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general description the MAX1774 is a complete power-supply solution for pdas and other hand-held devices. it integrates two high-efficiency step-down converters, a boost converter for backup battery regulation, and four voltage detec- tors in a small 32-pin qfn or 28-pin qsop package. the MAX1774 accepts inputs from +2.7v to +28v and provides an adjustable main output from 1.25v to 5.5v at over 2a. the secondary core converter delivers an adjustable voltage from 1v to 5v and can deliver up to 1.5a. both the main and core regulators have separate shutdown inputs. when the ac adapter power is removed, an external p- channel mosfet switches input to the main battery. when the main battery is low, the backup step-up con- verter sustains the main output voltage. when the back- up battery can no longer deliver the required load, the system shuts down safely to prevent damage. four on- board voltage detectors monitor the status of the ac adapter power, main battery, and backup battery. the MAX1774 evaluation kit is available to help reduce design time. ________________________applications hand-held computers pdas internet access tablets pos terminals subnotebooks features dual, high-efficiency, synchronous-rectified step-down converters thin, small (1mm high) qfn package step-up converter for backup battery main power adjustable from +1.25v to +5.5v over 2a load current up to 95% efficiency core power adjustable from 1v to 5v internal switches up to 1.5a load current up to 91% efficiency automatic main battery switchover 100% (max) duty cycle up to 1.25mhz switching frequency input voltage range from +2.7v to +28v four low-voltage detectors 170? quiescent current 8? shutdown current digital soft-start independent shutdown inputs MAX1774 dual, high-efficiency, step-down converter with backup battery switchover ________________________________________________________________ maxim integrated products 1 32 31 30 29 28 27 26 bkup shdnc shdnm n.c. lxc ins lbo 25 n.c. 9 10 11 12 13 14 15 lxb lxb2 bin bk off aci dbi lbi 16 ref 17 18 19 20 21 22 23 n.c. gnd gnd gnd gnd fbm cs+ cs- fbc gnd inc 8 7 6 5 4 3 2 cvh pdrv in cvl ndrv pgnd pgndc MAX1774 32 7mm x 7mm qfn 1 mdrv 24 aco top view pin configurations 19-1810; rev 1; 1/02 evaluation kit available ordering information part temp range pin-package MAX1774eei -40? to +85? 28 qsop MAX1774emj -40? to +85? 32 7mm x 7mm qfn ac adapter main battery backup battery main (+3.3v) core (+1.8v) ac ok low main battery dead main battery MAX1774 functional diagram pin configurations continued at end of data sheet. 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.
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (figure 1, v in = v ins +12v, v inc = v cs- = v cs+ = +3.3v, v core = +1.8v, t a = 0? to +85?, unless otherwise noted. typical values are at t a = +25?.) 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. in, shdnm , mdrv , dbi, lbi, aci, cvh to gnd .......................................................-0.3v to +30v in to cvh, pdrv ......................................................-0.3v to +6v bin to cs-.................................................................-0.3v to +6v lxb to gnd ................................................-0.3v to (v bin + 0.7v) pdrv to gnd..................................(v cvh - 0.3v) to (v in + 0.3v) all other pins to gnd...............................................-0.3v to +6v pgnd to gnd .......................................................-0.3v to +0.3v continuous power dissipation 28-pin qsop (derate 10.8mw/? above +70?)........860mw 32-pin qfn (derate 23.2mw/? above +70?) ........1860mw operating temperature .......................................-40? to +85? storage temperature.........................................-65? to +150? temperature (soldering, 10s) ..........................................+300? parameter symbol conditions min typ max units input voltage v in 2.7 28 v input quiescent supply current i in v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 18 40 a cs- quiescent supply current i cs- v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 110 220 a core regulator quiescent supply current i inc v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 60 105 a backup mode bin quiescent supply current i bin v bin = +3.3v, cs- open v fbm = +1.5v, v shdnm = +3.3v, v bkoff = +1.5v, shdnc = gnd 60 105 a in shutdown supply current shdnm = shdnc = gnd 8 40 a main regulator main output voltage adjust range 1.25 5.5 v fbm regulation threshold v fbm v (cs+ - cs-) = 0 to +60mv, v in = +3.5v to +28v 1.21 1.25 1.29 v fbm input current i fbm v fbm = +1.3v -0.1 0.1 a current-limit threshold v cs+ - v cs- 60 80 100 mv minimum current-limit threshold v cs+ - v cs- 51525mv valley current threshold v cs+ - v cs- 40 50 60 mv zero current threshold v cs+ - v cs- 0515mv pdrv, ndrv gate drive resistance v cs- = +3.3v, i pdrv , i ndrv = 50ma 2 5.5 ? cs- to cvl switch resistance i cvl = 50ma 4.5 9.5 ? pdrv, ndrv dead time 50 ns
