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general description the MAX8704 high-current linear regulator uses an external n-channel mosfet to generate low-voltage supplies for notebook computers. this linear regulator delivers an output voltage as low as 0.5v from an input voltage as low as 1.0v. normally, this low input require- ment would make the design of such a regulator very dif- ficult. in this application, the 5v bias supply that is always available in the system powers the MAX8704 dri- ver and control circuitry. the MAX8704 includes a fixed current limit and an adjustable power limit to protect the external mosfet from overheating. additionally, the MAX8704 includes an internal thermal limit to prevent damage to the con- troller and provide remote thermal protection for the external mosfet. the MAX8704 features an adjustable soft-start function and generates a delayed power-good (pgood) signal that signals when the linear regulator is in regulation. the MAX8704 is available in a 10-pin ?ax package. applications v mch and v ccp cpu supplies notebook computers desktop computers servers vid power supplies low-voltage bias supplies features ? low-cost, high-current linear regulator ? external mosfet protection mosfet power limit 50mv (typ) current limit thermal limit ? 1.0v to 5.5v input supply voltage ? 1.2v or 1.5v preset, or adjustable output voltage ? power-good (pgood) open-drain output with 3ms startup delay ? programmable soft-start ? shutdown with output discharge MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet ________________________________________________________________ maxim integrated products 1 1 2 3 4 5 10 9 8 7 6 drv gnd csp csn plim pgood v cc v in MAX8704 max top view fb ss/en pin configuration ordering information 19-3420; rev 0; 9/04 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. evaluation kit available part temp range pin-package MAX8704eub -40 c to +85 c 10 ?ax ?ax is a registered trademark of maxim integrated products, inc.
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. v cc , v in to gnd.......................................................-0.3v to +6v csp, csn, drv to gnd ...........................................-0.3v to +6v fb, plim, ss/en, pgood to gnd.............-0.3v to (v cc + 0.3v) continuous power dissipation (t a = +70?) 10-pin ?ax (derated 5.6mw/? above +70?) .........444mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics (v in = 2.5v, v cc = 5.0v, plim = fb = gnd, csp = csn, ss/en floating, t a = 0c to +85c , unless otherwise noted. typical values are at t a = +25?.) parameter symbol conditions min typ max units v in 1.0 5.5 input voltage range v cc 4.5 5.5 v fb = v cc 1.462 1.50 1.538 preset output voltage (fixed) v out fb = gnd 1.170 1.20 1.230 v feedback voltage accuracy (adjustable) v fb fb = csn 490 500 510 mv load-regulation error v csp - v csn = 45mv -2.5 -2 % line-regulation error v in = 1v to 5.5v 0.01 % fb input bias current i fb v fb = 0.6v -1 +1 ? csn input bias current i csn v csn = 1.6v 50 100 ? output high v cc - 1.0 v cc - 0.7 drv output voltage swing v drv output low 0.7 1.0 v drv slew rate c drv = 40nf 0.2 v/? quiescent supply current (v cc )i cc fb forced above the regulation point, v csn = 1.6v 1.5 3 ma quiescent supply current (v in )i in fb forced above the regulation point, v csn = 1.6v 510a shutdown supply current (v cc ) ss/en = gnd 35 70 ? shutdown supply current (v in ) ss/en = gnd 5 10 a fault detection thermal-shutdown threshold t shdn rising edge, 20 c hysteresis +140 c v cc undervoltage-lockout threshold rising edge, 15mv hysteresis 4.2 4.45 v current-limit threshold v cslimit plim = gnd 45 50 57 mv power-limit threshold v pwrlimit rising edge 0.96 1.0 1.04 v power-limit conversion gain k plim v csp - v csn = 30mv, v csn = 0.5v, v in = 3.5v 155 200 233 ?/v 2 power-limit conversion gain variation v csp - v csn = 25mv to 45mv, v csn = 0.5v, v in = 2v to 4.5v ?2 %
