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typical operating circuit n m1 max4271 max4272 v out c board ctim cspd c tim on on gnd stat v cc r sense stat removable card 2.7v to 13.2v backplane in sense gate gnd general description the max4271/max4272/max4273 comprise a com- plete family of integrated 3v to 12v hot-swap con- trollers. they allow the safe insertion and removal of circuit cards into live backplanes. the discharged filter capacitors of the circuit card pro- vide a low impedance to the live backplane. high in- rush currents from the backplane to the circuit card can burn up connectors and components, or momentarily collapse the backplane power supply leading to a sys- tem reset. this family of hot-swap controllers prevents such problems by regulating the current to a preset limit when the board is plugged in, allowing the system to stabilize safely. after the startup cycle is completed, two on-chip comparators provide dualspeed/bilevel protection against short circuits, load glitches, and overcurrent conditions. in the event of a fault condition, the load is disconnected. fault recovery is handled by unlatching (max4271), autoretry (max4272), or pro- grammed (max4273) methods. the max4271 family includes many integrated features that reduce component count and design time. an on- board charge pump provides the gate drive for a low- cost, external n-fet. integrated features like startup current regulation and current glitch protection eliminate external timing resistors and capacitors. also featured are an open-drain status output to indicate a fault condi- tion, and an adjustable overcurrent response time. the max4271 (latched fault protection) and max4272 (autoretry fault protection) come in 8-pin so packages. the max4273 (full function) comes in the space-saving 16-pin qsop package and 16-pin so package. all parts are specified across the extended temperature range, and have an absolute maximum rating of 15v to provide extra protection against inductive kickback dur- ing board removal. features ? provide safe hot swap for +3v to +12v power supplies with few external components ? unique current regulation architecture minimizes n-fet linear mode duration ? autoretry feature (max4272/max4273) ? dualspeed/bilevel current limit protects against current glitches and short circuits ? power-on reset (max4273) ? 15v absolute maximum rating protects against inductive kickbacks during board removal ? internal charge pump generates gate drive for external n-mosfet ? status output pin indicates fault/safe condition ? space-saving 8-pin so, 16-pin qsop packages ________________________applications max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ________________________________________________________________ maxim integrated products 1 19-1694; rev 2; 12/07 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. evaluation kit available dualspeed/bilevel is a trademark of maxim integrated products. pin configurations appear at end of data sheet. base stations raid remote-access servers network routers and switches isdn ordering information part temp range pin- package pkg code max4271 esa -40 c to +85 c 8 so s8-1 max4272 esa -40 c to +85 c 8 so s8-1 max4273 eee -40 c to +85 c 16 qsop e16-1 max4273ese -40 c to +85 c 16 so e16-1
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v in = +2.7v to +13.2v, t a = -40? to +85?, unless otherwise noted. typical values are at v in = +5v and t a = +25?.) (note 3) 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. note 1: gate can be pulled below llmon, but current must be limited to 2ma. note 2: inc and on can be pulled below ground. limiting the current to 2ma ensures that these pins are never lower than about -0.8v. in to gnd ............................................................................+15v stat, outc, llmon, auxvcc to gnd ...............-0.3v to +14v gate to gnd ..............................................-0.3v to (v in + 8.5v) gate to llmon (note 1).........................................-1v to +8.5v inc, on to gnd (note 2) .........................................-1v to +14v cext to gnd ...............................................-8.5v to (v in + 0.3v) cspd, cton, ref to gnd ...........-0.3v to the lower of (v in + 0.3v) or +12v vsense, rth, ctim to gnd.......................-0.3v to (v in + 0.3v) current into inc, on (note 2) ............................................?ma current into any other pin ...............................................?0ma continuous power dissipation (t a = +70?) 8-pin so (derate 5.9mw/? above +70?)..................471mw 16-pin qsop (derate 8.3mw/? above +70?)...........667mw 16-pin so (derate 8.7mw/? above +70?)................696mw operating temperature range ...........................-40? to +85? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? parameter symbol conditions min typ max units power supplies input voltage range v in 2.7 13.2 v supply current i q v on = v in 0.6 1 ma current control t a = +25 c 455055 slow comparator threshold v sc , th v in - v sense t a = t min to t max 43.5 56 mv c spd = floating 10 20 40 s slow comparator response time t cspd c spd = 100nf to gnd 10 20 40 ms t a = +25 c 455055 5k on rth to v in t a = t min to t max 43.5 56 t a = +25 c 675 750 825 75k on rth to v in t a = t min to t max 650 840 max4273 only rth = gn d fast comparator threshold v fc , th max4271/max4272 180 200 220 mv fast comparator response time t fcd 10mv overdrive, from overload condition to gate discharging 350 ns sense input bias current i b , sense v sense = v in 0.2 10 a mosfet driver max4271/max4272 c tim = 100nf 21 31 41 max4273 c ton = 100nf 21 31 41 ms startup period (notes 4, 5) t start no capacitor 5.5 s gate charge current i gate gate = in (note 6) 100 a
