvs) rated below the in absolute maximum rating (figure 9). add a resistor in series with in to limit transient current going into the input for the max6398 only. i c c ii inrush out gate gate load =+ overvoltage protection switch/limiter controllers operate up to 72v 12 ______________________________________________________________________________________ max6397 max6398 load v batt gnd in out gate c gate c out figure 8. max6397/max6398 controlling gate inrush current max6397 max6398 load gnd in out gate v batt 60v tvs 1k ? figure 9. protecting the max6397/max6398 input from high- voltage transients
mosfet selection select external mosfets according to the application current level. the mosfet? on-resistance (r ds(on) ) should be chosen low enough to have minimum voltage drop at full load to limit the mosfet power dissipation. determine the device power rating to accommodate an overvoltage fault when operating the max6397/ max6398 in overvoltage limit mode. during normal operation, the external mosfets dissipate little power. the power dissipated in normal operation is: p q1 = i load 2 x r ds(on). the most power dissipation will occur during a pro- longed overvoltage event when operating the max6397/max6398 in voltage limiter mode, resulting in high power dissipated in q1 (figure 10) where the power dissipated across q1 is: p q1 = v q1 x i load where v q1 is the voltage across the mosfet? drain and source. thermal shutdown the max6397/max6398 thermal-shutdown feature shuts off the linear regulator output, reg, and gate if it exceeds the maximum allowable thermal dissipation. thermal shutdown also monitors the pc board tempera- ture of the external nfet when the devices sit on the same thermal island. good thermal contact between the max6397/max6398 and the external nfet is essential for the thermal-shutdown feature to operate effectively. place the nfet as close as possible to out. when the junction temperature exceeds t j = +150?, the thermal sensor signals the shutdown logic, turning off reg? internal pass transistor and the gate output, allowing the device to cool. the thermal sensor turns the pass transistor and gate on again after the ic? junction temperature cools by 20?. thermal-overload protection is designed to protect the max6397/ max6398 and the external mosfet in the event of cur- rent-limit fault conditions. for continuous operation, do not exceed the absolute maximum junction-tempera- ture rating of t j = +150?. thermal shutdown overvoltage limiter mode when operating the max6397/max6398 in overvoltage limit mode for a prolonged period of time, a thermal shutdown is possible due to device self-heating. the thermal shutdown is dependent on a number of differ- ent factors: the device? ambient temperature (t a ) the output capacitor (c out ) the output load current (i out ) the overvoltage threshold limit (v ov ) the overvoltage waveform period (t ovp ) the power dissipated across the package (p diss ) max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v ______________________________________________________________________________________ 13 max6397 max6398 load gnd in out gate v batt 60v tvs set v q1 +- i load v batt v max v ov figure 10. power dissipated across mosfets during an overvoltage fault (overvoltage limiter mode) t 2 t 1 t 3 t ovp gate out figure 11. max6397/max6398 timing diagram
max6397/max6398 when out exceeds the adjusted overvoltage threshold, an internal gate pulldown current is enabled until out drops by 5%. the capacitance at out is discharged by the internal current sink and the external out load cur- rent. the discharge time ( ? t1) is approximately: where v ov is the adjusted overvoltage threshold, i out is the external load current and i gatepd is the gate? internal 100ma (typ) pulldown current. when out falls 5% below the overvoltage threshold point, the internal current sink is disabled and the max6397/max6398? internal charge pump begins recharging the external gate voltage. the out volt- age continues to drop due to the external out load current until the mosfet gate is recharged. the time needed to recharge gate and re-enhance the external nfet is approximately: where c iss is the mosfet? input capacitance, v gs(th) is the mosfet? gate-to-source threshold voltage, v f is the internal clamp diode forward voltage (v f = 1.5v typ), and i gate is the max6397/max6398 charge-pump cur- rent (75? typ). during ? t2, c out loses charge through the output load. the voltage across c out ( ? v2) decreases until the mosfet reaches its v gs(th) threshold and can be approximated using the following formula: once the mosfet v gs ( th ) is obtained, the slope of the output voltage rise is determined by the mosfet q g charge through the internal charge pump with respect to the drain potential. the time for the out voltage to rise again to the overvoltage threshold can be approxi- mated using the following formula: where ? v out = ( v ov x 0.05) + ? v2. the total period of the overvoltage waveform can be summed up as follows: t ovp = ? t1 + ? t2 + ? t3 the max6397/max6398 dissipate the most power dur- ing an overvoltage event when i out = 0 (c out is dis- charged only by the internal current sink). the maximum power dissipation can be approximated using the follow- ing equation: the die temperature (t j ) increase is related to jc (8.3?/w and 8.5?