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general description the max6397/max6398 are small, high-voltage over - voltage protection circuits. these devices disconnect the output load or limit the output voltage during an input overvoltage condition. these devices are ideal for applications that must survive high-voltage transients such as those found in 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 threshold, 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 37a 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- shutdown 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 -40c to +125c. applications 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 (37a) linear regulator sources up to 100ma (max6397) fully specified from -40c to +125c (t j ) small, thermally enhanced 3mm x 3mm tdfn package 19-3668; rev 6; 7/14 *replace -t with +t for lead(pb)-free/rohs-compliant 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 . firewire is a registered trademark of apple computer, inc. pin configurations continued at end of data sheet. selector guide and typical operating circuit appear at end of data sheet. part temp range pin-package max6397 _ata-t* -40c to +125c 8 tdfn-ep** max6398 att-t* -40c to +125c 6 tdfn-ep** 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. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v pin conigurations ordering information evaluation kit available downloaded from: http:///
(all pins referenced to gnd, unless otherwise noted.) in, gate, out ...................................................... -0.3v to +80v shdn ......................................................... -0.3v to (v 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 = +70c) 6-pin tdfn (derate 18.2mw/c above +70c) ........ 1455mw 8-pin tdfn (derate 18.2mw/c above +70c) ........ 1455mw operating temperature range (t a ) ................. -40c to +125c junction temperature ...................................................... +150c storage temperature range ............................ -65c to +150c lead temperature ........................................................... +300c (v in = 14v; c gate = 6000pf, c reg = 4.7f, t a = t j = -40c to +125c, unless otherwise noted. typical values are at t a = t j = +25c.) (note 1) parameter symbol conditions min typ max units supply voltage range v in 5.5 72 v input supply current shdn = high, no load (max6397) 118 140 a shdn = high, (max6398) 104 130 shdn = low, no load (max6397) 37 45 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 s gate rise time gate rising from gnd to v out + 8v, c gate = 6000pf, out = gnd 1 ms set-to-gate propagation delay t ov set rising from v th - 100mv to v th + 100mv 0.75 s gate output high voltage v oh v out = v in = 6v, r gate to in = 1m? v in + 3.8v v in + 4.2v v in + 4.6v v v out = v in ; v in 14v, r gate to in = 1m? v in + 8.5v v in + 9.2v v in + 11.5v gate output low voltage v ol gate sinking 20ma, v out = gnd 0.38 v gate charge-pump current i gate gate = gnd 75 a 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 1 a thermal shutdown (note 3) +150 c thermal-shutdown hysteresis 20 c regulator (max6397) ground current i gnd shdn = gnd i reg = 1ma 40 48 a i reg = 100ma 60 max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 2 absolute maximum ratings stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. electrical characteristics downloaded from: http:///
(v in = 14v; c gate = 6000pf, c reg = 4.7f, t a = t j = -40c to +125c, unless otherwise noted. typical values are at t a = t j = +25c.) (note 1) note 1: specifications to t a = -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 reg output voltage (v in v reg + 1.8v) v reg max6397l/m i reg = 1ma 4.925 5 5.120 v 1ma < i reg < 100ma 4.85 5.15 max6397s/t i reg = 1ma 3.243 3.3 3.36 1ma < i reg < 100ma 3.201 3. 36 max6397y/z i reg = 1ma 2.246 2.5 2.542 1ma < i reg < 100ma 2.41 2.55 max6397v/w i reg = 1ma 1.76 1.8 1.837 mv/v 1ma < i reg < 100ma 1.715 1.837 dropout voltage (note 4) ?v do 5.5v v in 72v, i reg = 1ma, v reg = 5v 0.12 5.5v v in 72v, i reg = 100ma, v reg = 5v 1.2 current limit v in = 14v 150 300 ma overvoltage-protection threshold v ovp 105 % of v reg overvoltage-protection sink current i ovp v reg = 1.1 x v reg (nominal) 15 ma line regulation (note 5) ?v reg / ?v reg 6.5v v in 72v, i reg = 10ma, v reg = 5v 0.22 mv/ma 5.5v v in 72v, i reg = 1ma, v reg = 5v 0.05 5.5v v in 72v, i reg = 100ma, v reg = 5v 1.5 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 = 4.7f 180 s pok assertion threshold (max6397 only) v pok_th l 4.500 4.67 4.780 v 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 v 1.524 1.575 1.625 reg-to-pok delay v reg rising or falling 35 s 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 www.maximintegrated.com maxim integrated 3 electrical characteristics (continued) downloaded from: http:///