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection _______________________________________________________________________________________ 3 electrical characteristics (continued) (v in = +2.7v to +13.2v, t a = -40? to +85?, unless otherwise noted. typical values are at v in = +5v and t a = +25?.) (note 3) parameter symbol conditions min typ max units slow turn-off time t slow , off time from trigger to v gate < 0.1v (triggered by either the on input or the slow comparator), c gate = 1000pf to gnd 60 ? fast turn-off time t fast , off time from current overload to v gate < 0.1v (triggered by the fast comparator, during normal operation), c gate = 1000pf to gnd 15 s maximum gate protection voltage voltage at which internal zener clamp circuitry is triggered, measured with respect to v in (max4271/max4272), measured with respect to v llmon (max4273) 6.7 7.5 v v in 5v 5 minimum gate drive voltage measured with respect to v in, i gate = 8.5? v in 2.7v 2.7 v during fast discharge, due to a fast comparator fault in normal operation 0.4 1 2.75 ma during startup (current regulation provided by fast comparator) 25 70 195 gate discharge current i gate , dis during normal discharge, due to a slow comparator fault in normal mode, or by on going low 75 200 550 a llmon overvoltage threshold startup is initiated only after v llmon is less than this voltage (max4273) 0.1 v gate overvoltage threshold startup is initiated only after v gate is less than this voltage 0.1 0.6 v llmon impedance im p ed ance to gn d , after a faul t ( m ax 4273) 1 k reference (max4273) output voltage v ref no load, v in = 5v 1.164 1.2 1.236 v line regulation v ref , line 2.7v v in 13.2v, no load 1 8 mv load regulation v ref, load i ref = 0 to 100 a, v in = 5v 0.6 3 mv on and reset comparators threshold voltage v in = 5v, rising threshold at on or inc 0.575 0.6 0.625 v hysteresis v hyst 3mv power-supply rejection ratio psrr 2.7v v in 13.2v ? mv/v 10mv overdrive, on going positive or negative, inc going negative 10 s propagation delay t d , comp inc going positive 100 150 200 ms
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 4 _______________________________________________________________________________________ parameter symbol conditions min typ max units input voltage range input can be driven to the absolute maximum limit without false output inversion -0.1 13.2 v input bias current i b , comp 0.001 1 a on pulse width low (note 7) t restart to unlatch a fault max4271, max4273 with c tim = in 20 s digital outputs (stat, outc) output leakage current v stat 13.2, v outc 13.2 1 a output voltage low v ol i sink = 1ma 0.4 v retry timing (max4272, max4273) retry timeout period t retry 100nf capacitor on c tim (note 5) 0.5 1 2 s default retry timeout period t retry (default) c tim = no connection 176 ? undervoltage lockout (uvlo) threshold v uvlo startup is initiated when this threshold is exceeded at in 2.25 2.67 v hysteresis v u v lo , h ys t 100 mv delay t d , uvlo time the input voltage must exceed undervoltage lockout before startup is initiated 100 150 200 ms electrical characteristics (continued) (v in = +2.7v to +13.2v, t a = -40? to +85?, unless otherwise noted. typical values are at v in = +5v and t a = +25?.) (note 3) note 3: all devices are 100% tested at t a = +25?. all temperature limits are guaranteed by design. note 4: startup period is the time during which the slow comparator is ignored and the fast comparator regulates the sense current. it is measured from the time on is brought high. note 5: inferred from test with c ton = 10nf (max4273) and c tim = 1nf. note 6: the current available at gate is a function of v gate (see typical operating characteristics ). note 7: guaranteed by design.
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection _______________________________________________________________________________________ 5 typical operating characteristics (v in = 5v, r sense = 100m , c board = 47?, t a = +25?, unless otherwise noted.) 0 0.3 0.2 0.1 0.5 0.4 0.9 0.8 0.7 0.6 1.0 02468101214 supply current vs. input voltage max4271/3-01 v in (v) supply current (ma) on = v in i gate = 10 a on = gnd -40 -15 10 35 60 85 supply current vs. temperature max4271/3-02 temperature ( c) 0 0.3 0.2 0.1 0.5 0.4 0.9 0.8 0.7 0.6 1.0 supply current (ma) on = v in v in = 12v v in = 5v v in = 3v 180 200 190 220 210 230 240 18 20 19 22 21 23 24 068 2 4 10 12 14 slow comparator response time vs. input voltage max4271/3-03 v in (v) t cspd ( s) t cspd (ms) cspd = 110nf time in ms cspd = 0 time in s 49.0 49.6 49.4 49.2 50.0 49.8 50.8 50.6 50.4 50.2 51.0 02468101214 slow comparator threshold vs. input voltage max4271/3-04 v in (v) v sc,th (mv) t a = -40 c t a = +25 c t a = +85 c 200 250 350 300 400 450 1 10 100 1000 fast comparator response time vs. overdrive voltage max4271/3-05 overdrive voltage (mv) response time (ns) v in = 12v v in = 5v v in = 3v 200 300 400 350 450 500 -40 10 -15 35 60 85 fast comparator response time vs. temperature max4271/3-06 temperature ( c) response time (ns) v in = 12v v in = 5v v in = 3v 250 190 196 194 192 200 198 208 206 204 202 210 fast comparator threshold vs. input voltage max4271/3-07 v fc,th (mv) 02468101214 v in (v) t a = -40 c t a = +25 c t a = +85 c 0.1 1 1k 100 10 10k 100k 0.01 1 0.1 10 100 1000 time to fast discharge gate vs. c gate max4271/3-08 c gate (nf) time to discharge gate ( s) v in = 12v v in = 5v v in = 3v no external mosfet discharge to v gate = 0.1v 250 270 310 290 330 350 04 2 6 8 101214 startup period vs. input voltage max4271/3-09 v in (v) t start ( s) 25 27 31 29 33 35 t start (ms) c tim = 100nf time in ms c tim = 1nf time in s
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 6 _______________________________________________________________________________________ typical operating characteristics (continued) (v in = 5v, r sense = 100m , c board = 47?, t a = +25?, unless otherwise noted.) 