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet _______________________________________________________________________________________ 3 electrical characteristics (continued) (v in = 2.5v, v cc = 5.0v, plim = fb = gnd, csp = csn, ss/en floating, t a = 0c to +85c , unless otherwise noted. typical values are at t a = +25?.) parameter symbol conditions min typ max units plim output current v csp - v csn = 30mv, v in = 3.5v, v csn = 0.5v 14 18 21 ? plim output current offset csp = csn, v in = 1.0v, v csn = 0.5v 0.5 2 a csp input current v csn = 1.50v, v csp = 1.55v -1 +1 ? soft-start and shutdown soft-start charge current i ss v ss/en = 1.5v 4 5 6 ? ss/en full current threshold 2v ss/en enable threshold rising edge 0.4 0.5 0.6 v ss/en discharge current i ss/en v ss/en = 1.5v, thermal fault, bias fault condition, or uvlo 10 20 ? discharge-mode on-resistance r csn 10 ? inputs and outputs pgood trip threshold with respect to error-comparator threshold, 2% hysteresis -10 -8 -6 % pgood startup delay 135ms pgood output low voltage i sink = 4ma 0.3 v pgood leakage current i pgood v fb = 1.0v (pgood high impedance), pgood forced to 5v -1 +1 ? electrical characteristics (v in = 2.5v, v cc = 5.0v, plim = fb = gnd, csp = csn, ss/en floating, t a = -40c to +85c , unless otherwise noted.) (note 1) parameter symbol conditions min max units v in 1.0 5.5 input voltage range v cc 4.5 5.5 v fb = v cc 1.455 1.545 preset output voltage (fixed) v out fb = gnd 1.158 1.242 v feedback voltage accuracy (adjustable) v fb fb = csn 485 515 mv output high v cc - 1.1 drv output voltage swing v drv output low 1.1 v quiescent supply current (v cc )i cc fb forced above the regulation point, v csn = 1.6v 3ma quiescent supply current (v in )i in fb forced above the regulation point, v csn = 1.6v 10 ? shutdown supply current (v cc ) ss/en = gnd 70 ? shutdown supply current (v in ) ss/en = gnd 10 ?
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 4 _______________________________________________________________________________________ t ypical operating characteristics (circuit of figure 1, v out = 1.5v, t a = +25?, unless otherwise noted.) -3.0 -2.0 -2.5 -1.0 -1.5 -0.5 0 02 1345 output voltage deviation vs. load current MAX8704 toc01 load current (a) v out deviation (%) v in = 3.3v v in = 1.8v 0 0.3 0.9 0.6 1.2 1.5 02 1345 plim voltage vs. load current MAX8704 toc02 load current (a) plim voltage (v) v in = 3.3v power limit v in = 2.5v v in = 1.8v current limit 0 0.4 0.2 1.0 0.8 0.6 1.6 1.4 1.2 1.8 13 245 output voltage vs. input voltage MAX8704 toc03 input voltage (v) v out (v) 10ma load 1a load electrical characteristics (continued) (v in = 2.5v, v cc = 5.0v, plim = fb = gnd, csp = csn, ss/en floating, t a = -40c to +85c , unless otherwise noted.) (note 1) parameter symbol conditions min max units fault detection v cc undervoltage-lockout threshold rising edge, 15mv hysteresis 4.45 v current-limit threshold v cslimit plim = gnd 43 60 mv power-limit threshold v pwrlimit rising edge 0.90 1.10 v plim output current v csp - v csn = 30mv, v in = 3.5v, v csn = 0.5v 13 22 ? soft-start and shutdown soft-start charge current i ss v ss/en = 0 4 6 a ss/en enable threshold rising edge 0.4 0.6 v inputs and outputs pgood trip threshold with respect to error-comparator threshold, 2% hysteresis -11 -5 % pgood startup delay 0.5 5.5 ms pgood output low voltage i sink = 4ma 0.3 v note 1: specifications to -40? are guaranteed by design, not production tested.