(v in = 14v, c reg = 4.7f, i reg = 0, unless otherwise noted.) supply current vs. temperature max6397 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 0 20 40 60 80 supply current vs. input voltage max6397 toc03 input voltage (v) supply current (a) max6398gate on 80 100 90 120110 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 max6398gate on 20 3025 4035 45 50 0 40 20 60 80 shutdown supply current vs. input voltage (max6397) max6397 toc05 input voltage (v) supply current (a) 10 30 50 70 regulator ongate off 0 64 2 8 10 12 14 16 18 20 0 20 40 60 80 shutdown supply current vs. input voltage max6397 toc06 input voltage (v) supply current (a) max6398gate off 0 64 2 8 10 12 4 12 10 6 8 14 16 18 20 22 24 gate-drive voltage vs. input voltage max6397 toc07 input voltage (v) v gate - v out (v) v out = v in 40 60 80 100 120 140 160 0 20 10 30 40 50 60 70 80 supply current vs. input voltage max6397 toc01 input voltage (v) supply current (a) max6397gate on 4.0 4.64.4 4.2 5.04.8 5.85.6 5.4 5.2 6.0 -50 -25 0 25 50 75 100 125 uvlo threshold vs. temperature max6397 toc08 temperature (c) v uvlo (v) set threshold vs. temperature max6397 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 1.2401.200 -50 125 max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v maxim integrated 4 www.maximintegrated.com typical operating characteristics downloaded from: http:///
(v in = 14v, c reg = 4.7f, i reg = 0, unless otherwise noted.) 16.0 16.316.2 16.1 16.516.4 16.916.8 16.7 16.6 17.0 -50 -25 0 25 50 75 100 125 gate-to-out clamp voltage vs. temperature max6397 toc10 temperature (c) gate-to-out clamp voltage (v) 0 0.40.2 0.80.6 1.21.0 1.4 1.81.6 2.0 0 40 60 80 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 = +125c t a = +25c t a = -40c 4.90 5.004.95 5.105.05 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 toc12 temperature (c) reg output voltage (v) i load = 100ma i load = 10ma i load = 50ma max6397l 4.0 4.64.4 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 toc13 load current (ma) reg output voltage (v) thermal shutdown t a = +25c t a = +125c t a = -40c gate-drive voltage vs. temperature max6397 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 10.50010.450 -50 125 power-supply rejection ratio vs. frequency max6397 toc15 frequency (hz) psrr (db) 1m 100k 10k 1k 100 -60 -50 -40 -30 -20 -10 0 -70 10 10m c reg = 10f i reg = 10ma 4ms/div startup waveform (r load = 100 ? , c in = 10 f, c out = 10 f) v in 10v/div max6397 toc16 v gate 10v/div v out 10v/div i out 200ma/div 400s/div startup waveform from shutdown (c in = 10 f, c out = 10 f) v shdn 2v/div max6397 toc17 v gate 10v/div v out 10v/div i out 200ma/div r load = 100 ? max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v maxim integrated 5 www.maximintegrated.com typical operating characteristics (continued) downloaded from: http:///
(v in = 14v, c reg = 4.7f, i reg = 0, unless otherwise noted.) 1ms/div voltage limit fault v in 20v/div max6397 toc19 v gate 20v/div v out 20v/div v reg 5v/div v ov = 30v 400s/div transient response max6397 toc20 v in 10v/div v reg 100mv/div c reg = 10f i reg = 10ma 1ms/div reg load-transient response v reg ac-coupled500mv/div max6397 toc21 i reg 100ma/div c reg = 10f 1ms/div regulator startup waveform v in 10v/div max6397 toc22 v pok 2v/div v reg 2v/div i reg = 10ma 200s/div overvoltage switch fault v in 20v/div max6397 toc18 v gate 20v/div v out 20v/div v reg 5v/div v ov = 30v 100s/div regulator pok assertion v reg 2v/div max6397 toc23 i reg 200ma/div v pok 2v/div i reg = 0 0v0v 0a max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v maxim integrated 6 www.maximintegrated.com typical operating characteristics (continued) downloaded from: http:///
detailed description the max6397/max6398 are ultra-small, low-current, high-voltage protection circuits for applications that must survive 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 controller allows system designers to size the external mosfet to their load current and board size. the max6397/max6398 drive the mosfets 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 thresh - old, gate pulls to out, turning off the mosfet. the max6397/max6398 are configurable to operate as overvoltage-protection switches or as closed-looped voltage limiters. in overvoltage-protection switch mode, the input voltage is monitored. when an overvoltage condition 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 volt - age 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-lows 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 100s delay before gate enhancement commences. the max6397 integrates a high input voltage, low- quiescent-current linear regulator, in addition to an overvoltage-protector circuit. the linear regulator remains enabled at all times to power low-current always-on applications (independent of the state of the external mosfet). the regulator is offered with several standard output voltage options (5v, 3.3v, 2.5v, or 1.8v). an open- drain power-good output notifies the system if the regulator pin name function max6397 max6398 1 1 in supply voltage input. bypass with a minimum 10f capacitor to gnd. 2 2 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 1a source. connect to in for normal operation. 