0 0.4 0.2 0.8 0.6 1.0 1.2 1.6 1.4 1.8 02468101214161820 fast gate discharge current vs. v gate max4271/3-10 v in (v) fast gate discharge current (ma) v in = 12v v in = 3v v in = 5v 0 60 40 20 80 100 120 08 6 2 4 10 12 14 16 18 20 gate charge current vs. gate voltage max4271/3-11 v gate (v) i gate ( a) v in = 12v v in = 5.0v v in = 3.0v 0 50 25 100 75 125 150 -40 10 35 -15 60 85 gate charge current vs. temperature max4271/3-12 temperature ( c) i gate ( a) v in = 12v v gate = 0 v in = 5v v in = 3v 0 5 15 10 20 25 04 2 6 8 101214 gate voltage vs. input voltage max4271/3-13 v in (v) v gate (v) i gate = 10 a t a = +85 c t a = +25 c t a = -40 c 0 100 50 150 300 350 250 200 400 0 46810 2 1214161820 slow gate discharge current vs. gate voltage max4271/3-13a v gate (v) i gate ( a) v in = 12v v in = 3v triggered by a fault or by on falling v in = 5v 0.01 0.1 1 10 100 1000 time to charge gate vs. c gate max4271/3-14 c gate (nf) time to charge gate (ms) 1000 0.001 0.01 1 0.1 100 10 no external mosfet v in = 12v to v gate = 17v v in = 5v to v gate = 10v v in = 3v to v gate = 6v 1.0 2.5 2.0 1.5 3.0 3.5 4.0 3.0 3.8 3.6 3.2 3.4 4.0 4.2 4.4 4.6 4.8 5.0 fast gate discharge current vs. v in max4271/3-15 fast gate discharge current (ma) rising v in falling v in v in (v) 0 100 50 200 150 350 300 250 400 -40 10 -15 35 60 85 slow gate discharge current vs. temperature max4271/3-16 temperature ( c) i gate ( a) v in = 3v v in = 5v, 12v v gate = v in triggered by a fault or by on falling 0.01 0.1 1 10 100 1000 time to slow discharge gate vs. c gate max4271/3-17 c gate (nf) time to discharge gate (ms) 1000 0.001 1 0.1 0.01 10 100 v in = 12v v in = 5v v in = 3v discharge to v gate = 0.1v no external mosfet
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection _______________________________________________________________________________________ 7 typical operating characteristics (continued) (v in = 5v, r sense = 100m , c board = 47?, t a = +25?, unless otherwise noted.) 1.0 1.4 1.2 1.8 1.6 2.2 2.0 2.4 -40 10 -15 356085 fast gate discharge current vs. temperature max4271/3-18 temperature ( c) fast gate discharge current (ma) v gate = v in v in = 3v v in = 5v, 12v c board = 470 f, r sense = 100m , c gate = 0 i load (1a/div) v out (2v/div) v gate (2v/div) 0 50 s/div max4271/max4272 turn-off time (c board = 470 f) max4271/3-19 on 0 c board = 470 f, r sense = 100m , c gate = 22nf, r s = 0 i load 1a/div v out 2v/div v gate 2v/div 0 0 200 s/div max4271/max4272 turn-off time (external c gate = 22nf, c board = 470 f) max4271/3-20 on 0.595 0.597 0.601 0.599 0.603 0.605 04 2 6 8 101214 on comparator threshold vs. input voltage max4271/3-21 v in (v) on comparator threshold (v) rising falling 0.5950 0.6000 0.5975 0.6050 0.6025 0.6075 0.6100 -40 35 60 -15 10 85 on-comparator threshold vs. temperature max4271/3-22 temperature ( c) on comparator threshold (v) rising falling } } v in = 12v v in = 5v v in = 3v 2.30 2.40 2.35 2.50 2.45 2.55 2.60 -40 35 60 -15 10 85 uvlo threshold voltage vs. temperature max4271/3-26 temperature ( c) uvlo threshold (v) rising falling 140 145 150 155 160 uvlo delay vs. temperature max4271/3-27 temperature ( c) uvlo delay (ms) -40 35 60 -15 10 85 v in = 3v v in = 5v, 12v max4273 turn-off time (external c gate = 22nf, c board = 470 f) max4271/3-20a i load 1a/div 0 on v out 2v/div v gate 2v/div 0 20 s/div max4273 turn-off time (c board = 470 f) max4271/3-20b v out 2v/div v gate 2v/div 0 i load 1a/div 0 on 50 s/div
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 8 _______________________________________________________________________________________ typical operating characteristics (continued) (v in = 5v, r sense = 100m , c board = 47?, t a = +25?, unless otherwise noted.) 0.625 0.605 0.615 0.595 0.585 0.575 410 6 8 12 14 16 inc threshold vs. input voltage max4271/3-28 input voltage (v) threshold (v) rising falling 600 610 608 606 604 602 614 612 618 616 620 -40 10 -15 356085 inc threshold vs. temperature max4271/3-29 temperature ( c) threshold (mv) rising falling 1.2020 1.2024 1.2022 1.2030 1.2028 1.2026 1.2038 1.2036 1.2034 1.2032 1.2040 379 5 111315 max4273 reference voltage vs. input voltage max4271/3-30 input voltage (v) reference voltage (v) 1.2000 1.2005 1.2010 1.2015 1.2020 1.2025 1.2030 1.2035 1.2040 -40 -15 10 35 60 85 reference voltage vs. temperature max4271/3-31 temperature ( c) reference voltage (v) 920 930 925 940 935 945 950 3579111315 retry timeout period vs. input voltage max4271/3-32 input voltage (v) retry timeout (ms) c tim = 100nf 965 970 975 980 985 990 995 -40 -15 10 35 60 85 retry timeout period vs. temperature max4271/3-33 temperature ( c) retry period (ms) 12v 5v 3v c board = 470 f, r sense = 100m , c tim = 10nf, c gate = 0 i load 1a/div v out 2v/div v gate 2v/div on 500 s/div startup time (c board = 470 f) max4271/3-34
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection _______________________________________________________________________________________ 9 typical operating characteristics (continued) ( v in = 5v, r sense = 100m , c board = 47?, t a = +25?, unless otherwise noted. ) c board = 0, r sense = 100m , c gate = 0, r s = 0 i load 1a/div v out 2v/div v gate 2v/div 0 50 s/div turn-off time (c board = 0) max4271/3-37 on 0 pin description c board = 0, r sense = 100m , c tim = 10nf, c gate = 0 v out 2v/div v gate 2v/div on 100 s/div startup time (c board = 0) max4271/3-35 pin max4271 max4272 max4273 name function 1 rth current-sense threshold setting input. connect a resistor from rth to v in to set the fast comparator threshold. bypass to v in with 0.1 f. connect rth to v in to disable both the fast comparator and current regulation at startup. short rth to gnd for a 200mv threshold. see fast comparator threshold section. ? auxvcc auxiliary supply input. supply input for short-circuit switchover. to use this, connect a 1 f capacitor from auxvcc to gnd; otherwise, leave floating. see auxiliary v cc section. 13in input voltage. connect to +2.7v to +13.2v supply; 15v absolute maximum voltage rating. 2 4 sense current-sense resistor voltage input. r sense is connected from in to sense. 3 5 gate gate drive output. connect to gate of external n-channel pass transistor. ? cext external gate capacitance connection. connect a capacitor from cext to gate to increase the gate charging time. this pin goes high impedance during a fast comparator fault for fast discharge. ? llmon load line monitor. connect to the source of the external n-channel mosfet. the external fet is turned on only when the load voltage is less than 100mv. 4 8 gnd ground 9 cton startup timer input. leave floating or connect the timing capacitor from cton to gnd. see startup and retry timers section. 5 10 cspd slow comparator speed setting. leave floating or connect the timing capacitor from cspd to gnd. see slow comparator response time section.