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet _______________________________________________________________________________________ 5 0 0.3 0.9 0.6 1.2 1.5 12345 plim voltage vs. input voltage MAX8704 toc04 input voltage (v) plim voltage (v) output short 1a load 10ma load 0 1 3 2 4 5 12345 5v bias supply current vs. input voltage MAX8704 toc05 input voltage (v) bias supply current (ma) 1a load dropout power limit 0 0.5 1.5 1.0 2.0 2.5 12345 mosfet power dissipation vs. drain-to-source voltage MAX8704 toc06 v ds (v) power (w) 1a load output short 0 2 1 4 3 5 6 0 1.0 1.5 0.5 2.0 2.5 3.0 output current limit vs. ss/en voltage MAX8704 toc07 ss/en voltage (v) current limit (a) 200 s/div soft-start (c ss = 10nf) 2v 1v b a c MAX8704 toc08 1a 0 0 1.5v 0 a. en/ss, 1v/div b. ldo output, 1v/div 1.5 ? load, v in = 1.8v c. fet current, 1a/div 1ms/div shutdown sequence (no load) 5v 0 b a c MAX8704 toc09 1a 0 5v 0 0 a. pgood, 5v/div b. en/ss, 5v/div no load, c ss = 1nf, v in = 1.8v 1.5v d c. ldo output, 1v/div d. fet current, 2a/div t ypical operating characteristics (continued) (circuit of figure 1, v out = 1.5v, t a = +25?, unless otherwise noted.)
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 6 _______________________________________________________________________________________ t ypical operating characteristics (continued) (circuit of figure 1, v out = 1.5v, t a = +25?, unless otherwise noted.) 40 s/div line transient (1.8v to 2.5v) 2.5v 3.5v b a c MAX8704 toc13 1.50v 1.49v 3.3v 1.51v a. input: 1.8v to 2.5v, 0.5v/div b. drv, 200mv/div r out = 1.5 ? c. output, 10mv/div 2.0v 1.5v gain and phase vs. frequency MAX8704 toc14 phase 0.001 0.01 0.1 1 10 180 90 0 -90 -180 gain (db) 0.001 0.01 0.1 1 10 fb = csn, v out = 0.5v, v in = 1.0v c out = 2 x 22 f 1206 ceramic, i out = 0.5a frequency (mhz) 60 40 20 0 -20 gain and phase vs. frequency MAX8704 toc15 phase 0.001 0.01 0.1 1 10 180 90 0 -90 -180 gain (db) 0.001 0.01 0.1 1 10 fb = csn, v out = 0.5v, v in = 1.0v c out = 100 f 70m ? sanyo 4tpb100m, i out = 0.5a frequency (mhz) 60 40 20 0 -20 frequency (mhz) psrr 40 -80 MAX8704 toc16 -120 0 -40 0.001 0.01 0.1 1 10 fb = v cc , v out = 1.5v, v in = 2.5v c out = 2 x 22 f 1206 ceramic, i out = 0.5a psrr (db) 20 s/div 3a load transient (irf7401) 3.5a 3.0v b a c MAX8704 toc11 1.50v 1.45v 1.8v 1.7v a. load: 0.5a - 3.5a, 3a/div b. drv, 0.5v/div v in = 1.8v d c. input, 100mv/div d. output, 50mv/div 0.5a 3.5v 20 s/div 3a load transient (fds6570a) 3.5a 2.5v b a c MAX8704 toc12 1.50v 1.45v 1.8v 1.7v a. load: 0.5a - 3.5a, 3a/div b. drv, 0.5v/div v in = 1.8v d c. input, 100mv/div d. output, 50mv/div 0.5a 3.0v 4ms/div power limit 5v 0 b a c MAX8704 toc10 4a 0 0 0 a. en/ss, 2v/div b. plim, 0.5v/div 0.4 ? load, c ss = 10nf, v in = 1.8v, r plim = 200k ? , c plim = 0.1 f 1.5v d c. ldo output, 1v/div d. fet current, 5a/div
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet _______________________________________________________________________________________ 7 detailed description the MAX8704 is a low-dropout, external n-channel mosfet linear regulator for low-voltage notebook power supplies. the regulator uses two separate sup- plies?he notebook? 5v bias supply (v cc ) for driving the external n-channel mosfet, and the lowest system supply available for the power input (v in ). by using separate bias and power inputs, the MAX8704 maxi- mizes the gate drive while minimizing the power loss. the regulator provides an accurate (-2% typ load regu- lation) output that delivers up to 5a for powering the low-voltage (1.0v, 1.2v, 1.5v, and 1.8v) supplies required by notebook chipsets. figure 1 shows the standard application circuit, and figure 2 shows the functional diagram. the MAX8704 standard application circuit delivers up to 5a and oper- ates with input voltages up to 5.5v, but not simultane- ously. continuous high output currents can only be achieved when the input-to-output differential voltage is low (figure 1). 5.0v bias supply (v cc ) the v cc input powers the control circuitry and provides the gate drive to the external n-channel mosfet. this improves efficiency by allowing v in to be powered from a low-voltage system supply. power v cc from a well- regulated 5v supply. current drawn from the v cc sup- ply remains relatively constant with variations in v in and pin description pin name function 1v in input voltage sense. the MAX8704 senses the voltage across the external mosfet (v in - v csn ) to determine the mosfet? power dissipation. 