3 3 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. 5 4 gnd ground 6 5 gate gate-drive output. connect gate to the gate of an external n-channel mosfet. gate is a charge pump with a 75a 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. 7 6 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 4.7f capacitor to gnd. ep exposed pad. connect to ground plane. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 7 pin description downloaded from: http:///
output falls to 92.5% or 87.5% of its nominal voltage. the max6397s reg output operates independently of the shdn logic input. the max6397/max6398 include internal thermal- shutdown protection, disabling the external mosfet and linear regulator if the chip reaches overtemperature condi - tions. 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 75a 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 overvoltage- switch modes are quite similar but have distinct charac - teristics. in overvoltage-switch mode (figure 2a), gate is enhanced to v in + 10v while the monitored in volt - age remains below the overvoltage-fault threshold (set < 1.215v). when an overvoltage fault occurs (set 1.215v), gate is pulled one diode below out, turning 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% hysteresis), gate is again enhanced to v in + 10v. figure 1. functional diagram figure 2a. max6397/max6398 gate waveform during overvoltage switch mode max6397max6398 10vcharge pump linear regulator v pok_th max6397 only thermal protection 5v 1.23v uvlo gate out shdn reg pok gnd in set v gate 10v/divv out 10v/div v in 10v/div 10ms/div max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 8 downloaded from: http:///
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. 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 overvoltage limiters sawtooth gate output operates the mosfet in a switched-linear mode while the input voltage 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, sets internal comparator, the internal gate charge pump, and the external n-channel mosfet resulting in a switch-on/ off function. when the programmed overvoltage threshold is tripped, the internal fast comparator turns off the external mosfet, pulling gate to out within t ov and disconnect - ing the power source from the load. when in decreases below the adjusted overvoltage 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, sets 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, discharging the external gate, regulating out at the set overvoltage threshold. out remains active during figure 2b. max6397/max6398 gate waveform during overvoltage limit mode figure 3. overvoltage switch protection configurationfigure 4. overvoltage limiter protection switch configuration v gate 10v/divv out 10v/div v in 10v/div 4ms/div max6397max6398 in set gate out gnd v batt r1r2 max6397max6398 in set gate out gnd v batt r1r2 c out max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 9 downloaded from: http:///
the overvoltage transients and the mosfet continues to conduct during the overvoltage event, operating in switched-linear mode. as the transient begins decreasing, out fall time will depend on the mosfets 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 capacitance. for a 6000pf c gd , a 0.1f 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 mosfets maximum gate voltage, the fet will dissipate power continuously. to prevent damage to the external mosfet, proper heatsinking should be implemented. applications information 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 5). 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 current equivalent to a minimum 100 x i set (sets 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 sets 50na input bias current. r total < 4m use the following formula to calcue r2: total th ov r r2 v v = 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. figure 5. setting the max6397/max6398 overvoltage threshold max6397max6398 in set gate out gnd in r1r2 max6397max6398 in set gate out gnd r1r2 in max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 10 downloaded from: http:///