detailed description the max4271/max4272/max4273 are circuit breaker ics designed for hot-swap applications where a line card is inserted into a live backplane. normally, when a line card is plugged into a live backplane, the card? discharged filter capacitors provide a low impedance that can momentarily cause the main power supply to collapse. the max4271/max4272/max4273 are designed to reside either in the backplane or in the removable card to provide inrush current limiting and short-circuit protection. this is achieved by using an external n-channel mosfet, an external current-sense resistor, and two on-chip comparators. figures 1 and 2 show the functional diagrams. the timing and voltage levels for several critical parame- ters can be adjusted with external resistors, external capacitors, or by pin strapping. the timing components are optional; without them, the part is set to its nominal values, as shown in the electrical characteristics . the parameters that can be adjusted are: ?current-limit threshold ?slow comparator response time ?startup timer ?fast comparator threshold ?autoretry timeout period (time the part is shut down after an overcurrent event) ?fault management (latched/autoretry) ?current overload threshold startup mode ctim (max4271/max4272) or cton (max4273) sets the startup period (see startup and retry timers ). the startup period begins after three conditions are met: 1) 150ms after v in exceeds the uvlo threshold (see over/undervoltage lockouts ) and 2) 10? after v on >0.6v and 3) the device is no longer in retry mode. during startup, the slow comparator is disabled and the inrush current can be limited in two ways: 1) slow ramping of the current to the load by control- ling the external mosfet gate voltage or 2) limiting the current to the load by regulating the volt- age across the external current-sense resistor max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 10 ______________________________________________________________________________________ pin max4271 max4272 max4273 name function 6 11 ctim startup and retry timers input. controls the startup time and the autoretry time (32x startup time) in the max4272, only the autoretry time in the max4273, and only the startup time in the max4271. leave floating or connect the timing capacitor from ctim to gnd. connect to in for latched mode (this prevents autoretry in max4273). see startup and retry timers section. 7 12 stat status output. high indicates startup completed with no fault (table 1). stat is an open-drain output. 813on on comparator input. connect high for normal operation; connect low to force the mosfet off. comparator threshold v th,on = 0.6v allows for precise control over shutdown feature. pulse on low for 20 s min to unlatch after a fault (max4273 in latched mode, or max4271). negative pulses are ignored during autoretry (max4273 in autoretry mode, or max4272). see on and reset comparators section. 14 ref 1.2v reference output. do not bypass with a capacitor to gnd. 15 inc uncommitted comparator input. controls outc. 16 outc uncommitted comparator output. outc goes high 150ms after inc goes high. outc goes low immediately after inc goes low. outc is an open-drain output. pin description (continued)
unlike other circuit breaker ics, the max4271/ max4272/max4273 regulate the current to a preset level instead of completely turning off the external n-fet if an overcurrent condition occurs during startup. in startup mode, the gate drive current is limited to 100? and decreases with the increase of the gate voltage (see gate charge current vs. gate voltage in the typical operating characteristics ). this allows the controller to slowly enhance the mosfet. if the fast comparator detects an overcurrent, the gate voltage is momentarily discharged with a fixed 70? current until the load current through the sense resistor (r sense ) decreases below its threshold point. this effectively regulates the inrush current during startup. figure 3 shows the startup waveforms. stat goes high at the end of the startup period if no fault condition is present. normal operation (dualspeed/bilevel) in normal operation (after the startup timer has expired), protection is provided by turning off the exter- nal mosfet when a fault condition is encountered. dualspeed/bilevel fault protection incorporates two comparators with different thresholds and response times to monitor the current: 1) slow comparator . this comparator has an externally set response time (from 20? to seconds) and a fixed 50mv threshold voltage. the slow comparator ignores low-amplitude momentary current glitches. after an extended overcurrent condition, a fault is generated and the mosfet gate is slowly dis- charged. 2) fast comparator . this comparator has a fixed 350ns response time and a 200mv threshold voltage (adjustable from 50mv to 750mv in max4273). the fast comparator turns off the mosfet immediately after it detects a large amplitude event such as a short circuit. max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 11 figure 1. max4271/max4272 functional diagram max4271 max4272 6 a slow comparator fast comparator 6 a 2.45v c spd cspd input uvlo logic control discharge enable v in stat gnd ctim c tim 4 a 0.1v 0.6v on r sense n m1 gate sense in gate ovlo on comparator gate drive v sc,th 50mv v sc,th 200mv 150ms delay on v in rising v out v in charge pump (1mhz)
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 12 ______________________________________________________________________________________ figure 2. max4273 functional diagram max4273 6 a v in slow comparator fast comparator 6 a 2.45v input uvlo logic control discharge enable stat ref ref 1.2v auxvcc auxvcc gnd 0.1v 0.6v on llmon inc v out r sense v in c spd n q1 gate cext cext sense rth rth in output ovlo 0.1v gate ovlo on comparator reset comparator gate drive v fc,th = 50mv v fc,th = 200mv v fc,th = 50mv to 750mv 150ms delay on v in rising outc v in ctim ctim 4 a 10 a auxvcc v in cton cton 4 a 150ms delay on inc rising 0.6v charge pump 1mhz
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 13 in each case, when a fault is encountered, the status pin (stat) goes low, and for the max4273, the device discharges the output voltage through a 1k resistor from llmon to gnd. after a fault, the max4271 stays latched off and the max4272 enters retry mode, while the max4273 has selectable latched or retry mode. figure 4 shows the waveforms of a fault condition. bilevel fault protection slow comparator the slow comparator is disabled at startup while the external mosfet is turning on. this allows the part to ignore the higher-than-normal inrush current charging the board capacitors when a card is first plugged in. if the slow comparator detects an overload condition while in normal operation (after startup is completed), it turns off the external mosfet by discharging the gate capacitance with a 200? current. the slow compara- tor threshold is set at 50mv and has a default delay of 20? (cspd floating), allowing it to ignore power-sup- ply glitches and noise. the response time can be lengthened with an external capacitor at cspd (figure 10). if the overcurrent condition is not continuous, then the duration above the threshold minus the duration below must be greater than 20? (or the external programmed value) for the device to trip. when the current is above the threshold, cspd is charged with a 6a current source. a fault is detected when cspd is charged to the trip point of 1.2v. therefore, a pulsing current with a duty cycle of 50% or greater (i.e., the current is above the threshold level > 50% of the time) is considered a fault condition even if it is never higher than the thresh- old for longer than the slow comparator? set response