2v cc analog and driver supply input. connect to the system supply voltage (+5.0v). bypass v cc to analog ground with a 1? or greater ceramic capacitor. 3 pgood open-drain power-good output. pgood is low when the output voltage is more than 8% (typ) below the nominal regulation voltage. pgood is also pulled low during soft-start and in shutdown. approximately 3ms (typ) after the ldo reaches the regulation voltage, pgood becomes high impedance as long as the output remains in regulation. 4 plim power-limit adjustment. the plim output sources a current directly proportional to the mosfet? power dissipation. if the plim voltage exceeds the 1.0v power-limit threshold, the regulator reduces the power dissipation by folding back the current limit. an external resistor between plim and gnd sets the maximum mosfet? power dissipation. additionally, an external capacitor filters the plim voltage, allowing short high-power transients to occur periodically. 5 ss/en soft-start and enable input. connect ss/en to an open-drain output. when ss/en is pulled low, the linear regulator shuts down and pulls the output to ground. connect a soft-start capacitor from ss/en to gnd to slowly ramp up the current limit during startup (see the soft-start and enable section). 6fb feedback input. connect fb to v cc for a fixed 1.5v output, or connect fb to gnd for a fixed 1.2v output. for an adjustable output, connect fb to a resistive divider from the output voltage. the fb regulation level is 0.5v. 7 csn negative current-sense input and output sense input. connect to the negative terminal of the current-sense element as shown in figure 1. csn serves as the feedback input in fixed-voltage mode (fb = gnd or v cc ). when the MAX8704 is disabled, the output is discharged through a 10 ? resistor to gnd. 8 csp positive current-sense input. connect to the positive terminal of the current-sense element as shown in figure 1. the MAX8704 driver reduces the gate voltage when the current-limit threshold is exceeded. 9 gnd ground 10 drv gate drive for the external n-channel mosfet
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 8 _______________________________________________________________________________________ load current. bypass v cc with a 1? or greater ceramic capacitor as close to the MAX8704 as possible. undervoltage lockout (uvlo) the v cc input undervoltage-lockout (uvlo) circuitry ensures that the regulator starts up with a gate-drive voltage that can adequately bias the external n-channel mosfet. the uvlo threshold is 4.2v (typ), and v cc must remain above this level for proper operation. power-supply input (v in ) the power input supply (v in ) sources the current required by the linear regulator? output (v out ). v in connects to the drain of the external n-channel power mosfet. v in may be as low as 1.0v, minimizing the power dissipation across the n-channel mosfet. bypass v in with a 10? or greater capacitor as close to the external mosfet as possible. to avoid input volt- age sag during a load transient, the input supply should provide a low source impedance. if a high- impedance source is used, additional input bulk capacitance is required near the MAX8704. soft-start and enable (ss/en) as shown in figure 2, a capacitor on ss/en allows a gradual buildup of the MAX8704 current limit, reducing the initial inrush current peaks at startup. the input sup- ply uvlo and thermal-overload fault trigger the internal ss/en pulldown resistor (r ss/en = 1k ? ), automatically forcing the MAX8704 into shutdown. when properly powered (v cc above uvlo), the MAX8704 charges the soft-start capacitor with a constant 5? current source (see the soft-start capacitor selection section). once the ss/en voltage rises above 0.5v, the linear regulator c ss 0.01 f r plim 200k ? c plim 0.1 f r2 10k ? r1 20k ? c out 2 x 22 f output (v out ) 1.5v at 5a (max) r sense 10m ? n1 irf7401 c in2 10 f c in1 100 f input 1.8v to 5.5v drv v in v cc pgood ss/en plim gnd fb csn csp 5v bias supply c1 1.0 f r3 100k ? power- good off on MAX8704 figure 1. standard application circuit r ds(on) (m ? ) mosfet 2.5v 1.8v v ds (v) c iss * ( nf) package vendor fds6574a 7 9 20 8 so-8 (2.5w) fairchild si4836dy 4 5 12 7 so-8 (2.5w) siliconix (vishay) table 1. mosfet selection (>1.5v output-voltage applications) r ds(on) (m ? ) mosfet 4.5v 2.5v v ds (v) c iss * ( nf) package vendor irf7401 22 30 20 2.7 so-8 (2.5w) international rectifier nds8425 22 28 20 1.4 so-8 (2.5w) fairchild fds6572a 6 8 20 6.2 so-8 (2.5w) fairchild fds7064n 7.5 30 3.7 bottomless so-8 (3w) fairchild si9426dy 13.5 16 20 3.5 so-8 (2.5w) siliconix (vishay) si4866dy si7882dp 5.5 8 12 3.2 so-8 (2.5w) powerpak (5w) siliconix (vishay) table 2. mosfet selection (0.5v to 1.5v output-voltage applications) * c iss when v ds = 1v * c iss when v ds = 1v