/w for the max6397 and max6398, respectively) of the package when mounted correctly with a strong thermal contact to the circuit board. the max6397/max6398 thermal shutdown is governed by the equation: t j = t a + p diss x ( jc + ca ) < 170? (typical thermal-shutdown temperature) for the max6397, the power dissipation of the internal linear regulator must be added to the overvoltage pro- tection circuit power dissipation to calculate the die temperature. the linear regulator power dissipation is calculated using the following equation: p reg = (v in ?v reg ) (i reg ) for example, using an irfr3410 100v n-channel mosfet, figure 12 illustrates the junction temperature vs. output capacitor with i out = 0, t a = +125?, v ov < 16v,v f = 1.5v, i gate = 75ma, and i gatepd = 100ma. figure 12 shows the relationship between output capacitance versus die temperature for the conditions listed above. pv i t t diss ov gatepd ovp . = 0 975 1 ? ? ? ? t q v v i gd gs qgd out gate 3 _ ? ? ? vi t c out out 2 2 = ? tc vv i iss gs th f gate 2 () = + ? tc v ii out ov out gatepd 1 005 . = + overvoltage protection switch/limiter controllers operate up to 72v 14 ______________________________________________________________________________________ output capacitance ( f) junction temperature ( c) 100 10 120 130 140 150 160 170 180 1 1000 i out = 0 t a = +125 c thermal shutdown c gate = 0 c gate = inf c gate = 10nf c gate = additional capacitance from gate to gnd figure 12. junction temperature vs. c out
an additional capacitor can be added to gate and gnd to shift the curves as this increases ? t1. these val- ues are used for illustration only. customers must verify worst-case conditons for their specific application. output current calculation the max6397 high input voltage (+72v max) provides up to 100ma of output current at reg. package power dissi- pation limits the amount of output current available for a given input/output voltage and ambient temperature. figure 13 depicts the maximum power dissipation curve for the max6397. the graph assumes that the exposed metal pad of the max6397 package is soldered to 1in 2 of pc board copper. use figure 11 to determine the allow- able package dissipation for a given ambient tempera- ture. alternately, use the following formula to calculate the allowable package dissipation: p diss = 1.455w for t a +70? maximum power dissipation = 1.455 - 0.0182 (t a - 70?) for +70? t a +125? where, 0.0182 w/? is the max6397 package thermal derating. after determining the allowable package dissipation, calculate the maximum output current using the follow- ing formula: i p vv ma out max d iss in reg () = ? 100 max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v ______________________________________________________________________________________ 15 temperature ( c) p d (w) 140 120 80 100 40 60 20 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 0 160 derate 18.2mw/ c above +70 c 1.455w figure 13. maximum power dissipation vs. temperature max6397 gate pok reg in shdn gnd set out always-on c v cc reset gpio 12v in dc-dc converter out in gnd c t ypical application circuit
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v 16 ______________________________________________________________________________________ max6397 max6398 in set reg reg gate out gnd v batt r1 r2 max6397 max6398 in set gate out gnd r1 r2 v batt c out c out dc-dc converter dc-dc converter overvoltage limiter controller overvoltage switch controller t ypical operating circuit chip information transistor count: 590 process: bicmos 6 gate 5 out 4 set 12 gnd 3 shdn max6398 in tdfn top view *ep *exposed pad. connect to gnd. pin configurations (continued) selector guide part reg output voltage (v) pok assertion threshold (%) top mark MAX6397LATA 5.0 92.5 ann max6397mata 5.0 87.5 ano max6397sata 3.3 87.5 anq max6397tata 3.3 92.5 anp max6397yata 2.5 87.5 ank max6397zata 2.5 92.5 anj max6397vata 1.8 87.5 anm max6397wata 1.8 92.5 anl max6398att ajd
max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v 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 ____________________ 17 2007 maxim integrated products is a registered trademark of maxim integrated products, inc. 6, 8, &10l, dfn thin.eps h 1 2 21-0137 package outline, 6,8,10 & 14l, tdfn, exposed pad, 3x3x0.80 mm common dimensions symbol min. max. a0.700 .80 d2.903 .10 e2.903 .10 a1 0.00 0.05 l0.200 .40 pkg. code n d2 e2 e jedec spec b [(n/2)-1] x e package variations 0.25 min. k a2 0.20 ref. 2.300.10 1.500.10 6 t633-1 0.95 bsc mo229 / weea 1.90 ref 0.400.05 1.95 ref 0.300.05 0.65 bsc 2.300.10 8 t833-1 2.00 ref 0.250.05 0.50 bsc 2.300.10 10 t1033-1 2.40 ref 0.200.05 - - - - 0.40 bsc 1.700.10 2.300.10 14 t1433-1 1.500.10 1.500.10 mo229 / weec mo229 / weed-3 0.40 bsc - - - - 0.200.05 2.40 ref t1433-2 14 2.300.10 1.700.10 t633-2 6 1.500.10 2.300.10 0.95 bsc mo229 / weea 0.400.05 1.90 ref t833-2 8 1.500.10 2.300.10 0.65 bsc m o229 / weec 0.300.05 1.95 ref t833-3 8 1.500.10 2.300.10 0.65 bsc m o229 / weec 0.300.05 1.95 ref -drawing not to scale- h 2 2 21-0137 package outline, 6,8,10 & 14l, tdfn, exposed pad, 3x3x0.80 mm 2.300.10 mo229 / weed-3 2.00 ref 0.250.05 0.50 bsc 1.500.10 10 t1033-2 package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .) revision history pages changed at rev 3: 1, 14, 15, 17

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