time. the discharge rate depends on the n-fet gate capaci- tance and the external capacitance at gate. in the max4273, cext remains connected and capacitance to this point has to be discharged by the same current. this increases the discharge time. once the fault con- dition is detected, the stat pin goes low and the device goes into retry or latched mode. fast comparator the fast comparator behaves differently according to the operating mode. during startup, the fast comparator is part of a simple current regulator. when the sensed current is above the fast comparator threshold, the gate is discharged with a 70? current source. when the sensed current drops below the threshold, the charge pump turns on again. the sensed current will rise and fall near the threshold due to the fast comparator and charge-pump propagation delay. the gate voltage will be roughly sawtooth shaped, and the load current will present a 20% ripple. the ripple can be reduced by adding a capacitor from gate to gnd. figure 3. startup waveforms t start t on 2.7v to 6v v gate i fast, set v gate stat on v th v out c board = large c board = 0 v out i load figure 4. response to a fault condition t off v gate stat v out i load i lim 2.7v to 6v
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 14 ______________________________________________________________________________________ table 1. status output truth table if the sensed current is still high after the startup timer expires, the mosfet gate is discharged completely as described below. in normal operation (after startup), the fast comparator is used as an emergency off switch. if the load current reaches the fast comparator threshold, the device quickly forces the mosfet off completely. this could happen in the event of a serious current overload or a dead short. the fast comparator has a 350ns response time and dis- charges gate with a 1ma current. given a 1000pf gate capacitance and 12v gate voltage, the mosfet will be off in about 12?. any additional capacitance connected between gate and gnd to slow down the startup time also increases the turn-off time. in the max4273, cext goes high impedance during the fast discharge. this reduces the effective capaci- tance on gate if a capacitor is used between gate and cext, and allows the mosfet to quickly turn off. in turn, this allows adjustment of the mosfet charging time without affecting the fast discharge rate, although it does affect the slow discharge rate. the max4271/max4272 fast comparator threshold is set to four times the slow comparator threshold (i.e., 200mv). the max4273 fast comparator threshold is set to 200mv by connecting rth to gnd, is disabled by connecting rth to in, or is adjustable by an external resistor con- nected to in (see fast comparator threshold (rth) ). latched/autoretry the max4271 mosfet driver stays latched off after a fault condition until it is reset by a negative-going pulse on the on pin. the max4272 is periodically turned on after a fault condition with a timeout duration set by an external timing capacitor on ctim. the max4273 has a selectable latched mode or retry mode. connect ctim to in to set the device in latched mode, or use an exter- nal capacitor at ctim to set the retry timeout. pulse on low for 20? (min) to restart after a fault (max4271/max4273 in latched mode). negative pulses are ignored during autoretry (max4273 in autoretry mode, or max4272). the capacitor on ctim affects the max4272? retry timeout period (time the part is shut down after an over- current event) and the startup time (see the electrical characteristics ). the retry timeout period is fixed at 32 times the startup time in order to minimize power dissi- pation in the external mosfet in case of a short-circuit condition (see mosfet thermal considerations ). this is not an issue for parts latched off during a fault condi- tion since they stay off until commanded on. the max4273 configured in retry mode has a separate startup timer capacitor (cton) and retry timeout capacitor (ctim). this allows the user to change the ratio between startup time and retry timeout period. status output the status output is an open-drain output that goes low under the following conditions: ?during the uvlo delay period ?in startup ?forced off (on <0.6v) ?in an overcurrent condition ?in the retry timeout period (or latched off, for the latched parts) stat is high only if the part is in normal mode and no faults are present (table 1). figure 5 shows the status (stat) output timing diagram. over/undervoltage lockouts the uvlo prevents the max4271/max4272/max4273 from turning on the external mosfet until v in exceeds the lockout threshold (2.25v min) for 150ms. the uvlo protects the external mosfet from insufficient gate drive voltage. the 150ms timeout ensures that the board is fully plugged into the backplane and that v in in uvlo delay period part in startup on pin overcurrent condition part in retry-timeout period (or latched off due to overcurrent condition) stat pin (status) yes x x x x low x yes x x x low x x low x x low x x x yes x low x x x x yes low no no high no no high
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 15 is stable. any input voltage transient at in below the uvlo threshold will reset the device and initiate a start- up sequence. these devices also have an overvoltage lockout (ovlo) feature that prevents the device from restarting after a fault condition if the discharge has not been completed. v gate has to be discharged to below 0.1v. additionally, the max4273 llmon pin discharges the load line with a 1k pulldown and prevents startup until the load voltage is below 0.1v. since the max4271/max4272 do not monitor the output voltage, a startup sequence can be initiated while the board capacitance is still charged. a large board capacitance or a short startup period may prevent the max4272 from charging completely in one startup period. the max4272 responds to these condi- tions by charging the capacitor with bursts defined by a t on duty cycle and a period of t on + t retry . the charg- ing will be complete after several retries unless the resis- tive load or current load excessively discharges the board capacitance during the retry timeout. this feature applies to the max4273 if llmon is left floating or is connected to gnd. to prevent multiple charging bursts, ensure that the t on timer exceeds the minimum time required to complete the charge of the board capaci- tance (see component selection ). gate overvoltage protection new-generation mosfets have an absolute maximum rating of ?v for the gate-to-source voltage (v gs ). to protect these mosfets, the max4271/max4272 limit the gate-to-drain voltage (the max4273 limits the gate-to- source voltage) to +7.5v with an internal zener diode. no protection is provided for negative v gs (max4271/ max4272). if gate can be discharged to ground faster than the output voltage, an external small-signal protec- tion diode (d1) can be used, as shown in figure 6. the max4273 has the protection diode internal. applications information component selection n-channel mosfet select the external n-channel mosfet according to the application? current level. table 2 lists some rec- ommended components. the mosfet? on-resistance (r ds(on) ) should be chosen low enough to have a min- imum voltage drop at full load to limit the mosfet power dissipation. high r ds(on) can cause output rip- ple if the board has pulsing loads or triggers an exter- nal undervoltage reset monitor at full load. determine the device power-rating requirement to accommodate a short circuit on the board at startup with the device figure 5. status (stat) output timing diagram o 1.2v t start o o stat ctim (cton)* on fault condition, or on falling edge no fault conditions present *max4273 only v in v in table 2. recommended n-channel mosfets part number manufacturer description irf7413 11m , 8 so, 30v irf7401 22m , 8 so, 20v irl3502s international rectifier 6m , d2pak, 20v mmsf3300 20m , 8 so, 30v mmsf5n02h 30m , 8 so, 20v mtb60n05h motorola 14m , d2pak, 50v fds6670a 10m , 8 so, 30v nds8426a 13.5m , 8 so, 20v fdb8030l fairchild 4.5m , d2pak, 30v figure 6. external gate-source protection d1 gate r sense v out v gd v gs c board in sense m1 n max4271 max4272 gate drive charge pump