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover _______________________________________________________________________________________ 3 parameter symbol conditions min typ max units maximum duty cycle 100 % minimum on-time 200 400 650 ns minimum off-time 200 400 650 ns core regulator input voltage range v inc 2.6 5.5 v v inc rising 2.40 2.47 2.55 inc undervoltage lockout v inc falling 2.30 2.37 2.45 v c or e outp ut v ol tag e ad j ust rang e 1.0 5.0 v maximum core load current v core = 1.8v (note 1) 1 1.5 a fbc regulation threshold v fbc v inc = +2.5 to +5.5v, i o u t c = 0 to 200ma 0.97 1.0 1.03 v fbc input current i fbc v fbc = +1.3v -0.1 0.1 a dropout voltage i outc = 400ma 0.1 0.25 v lxc leakage current i lxc v inc = +5.5v, v l x c = 0 to +5.5v -10 10 a lxc p-channel, n-channel on- resistance 0.25 0.5 ? lxc p-channel current limit i clc 1200 1800 3000 ma lx c p - c hannel m i ni m um c ur r ent 100 250 400 ma lxc n-channel valley current 900 1400 2400 ma lxc n-channel zero-crossing current 40 110 170 ma lxc dead time 50 ns max duty cycle 100 % minimum on-time 170 400 690 ns minimum off-time 170 400 690 ns backup regulator backup battery input voltage v bbatt 0.9 5.5 v lxb n-channel on-resistance v cs- = +3.3v, i lxb = 50ma 1.9 3.5 ? lxb current limit 200 350 600 ma lxb leakage current v lxb = +5.5v, v fbm = +1.3v 1 a bin leakage current i bin v bin = +5.5v, cs- = bkoff = shdnc = shdnm = gnd 1 a bin, cs- switch resistance v cs- = +3.3v, bkoff = gnd, shdnm = cvl 7.5 15 ? bin switch zero-crossing threshold v bin = + 2.5v , bkoff = shdnc = shdnm = c v l 17 35 mv lxb maximum on-time 2.8 5.6 9.2 s zero crossing detector timeout 40 s electrical characteristics (continued) (figure 1, v in = v ins = +12v, v inc = v cs- = v cs+ = +3.3v, v core = +1.8v, t a = 0 c to +85 c, unless otherwise noted. typical values are at t a = +25 c.)
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 4 _______________________________________________________________________________________ parameter symbol conditions min typ max units reference reference voltage v ref 1.23 1.25 1.27 v reference load regulation i ref = 0 to 50? 10 mv reference line regulation v cs- = +2.5v to +5.5v, i ref = 50? 5 mv reference sink current 10 ? cvl, cvh regulators i cvl = 50ma, v cs- = 0 2.6 2.8 3.1 cvl output voltage v cvl i cvl = 50ma, v cs- = +3.3v 3.2 v cvl switchover threshold cs- rising, hysteresis = 100mv typical 2.40 2.47 2.55 v v in = +4v, i cvh = 25ma v in - 3.4 v in - 2.8 cvh output voltage v cvh v in = +12v, i cvh = 50ma v in - 4.2 v in - 3.7 v cvh switchover threshold v in v in rising, hysteresis = 350mv typ 5.5 v v cvl rising 2.40 2.47 2.55 cvl undervoltage lockout v cvl falling 2.30 2.37 2.45 v low-voltage comparators v bkoff rising 0.51 0.55 0.59 backup regulator shutdown threshold v bkoff v bkoff falling 0.46 0.50 0.54 v bkoff input bias current v bkoff = +5.5v 1 a lbi threshold v lbi v lbi falling, hysteresis = 50mv typical 1.17 1.20 1.23 v dbi threshold v dbi v dbi falling, hysteresis = 50mv typical 1.17 1.20 1.23 v bkup low-input threshold 0.4 v lbi, dbi input leakage current v lbi = v dbi = +1.3v 100 na lbo , bkup , aco , mdrv output low i sink = 1ma 0.4 v lbo , bkup , aco , mdrv output leakage current v lbi = +1.3v, v aci = +12v, v aco = v lbo = v bkup = +5.5v, v mdrv = +28v 1.0 ? aci threshold v aci ?v ins, aci falling 0.22 0.35 v aci input leakage current v aci = +1.3v 100 na ins input leakage current v ins = +3.3v 1.5 10 ? logic inputs shdnm , shdnc input low voltage 0.4 v shdnm , shdnc input high voltage 2.0 v electrical characteristics (continued) (figure 1, v in = v ins = +12v, v inc = v cs- = v cs+ = +3.3v, v core = +1.8v, t a = 0 c to +85 c, unless otherwise noted. typical values are at t a = +25 c.)