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet _______________________________________________________________________________________ 9 is enabled. as the voltage on ss/en continues to increase, the current-limit threshold slowly ramps up, effectively limiting the input inrush current during power-up ( figure 3). the MAX8704 reaches the full cur- rent limit when the ss/en voltage exceeds 2v. when ss/en is pulled low?ither by an external open- drain output or by the internal power-ok (pok) lockout signal?he MAX8704 pulls the driver (drv) low and dis- charges the output through a 10 ? discharge fet. drive ss/en with a push/pull output to bypass soft-start. output voltage and dual mode feedback the MAX8704? dual-mode operation allows the selec- tion of two common preset voltages without requiring external components. connect fb to v cc for a fixed 1.5v output, or connect fb to gnd for a fixed 1.2v out- put. alternatively, the output voltage can be adjusted using a resistive voltage-divider ( figure 2). the adjust- ed output voltage is: vv r r out fb =+ ? ? ? ? ? ? 1 1 2 MAX8704 control block current limit (figure 3) plim multiplier thermal shdn shdn 0.91 x ref fb ss/en pgood logic supply 5v bias supply input 1.0v to 5.5v power- good off on 5 a 0.5v c ss v cc v in drv n1 csp csn output (v out ) r1 r2 c plim r sense r plim c1 c in c out pok 1k ? 10 ? dual-mode feedback delay logic 0.5v gnd plim fb error amp shdn r3 r ss/en figure 2. functional diagram dual mode is a trademark of maxim integrated products, inc.
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 10 ______________________________________________________________________________________ where the feedback threshold (v fb ) equals 0.5v, as specified in the electrical characteristics table . the minimum adjustable output voltage is 0.5v (fb = csn). the maximum adjustable output voltage is limited by the gate driver? output-voltage swing range (see the electrical characteristics table ) and the gate threshold of the selected n-channel mosfet. fault protection current limit the MAX8704 features a current limit ( figure 3) that monitors the voltage across the current-sense resistor, typically limiting the csp to csn voltage to 50mv. when the csp to csn voltage reaches the current-limit threshold, the MAX8704 regulates the output current rather than the output voltage. during startup, the soft- start circuit ramps the current limit to reduce the input surge current (see the soft-start capacitor selection section). mosfet power-limit protection the MAX8704 includes a proprietary power-limit circuit to protect the external n-channel mosfet, especially under short-circuit conditions. the MAX8704 uses an internal multiplier circuit to generate an output current (i plim ) that is directly proportional to the mosfet power dissipation. when the plim voltage exceeds 1.0v, the MAX8704 folds back the current limit to reduce the power dissipation across the external com- ponents ( figure 3). the power limit allows an output short for an indefinite period of time without damaging the MAX8704 or its external components. thermal-overload protection thermal-overload protection prevents the MAX8704 from overheating. when the junction temperature exceeds +140 c, the linear regulator automatically pulls pgood low and enters shutdown?he MAX8704 pulls ss/en low with an internal 1k ? pulldown resistor. this disables the driver and discharges the output, allowing the device to cool. once the junction tempera- ture cools by 20 c, the thermal protection circuit releases the ss/en input, allowing the MAX8704 to automatically power up using the soft-start sequence. a continuous thermal-overload condition results in a pulsed output. power-good the MAX8704 provides an open-drain pgood output that goes high 3ms (typ) after the output initially reach- es regulation. pgood transitions low immediately after the output voltage drops below 92% (typ) of the nomi- nal regulation voltage, or when the MAX8704 enters shutdown. connect a pullup resistor from