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 16 ______________________________________________________________________________________ configured in automatic retry mode (see mosfet thermal considerations ) . using the max4271/max4273 in latched mode allows the consideration of mosfets with higher r ds(on) and lower power ratings. a mosfet can typically withstand single-shot pulses with higher dissipation than the specified package rating. low mosfet gate capaci- tance is not necessary since the inrush current limiting is achieved by limiting the gate dv/dt. table 3 lists some recommended manufacturers and components. sense resistor the slow comparator threshold voltage is set at 50mv. select a sense resistor that causes a 50mv voltage drop at a current level above the maximum normal operating current; typically, set the overload current at 1.2 to 1.5 times the nominal load current. the fast com- parator threshold is typically set at 200mv. this will set the fault current limit at four times the overload current limit. the max4273 fast comparator threshold can be set between 50mv and 750mv; see table 4 for a detailed listing. choose the sense resistor power rating according to the device configuration. if no retry mode is selected, p rsense = (i overload ) 2 x r sense ; if retry is selected, then p rsense = (i fault ) 2 x r sense x (t on /t retry ). fast comparator threshold (rth) (max4273) the fast comparator threshold is determined by the external resistor connected at rth. to select threshold voltages between 50mv and 750mv, use resistor val- ues between 5k and 75k according to figure 7. resistor values between 200 and 5k are not recom- mended. setting the threshold voltage of the fast com- parator below 50mv will effectively override the slow comparator operation. the max4273 fast comparator can be disabled by shorting the rth pin to v in (v in - 25mv or less). ground rth to set the threshold to 200mv internally. startup and retry timers (ctim, cton) the startup (t start ) and retry (t retry ) timers are deter- mined by the capacitors connected at ctim and cton. the capacitor connected to ctim has two func- tions for the three devices as follows: table 4. current levels vs. r sense r sense (m ) part number overload threshold set by slow comparator (a) fault current threshold set by fast comparator (a) max4271/max4272 5 20 10 max4273 5 5 to 75 max4271/max4272 1 4 50 max4273 1 1 to 15 max4271/max4272 0.5 2 100 max4273 0.5 0.5 to 7.5 table 3. component manufacturers component manufacturer phone web dale-vishay 402-564-3131 www.vishay.com sense resistors irc 704-264-8861 www.irctt.com fairchild 888-522-5372 www.fairchildsemi.com international rectifier 310-322-3331 www.irf.com mosfets motorola 602-244-3576 www.mot-sps.com/ppd/ device ctim function max4271 startup time max4272 startup time and sets retry timer max4273 sets retry timer
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 17 cton determines the maximum time allowed to com- plete turn-on for the max4273. the default values for turn-on time (t on ) and t retry are chosen by leaving these pins floating; they are 10? and 320?, respec- tively. these are also the minimum values (not con- trolled and dependent on stray capacitance). longer timings are determined by the size of the capacitor according to figure 8, and can be determined in table 5, which lists the startup and retry timing parameters. set the t on timer long enough to allow for the mosfet to be enhanced and the load capacitor to be charged com- pletely . there are two ways of completing the startup sequence. case a describes a startup sequence that does not use the current-limiting feature and slowly turns on the mos- fet by limiting the gate dv/dt. case b uses the current- limiting feature and turns on the mosfet as fast as possible while still preventing a high inrush current. case a. startup without current regulation there are three ways to turn on the mosfet without reaching the fast comparator current limit: 1) if the board capacitance is small, the inrush current is low. 2) if the gate capacitance is high, the mosfet turns on slowly. 3) the fast comparator can be disabled (max4273 only). in all three cases, t on is determined only by the charge required to enhance the mosfet. effectively, the small gate-charging current limits the output voltage dv/dt. this time can be extended by connecting an external capacitor between gate and gnd (max4271/ max4272) (figure 9) or between gate and cext (max4273). the turn-on time is dominated by the exter- nal gate capacitance if this value is considerably higher than the mosfet gate capacitance. table 6 shows the timing required to enhance the recommended mosfet with or without the external capacitor; figure 3 shows the related waveforms and timing diagrams. (see time to charge gate vs. c gate and startup time with c board = 0 in the typical operating characteristics .) remember that a high gate capacitance also increases the turn-off time (t off ), except in the case of a max4273 fast fault. if an external gate capacitor is not used, r s is not nec- essary. r s prevents mosfet self-oscillations that can occur when c gate is high while c board is low. electrical characteristics as specified by the manufac- turer? data sheet are: fds6670a: c iss = 3200pf, q t (max) = 50nc, r ds(on) = 8.2m irf7401: c iss = 1600pf, q t (max) = 48nc, r ds(on) = 22m mmsf5n03hd: c iss = 1200pf, q t (max) = 21nc, r ds(on) = 40m figure 7. fast comparator threshold vs. rth 0 200 100 400 300 500 600 700 800 02030 10 40 50 60 70 80 fast comparator threshold voltage vs. rth rth (k ) v th , fc (mv) rth (k ) = v th , fc (mv)/10 indeterminate, not recommended for rth < 5k disabled for rth < 200 *max4273 retry feature disabled by connecting ctim to v in . table 5. startup and retry timing parameters t on t retry part default (s) externally set default (s) externally set max4271 10 t on (ms) = 0.31 x ctim (nf) no retry available max4272 10 t on (ms) = 0.31 x ctim (nf) 320 t retry (ms) = 32 x t on = 10 x ctim (nf) max4273* 10 t on (ms) = 0.31 x cton (nf) 320 t retry (ms) =10 x ctim (nf)