reverse-battery protection use a diode or p-channel mosfet to protect the max6397/max6398 during a reverse-battery insertion (figures 6a, 6b). 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 6a) thus improving efficiency. connecting a positive battery voltage to the drain of q1 (figure 6b) produces forward bias in its body diode, which clamps the source voltage one diode drop below the drain voltage. when the source voltage exceeds q1s 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 fets body diode. the gate remains at the ground potential. the fet remains off and disconnects the reversed battery from the system. the zener diode and resistor combination prevent damage to the p-channel mosfet during an overvoltage condition. figure 6. reverse-battery protection using a diode or p-channel mosfet max6397max6398 load (b) v batt q1 gnd in out gate max6397max6398 load (a) v batt gnd in out gate max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 11 downloaded from: http:///
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 4.7f. 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 necessary to increase capacitance. under normal conditions, use a 10f capacitor at in. larger input capacitor values and lower esr provide better supply-noise rejection and line-transient response. inrush/slew-rate control inrush current control can be implemented by placing a capacitor at gate (figure 7) to slowly ramp up the gate, thus limiting the inrush current and controlling gates slew rate during initial turn-on. the inrush current can be approximated using the followng formula: out inrush gate load gate c i i i c = + where i gate is gates 75a 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 overvoltage 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 8). add a resistor in series with in to limit transient current going into the input for the max6398 only. mosfet selection select external mosfets according to the application current level. the mosfets 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. figure 7. max6397/max6398 controlling gate inrush current figure 8. protecting the max6397/max6398 input from high- voltage transients max6397max6398 load v batt gnd in out gate c gate c out max6397max6398 load gnd in out gate v batt 60v tvs 1k ? max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 12 downloaded from: http:///
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 prolonged overvoltage event when operating the max6397/max6398 in voltage limiter mode, resulting in high power dissipated in q1 (figure 9) where the power dissipated across q1 is: p q1 = v q1 x i load where v q1 is the voltage across the mosfets drain and source.thermal shutdown the thermal-shutdown feature of the max6397/ max6398 shuts off the linear regulator output (reg), and gate if it exceeds the maximum allowable thermal dissipation. thermal shutdown also monitors the pcb temperature 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 = +150c, the thermal sensor signals the shutdown logic, turning off regs 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 ics junction temperature cools by 20c. thermal-overload protection is designed to protect the max6397/max6398 and the external mosfet in the event of current-limit fault conditions. for continuous operation, do not exceed the absolute maximum junction-temperature rating of t j = +150c. thermal shutdownovervoltage limiter mode when operating the devices 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 different factors: ? the devices 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 ) 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: ov out out gatepd v 0.05 t 1 c i i ?= + where v ov is the adjusted overvoltage threshold, i out is the external load current and i gatepd is the gates internal 100ma (typ) pulldown current. figure 9. power dissipated across mosfets during an overvoltage fault (overvoltage limiter mode) figure 10. max6397/max6398 timing diagram max6397max6398 load gnd in out gate v batt 60v tvs set v q1 + - i load v batt v max v ov t 2 t 1 t 3 t ovp gate out max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 13 downloaded from: http:///