pgood to v cc for a logic-level output. use a 100k ? resistor to minimize current consumption. design procedure external mosfet selection the external mosfet selection depends on the gate threshold voltage, input-to-output voltage, and package power dissipation. the MAX8704 uses an external n- channel mosfet controlled by a 5v driver, so the max- imum gate-to-source voltage across the mosfet (v gs(max) ) is equivalent to: v gs(max) = v drv(max) - v csp where the maximum drive voltage is approximately v cc - 1v. the selected mosfet? on-resistance must be low enough to support the minimum input-to-output differential voltage (dropout voltage) and maximum load required by the application: for output voltages less than 1.5v, standard mosfets that provide on-resistance specifications with 2.5v gate-to-source voltages are sufficient. for output volt- ages greater than 1.5v, use low-threshold mosfets r vv v i ds on min in min cslimit out out max ()( ) () () = ?? current limit v cc plim ss/en to control block 1v 2v csn csp figure 3. current-limit functional diagram
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet ______________________________________________________________________________________ 11 that provide on-resistance specifications with a 1.8v gate-to-source voltage. mosfet power dissipation the maximum power dissipation of the MAX8704 depends on the thermal resistance of the external n- channel mosfet package, the board layout, the tem- perature difference between the die and ambient air, and the rate of airflow. the power dissipated in the mosfet is: p dis = i out x (v in - v csp ) the maximum power dissipation allowed is determined by the following formula: where t j(max) is the maximum junction temperature (+150?), t a is the ambient temperature, jc is the ther- mal resistance from the die junction to the package case, and ca is the thermal resistance from the case through the pc board, copper traces, and other materi- als to the surrounding air. standard so-8 mosfets are typically rated for 2w, while new power packages (powerpak, directfet, etc.) can achieve power dissipa- tion ratings as high as 5w. for optimum power dissipa- tion, use a large ground plane with good thermal contact to ground and use wide input and output traces. extra copper on the pc board increases thermal mass and reduces the thermal resistance of the board. setting the current limit the current-sense voltage threshold is preset to 50mv (typ), so the achievable peak source current (i peak ) is determined by the current-sense resistor. the current- sense resistor can be determined by: r sense = v cslimit / i peak for the best current-sense accuracy, use a 1% current- sense resistor between the source of the mosfet and the output. setting the power limit the MAX8704 includes a unique power-limit protection circuit that limits the maximum power dissipation in the external mosfet. an external resistor (r plim ) adjusts the actual power limit as defined by the following equation: where r sense is the current-sense resistor, p limit is the maximum mosfet power dissipation, the power-limit conversion gain (k plim ) equals 200?/v 2 , and the power-limit threshold (v pwrlimit ) equals 1.0v. an exter- nal capacitor (c plim ) adjusts the power-limit time con- stant ( plim = r plim x c plim ), allowing short high-power transients while protecting against thermal stress. short plim to ground to disable the power-limit protection. input capacitor selection (c in ) typically, the linear regulator is powered from the out- put of a step-down regulator, effectively providing a low-impedance source for the MAX8704. under these conditions, a local 10? or greater ceramic capacitor is sufficient for most applications. if the linear regulator is connected to a high-impedance input, low-esr poly- mer capacitors are recommended on the input. output capacitor selection (c out ) the MAX8704 requires 10?/a or greater ceramic capacitor for stable operation and optimized load-tran- sient response. for higher capacitance values, the reg- ulator remains stable with low-esr, polymer output capacitors as shown in the output capacitance vs. load current graph (see figure 4). when selecting the output capacitor to provide good transient response, the capacitor? esr should be minimized: ? v out = ? i out x esr where ? i out is the maximum peak-to-peak load current step, and ? v out is the transient output-voltage tolerance. r v pkr plim pwrlimit limit plim sense = r tt dis max j max a jc ca () () = ? + ? 02 1345 output capacitance vs. load current load current (a) c out ( f) 10 30 20 40 50 v in > v out + 0.2v figure 4. output capacitance vs. load current