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 18 ______________________________________________________________________________________ case b. startup with current regulation in applications where the board capacitance (c board ) at v out is high, the inrush current causes a voltage drop across r sense that exceeds the fast comparator threshold (v fc,th ). in this case, the current charging c board can be considered constant and the turn-on time is determined by: t on = c board ? v in / i fast,set where the maximum load current i fast,set = v fc,th / r sense . figure 3 shows the waveforms and timing dia- grams for a startup transient with current regulation. (see startup time (c board = 470?) in the typical operating characteristics. ) when operating under this condition, an external gate capacitor is not required. adding an exter- nal gate capacitor at gate to gnd reduces the regulat- ed current ripple but increases t off by increasing the gate delay (t d ) (figure 4). the actual startup time is determined by the longer of the two timings of case a and case b. set the startup timer t start at 2 ? t on to guarantee enough time for the output voltage to settle; also take into consideration device parameter variation. retry the retry timer defines the dead time before the ic tries to restart a startup sequence following a fault detection. this feature is available on the max4272/max4273. before selecting the retry timer value, determine how long a temporary high-current fault condition may be present. in the event of a permanent fault, the automat- ic retry will effectively force current pulses through the figure 8. startup and retry timeout t on and t retry vs. cton and ctim capacitance (nf) t on and t retry (ms) 10,000 0.1 0.01 1 10 100 1000 0.01 10 100 0.1 1 1000 t retry vs. ctim (max4272/max4273) t on vs. ctim (max4271/max4272) t on vs. cton (max4273) figure 9. operation with an external capacitor m1 c spd * optional (see text) c tim ctim gate sense gnd cspd on r sense v out c gate c board v in in r s * max4271 max4272 max4273 r pullup stat table 6. ?no-overcurrent? turn-on timing (startup without current limit) mosfet t on (ms) mosfet t off (ms) device c gate (nf) v in = 3v v in = 5v v in = 12v v in = 3v v in = 5v v in = 12v 0 0.22 0.16 0.19 0.07 0.13 0.145 fairchild fds6670a 22 2.3 2 3.2 0.54 1.1 1.95 0 0.175 0.130 0.16 0.075 0.13 0.16 international rectifier irf7401 22 1.9 1.8 3.5 0.54 1.1 2.0 0 0.101 0.074 0.073 0.033 0.067 0.085 motorola mmsf5n03hd 22 2.0 1.8 3.2 0.470 1.0 1.95
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 19 mosfet with a duty cycle equal to t on/ t retry and with a current equal to i fast,set . therefore, particular care has to be taken when choosing between immediate retry and board space needed to manage the power dissipation capabilities of the mosfet (see thermal considerations ). the duty cycle is fixed to 1/32 for the max4272, but can be varied in the max4273 by choos- ing ctim and c ton independently. additional external gate capacitance (cext) an external gate capacitance can be connected at gate. this increases the time required to enhance the mosfet and further limits the output rise time. in the max4271/max4272, connect the external capacitor between gate and gnd. in the max4273, the external capacitor can be connected between gate and cext or gnd. if the capacitor is connected to cext, it is dis- charged to ground during a slow comparator fault but it is left floating during a fast comparator fault; this allows the device to turn off the external mosfet faster during critical faults. (cext is biased at v in ; therefore, use a nonpolarized capacitor). capacitance connected from gate to cext does little to decrease the regulated cur- rent ripple. add a small capacitor (5nf) from gate to gnd. see the charging and discharging time vs. c gate graphs in the typical operating characteristics . slow comparator response time (cspd) the slow comparator threshold is set at 50mv, and its response time is determined by the external capacitor connected to cspd (figure 10). a minimum response time of 20? (typ) is achieved by leaving this pin floating. this time is determined inter- nally and is not affected by stray capacitance at cspd (up to 100pf). set the slow comparator response time to be longer than the normal operation load transients (see slow comparator ). on and reset comparators the on comparator controls the on/off function of these devices. the on comparator is a precision volt- age comparator that can be used for temperature moni- toring or as an additional uvlo (figure 11). the max4273 also features an uncommitted delayed com- parator. this comparator can be used for voltage moni- toring, power sequencing, or for generating a power-on reset signal for on-card microprocessors (figure 12). both comparator threshold voltages are set at v ref /2 = 0.6v with a 3mv (typ) hysteresis. the uncommitted comparator outc output is an open- drain output, and it is asserted low when its input volt- age (inc) is below the threshold voltage. it goes into a high-impedance state 150ms after the voltage has risen above the threshold. the delay for negative-going edges is 10?. figure 13 shows the max4273 used to monitor precise- ly the temperature of an external device such as the mosfet. this configuration uses the uncommitted comparator to set the uvlo at a higher level by running its output into the on comparator? input. the on comparator initiates startup when its input volt- age (v on ) rises above the threshold voltage and turns off the mosfet when the voltage falls below the thresh- old. the propagation delay is 10? going high or low. the on comparator is also used to reset the max4271/max4273 (when ctim = v in ) after a fault condition (see latched/autoretry ). figure 10. slow comparator response time vs. cspd 1000 0.01 0.1 1 10 100 1000 100 10 1 0.1 0.01 slow comparator response time vs. cspd cspd (nf) response time (ms) t cspd (ms) = 0.2 x cspd (nf) figure 11. temperature monitoring and protection logic control v ref r2 on ntc r1 = r2 ? (v ref / o.6 - 1) r2 = value of the ntc resistor at the limit temperature v ref = any reference voltage available or v in 0.6v r1 max4271 max4272 max4273
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 20 ______________________________________________________________________________________ the on and inc comparator inputs and the stat and outc can be pulled to voltages up to 14v indepen- dently of v in , thus allowing parts to be daisy-chained and not be turned on through the internal protection diodes. in some applications, it is useful to use connec- tors with staggered leads. in figure 14, the on pin forces the removable board to be powered up only when all connections are made. auxiliary v cc the auxiliary v cc is available on the max4273 and is used to sustain the input voltage required for the device to operate during a short-circuit condition on the board. when a short occurs, the main system power supply could collapse and the max4273 will not have enough voltage to keep the gate drive operational and turn off the external mosfet. if the fault is not removed, the system could remain in a sustained short-circuit state. connect a 1? capacitor from the auxvcc pin to gnd. this capacitor will deliver the necessary energy to the gate drive until the mosfet is turned off and the main supply recovers. the 1? capacitor is charged from v in through an internal switch during normal operation. maximum load capacitance the max4271/max4272/max4273 can be used on the backplane to regulate current upon insertion of a removable card (figure 16). this allows multiple cards with different input capacitance to be inserted into the same slot even if the card doesn? have on-board hot- swap protection. the max4271/max4272/max4273 current-limiting fea- ture is active during the startup period set by ctim. the startup period can be triggered if v in is connected to on through a trace on the card. once t start has expired (timed out), the load capacitance has to be charged or a fault condition is detected. to ensure startup with a fixed ctim, t start has to be longer than the time required to charge the board capacitance. the maximum load capacitance is calculated as follows: c board < t start ? i fast,set / v in input transients the voltage at v in must be above the uvlo during inrush and fault conditions. when a short condition occurs on the board, the fault current can be higher than the fast comparator current limit. the gate voltage figure 12. power-on reset outc reset p inc llmon in sense gate v in max4273 figure 14. fail-safe connector in on backplane 10k 1m reset v cc max4271 max4272 max4273 sense gate removable card figure 13. power-on reset and temperature gnd llmon outc on ref in inc sense gate ntc v in v out max4273