when out falls 5% below the overvoltage-threshold point, the internal current sink is disabled and the max6397/ max6398s internal charge pump begins recharging the external gate voltage. the out voltage 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: gs(th) f iss gate v v t2 c i + ?= where c iss is the mosfets input capacitance, v gs(th) is the mosfets 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 current (75a 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: out out t2 v 2 i c ? ?= 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 approximated using the following formula: gd out gs_qgd gate qv t3 vi ? ?? 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 during an overvoltage event when i out = 0 (c out is discharged only by the internal current sink). the maximum power dissipation can be approximated using the following equation: diss ov gatepd ovp t1 p v 0.975 i t ? = ? the die temperature (t j ) increase is related to jc (8.3c/w and 8.5c/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 following equation: t j = t a + p diss x ( jc + ca ) < 170c (typical thermal-shutdown temperature) for the max6397, the power dissipation of the internal linear regulator must be added to the overvoltage- protection 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 11 illustrates the junction temperature vs. output capacitor with i out = 0, t a = +125c, v ov < 16v,v f = 1.5v, i gate = 75ma, and i gatepd = 100ma. figure 11 shows the relationship between output capacitance versus die temperature for the conditions listed above. figure 11. junction temperature vs. c out output capacitance (f) junction temperature (c) 100 10 120 130 140 150 160 170 180 1 1000 i out = 0 t a = +125c thermal shutdown c gate = 0 c gate = inf c gate = 10nf c gate = additional capacitance from gate to gnd max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 14 downloaded from: http:///
an additional capacitor can be added to gate and gnd to shift the curves as this increases ?t1. these values 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 dissipation limits the amount of output current available for a given input/output voltage and ambient temperature. figure 12 depicts the maximum power dissipation curve for the max6397. the graph assumes that the exposed metal pad of the max6397 package is soldered to 1in2 of pcb copper. use figure 10 to determine the allowable package dissipation for a given ambient temperature. alternately, use the following formula to calculate the allowable package dissipation: p diss = 1.455w for t a +70c maximum power dissipation = 1.455 - 0.0182 (t a - 70c) for +70c t a +125c where 0.0182 w/c is the max6397 package-thermal derating. after determining the allowable package dissipation, calculate the maximum output current using the following formula: diss out(max) in reg p i 100ma vv = ? figure 12. maximum power dissipation vs. temperature 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/cabove +70c 1.455w 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 max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 15 typical application circuit downloaded from: http:///
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 max6397max6398 in set reg reg gate out gnd v batt r1r2 max6397max6398 in set gate out gnd r1r2 v batt c out c out dc-dc converter dc-dc converter overvoltage limiter controller overvoltage switch controller 6 gate 5 out 4 set 1 2 gnd 3 shdn max6398 in tdfn top view *ep *exposed pad. connect to gnd. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 16 typical operating circuit pin conigurations (continued) selector guide downloaded from: http:///
package type package code outline no. land pattern no. 6 tdfn t633+2 21-0137 90-0059 8 tdfn t833+2 21-0137 90-0058 max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v www.maximintegrated.com maxim integrated 17 package information for the latest package outline information and land patterns (footprints), go to www.maximintegrated.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. chip information process: bicmos downloaded from: http:///
revision number revision date description pages changed 0 5/05 initial release 3 1/07 changed formula and updated figure 13 caption title 1, 14, 15, 17 4 3/07 updated electrical characteristics table. 1, 3, 18 5 1/09 updated electrical characteristics table. 3 6 7/14 deleted automotive references in general description , applications , and detailed description sections; deleted load dump section and figure 5 (renumbering the remaining igures) 1, 7, 10C15 maxim integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim integrated product. no circuit patent licenses are implied. maxim integrated reserves the right to change the circuitry and speciications without n otice at any time. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. maxim integrated and the maxim integrated logo are trademarks of maxim integrated products, inc. max6397/max6398 overvoltage protection switch/limiter controllers operate up to 72v ? 2014 maxim integrated products, inc. 18 revision history for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim integrateds website at www.maximintegrated.com. downloaded from: http:///

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