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 12 ______________________________________________________________________________________ using larger output capacitance can improve efficiency in applications where the load current changes rapidly. the output capacitor acts as a reservoir for the rapid transient currents, reducing the peak current supplied by the input supply and effectively lowering the i 2 r power loss. soft-start capacitor selection (c ss ) a capacitor (c ss ) connected from ss/en to gnd caus- es the MAX8704 output current to slowly rise during startup, reducing stress on the input supply. the rise time to full current limit (t ss ) is determined by: t ss = c ss x 1.5v / i ss where i ss = 5? is the soft-start current. typical capac- itor values between 1nf to 100nf are sufficient. since the regulator ramps the current-limit threshold, the actual output-voltage slew rate depends on the load current and output capacitance. noise, psrr, and transient response the MAX8704 operates with low dropout voltage and low quiescent current in notebook computers while maintaining good noise, transient response, and ac rejection. see the typical operating characteristics for a graph of psrr vs. frequency. improved supply-noise rejection and transient response can be achieved by increasing the values of the input and output capaci- tors. use passive filtering techniques when operating from noisy sources. the MAX8704 load-transient response graphs (see the typical operating characteristics ) show two compo- nents of the output response: a dc load regulation and the transient response. a typical transient response for a step change in the load current from 0.5a to 3.5a is 25mv. lowering the output impedance?ncreasing the output capacitor? value and/or decreasing the esr attenuates the output undershoot and overshoot. pc board layout guidelines the MAX8704 requires proper layout to achieve the intended output power level and regulation characteris- tics. proper layout involves the use of a ground plane, appropriate component placement, and correct routing of traces using appropriate trace widths (figure 5). minimize high-current ground loops: connect the ground of the MAX8704, the input capacitor, and the output capacitor together at one point. minimize parasitic inductance: keep the input capacitor, external mosfet, and output capacitor close together. route the ground plane directly under the input and output power traces/planes. to optimize performance and power dissipation, a ground plane is essential. dedicated ground plane layers reduce trace inductance, ground imped- ance, and noise coupling (ground shield) between layers, and improve thermal conductivity throughout the board. connect the input filter capacitor less than 10mm from the mosfet. the connecting copper trace car- ries large currents and must be at least 5mm wide. use as much copper as necessary to decrease the thermal resistance of the mosfet. in general, more copper provides better heatsinking capabilities. chip information transistor count: 786 process: bicmos c in 5v bias ground c out output MAX8704 input ground ground r sense figure 5. recommended MAX8704 layout
MAX8704 high-current, low-voltage linear regulator with power-limited, external mosfet 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. 13 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2004 maxim integrated products printed usa is a registered trademark of maxim integrated products. 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 .) 10lumax.eps package outline, 10l umax/usop 1 1 21-0061 i rev. document control no. approval proprietary information title: top view front view 1 0.498 ref 0.0196 ref s 6 side view bottom view 0 0 6 0.037 ref 0.0078 max 0.006 0.043 0.118 0.120 0.199 0.0275 0.118 0.0106 0.120 0.0197 bsc inches 1 10 l1 0.0035 0.007 e c b 0.187 0.0157 0.114 h l e2 dim 0.116 0.114 0.116 0.002 d2 e1 a1 d1 min - a 0.940 ref 0.500 bsc 0.090 0.177 4.75 2.89 0.40 0.200 0.270 5.05 0.70 3.00 millimeters 0.05 2.89 2.95 2.95 - min 3.00 3.05 0.15 3.05 max 1.10 10 0.60.1 0.60.1 0 0.500.1 h 4x s e d2 d1 b a2 a e2 e1 l l1 c gage plane a2 0.030 0.037 0.75 0.95 a1

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