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 21 is discharged immediately, but note that the mosfet is not completely off until v gs < v th . if the main system power supply collapses below uvlo, the max4271/ max4272/max4273 will force the device to restart in startup mode with a 150ms delay once the supply has recovered. the main system power supply must be able to deliver this fault current without excessive volt- age drop. the mosfet is turned off in a very short time; there- fore, the resulting dv/dt can be considerable. the back- plane delivering the power to the external card must have a fairly low inductance to limit the voltage tran- sients caused by the removal of a fault. bypassing the input with a small capacitor alleviates false uvlo trips due to these transients. mosfet thermal considerations during normal operation, the mosfet dissipates little power, it is fully turned on, and its r ds(on) is minimal. the power dissipated in normal operation is p d = i load 2 x r ds(on) . a considerable amount of power is dissipated during the startup and turn-off transients. the design must take into consideration the worst-case scenario of a continuous short-circuit fault present on the board. two cases need to be considered: 1) the single turn-on with the device latched after a fault (when using max4271 or max4273 in latched mode) 2) the continuous automatic retry (when using the max4272 or max4273 in retry mode) use the following equation to calculate the maximum transient thermal resistance (in ?/w) required for an output short to ground: z ja (max) = (t jmax - t a ) / (v in ? i fast, set ) mosfet manufacturers typically include curves for the transient thermal resistance, z ja , of the package (figure 17). find the thermal impedance of the mos- fet by using t start as the pulse duration and by choosing the single pulse curve for latched mode parts or by choosing the duty cycle = 0.03 curve for the max4272 (the duty cycle is fixed at 32:1). if the z ja required is less than that of the package, reduce t start , reduce i fast,set , use a heatsink on the mos- fet, or choose one with better thermal characteristics. figure 16. using the max4271/max4272/max4273 on a backplane in v in on backplane max4271 max4272 max4273 sense gate ctim c board v out removable card with no hot-insertion protection figure 15. adjustable undervoltage lockout and output voltage reset generator outc reset p inc llmon on in sense gate v in max4273 100k
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 22 ______________________________________________________________________________________ for the max4273 in retry mode, the duty cycle can be adjusted. use the mosfet z ja curve and the t start pulse duration to choose a maximum duty cycle, d. calculate the retry time: t retry = t start / d use figure 8 to determine ctim given t retry . design procedure (max4273) given: hot-swap 5v supply to a 1000? card mosfet irf7413a: r ds(on) = 0.0135 operating current = 4a overload current = 5a system current limit = 10a retry enabled procedure: 1) select the current sense resistor: slow comparator threshold = 50mv overload current = 5a current-sense resistor value = 50mv/5a = 10m the device will initiate a slow fault if the load current is greater than 5a for longer than 20? after startup. 2) set fast comparator threshold (rth). the mosfet pulsed drain current limit is 58a. another consideration for setting the current limit is the system requirement. systems may glitch if 58a load transients are present. for this example, the load transient will be limited to 10a: system current limit = 10a current-sense resistor = 10m fast comparator threshold = 10m ? 10a = 100mv select rth from figure 7, rth = 10k 3) set startup timer. startup current = system current limit = 10a v in = 5v c board = 1000? t on = 1000? ? 5v / 10a = 500? give a factor of 2 guardband on the startup timer. t start = 2 ? t on = 1.0ms from figure 8, c ton = 3000pf. 4) select retry timeout. v in = 5v. i max = 10a. peak junction temperature, t j = 150? j peak ambient temperature, t a = 85? a 100 10 1 0.1 0.01 0.001 0.0001 0.1 1 10 100 thermal response (z ja ) t 1 , rectangular pulse duration (s) p dm t 1 t 2 notes: 1. duty factor d = t 1 / t 2 2. peak t j = p dm x z ja + t a d = 0.50 0.20 0.10 0.05 0.02 0.01 single pulse (thermal response) figure 17. example curves (from irf7413a) for maximum effective transient thermal impedance, junction to ambient
calculate the mosfet thermal resistance required for a short to ground. z ja = (150? - 85?) / (5v ? 10a) = 1.30?/w using t start = 1ms as the pulse duration, use figure 17 to select a duty cycle. the duty cycle should be about 0.01 or less. this implies t retry = t start / 0.01 = 100ms. from figure 8, c tim = 10nf. layout considerations to take full tracking advantage of the switch response time to an output fault condition, it is important to keep all traces as short as possible and to maximize the high-current trace dimensions to reduce the effect of undesirable parasitic inductance. place the max4271/ max4272/max4273 close to the card? connector. use a ground plane to minimize impedance and induc- tance. minimize the current-sense resistor trace length (<10mm), and ensure accurate current sensing with kelvin connections (figure 18). when the output is short circuited, the voltage drop across the external mosfet becomes large. hence, the power dissipation across the switch increases, as does the die temperature. an efficient way to achieve good power dissipation on a surface-mount package is to lay out two copper pads directly under the mosfet package on both sides of the board. connect the two pads to the ground plane through vias, and use enlarged copper mounting pads on the top side of the board. max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 23 ctim cspd gnd 1 2 8 7 on stat sense gate in so top view 3 4 6 5 max4271 max4272 pin configurations 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 rth outc inc ref on stat ctim cspd cton max4273 qsop/so auxvcc in cext sense gate llmon gnd figure 18. kelvin connections for the current-sense resistors sense resistor high-current path max4271 max4272 max4273
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 24 ______________________________________________________________________________________ soicn .eps package outline, .150" soic 1 1 21-0041 b rev. document control no. approval proprietary information title: top view front view max 0.010 0.069 0.019 0.157 0.010 inches 0.150 0.007 e c dim 0.014 0.004 b a1 min 0.053 a 0.19 3.80 4.00 0.25 millimeters 0.10 0.35 1.35 min 0.49 0.25 max 1.75 0.050 0.016 l 0.40 1.27 0.394 0.386 d d min dim d inches max 9.80 10.00 millimeters min max 16 ac 0.337 0.344 ab 8.75 8.55 14 0.189 0.197 aa 5.00 4.80 8 n ms012 n side view h 0.244 0.228 5.80 6.20 e 0.050 bsc 1.27 bsc c h e e b a1 a d 0 -8 l 1 variations: 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 .)
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection ______________________________________________________________________________________ 25 qsop.eps package information (continued) (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 .)
max4271/max4272/max4273 3v to 12v current-limiting hot-swap controllers with autoretry, dualspeed/bilevel fault pr otection 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. 26 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2007 maxim integrated products is a registered trademark of maxim integrated products, inc. revision history revision number revision date description pages changed 0 4/00 initial release 1 7/00 removed future product designation for max4273 1 1a 4/01 updated footer 1 2 12/07 included package codes in ordering information table, updated note 5 and package outlines 1, 4, 24

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