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover _______________________________________________________________________________________ 5 parameter symbol conditions min typ max units shdnm , shdnc input low current shdnm = shdnc = gnd -1 1 a shdnc input high current v shdnc = +5.5v 5 a shdnm input high current v shdnm = +5v 2 25 a electrical characteristics (figure 1, v in = v ins = +12v, v inc = v cs- = v cs+ = +3.3v, v core = +1.8v, t a = -40 c to +85 c, unless otherwise noted.) (note 2) parameter symbol conditions min max units input voltage v in 2.7 28 v input quiescent supply current i in v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 40 a cs- quiescent supply current i cs- v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 220 a core regulator quiescent supply current i inc v fbm = +1.5v, v fbc = +1.5v, v shdnm = v shdnc = +3.3v 105 a backup mode bin quiescent supply current i bin v bin = +3.3v, cs- open v fbm = +1.5v, v shdnm = +3.3v, v bkoff = +1.5v, shdnc = gnd 110 a in shutdown supply current shdnm = shdnc = gnd 40 a main regulator main output voltage adjust range 1.25 5.5 v fbm regulation threshold v fbm v (cs+ - cs-) = 0 to +60mv, v in = +3.5v to +28v 1.21 1.29 v fbm input current i fbm v fbm = +1.3v -0.1 0.1 a current-limit threshold v cs+ - v cs- 60 100 mv minimum current-limit threshold v cs+ - v cs- 525mv valley current threshold v cs+ - v cs- 40 60 mv zero current threshold v cs+ - v cs- 015mv pdrv, ndrv gate drive resistance v cs- = +3.3v, i pdrv , i ndrv = 50ma 5.5 ? cs- to cvl switch resistance i cvl = 50ma 9.5 ? maximum duty cycle 100 % minimum on-time 200 650 ns minimum off-time 200 650 ns electrical characteristics (continued) (figure 1, v in = v ins = +12v, v inc = v cs- = v cs+ = +3.3v, v core = +1.8v, t a = 0 c to +85 c, unless otherwise noted. typical values are at t a = +25 c.)
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 6 _______________________________________________________________________________________ parameter symbol conditions min max units core regulator input voltage range v inc 2.6 5.5 v v inc rising 2.39 2.55 inc undervoltage lockout v inc falling 2.29 2.45 v core output voltage adjust range 1.0 5.0 v maximum core load current v core = 1.8v (note 1) 1 a fbc regulation threshold v fbc v inc = +2.5 to +5.5v, i outc = 0 to 200ma 0.97 1.03 v fbc input current i fbc v fbc = +1.3v -0.1 0.1 a dropout voltage i outc = 400ma 0.25 v lxc leakage current i lxc v inc = +5.5v, v lxc = 0 to +5.5v -10 10 a lxc p-channel, n-channel on-resistance 0.5 ? lxc p-channel current limit 1200 3010 ma lxc p-channel minimum current 100 420 ma lxc n-channel valley current 880 2450 ma lxc n-channel zero-crossing current 40 170 ma max duty cycle 100 % minimum on-time 160 700 ns minimum off-time 170 690 ns backup regulator backup battery input voltage v bbatt 0.9 5.5 v lxb n-channel on resistance v cs- = +3.3v, i lxb = 50ma 3.5 ? lxb current limit 200 600 ma lxb leakage current v lxb = +5.5v, v fbm = +1.3v 1 a bin leakage current i bin v bin = +5.5v, cs- = bkoff = shdnc = shdnm = gnd 1a bin, cs- switch resistance v cs- = +3.3v, bkoff = gnd, shdnc = cvl 15 ? bin switch zero-crossing threshold v bin = +2.5v, bkoff = shdnc = shdnm = cvl 35 mv lxb maximum on-time 2.8 9.2 s reference reference voltage v ref 1.220 1.275 v reference load regulation i ref = 0 to 50a 10 mv reference line regulation v cs- = +2.5v to +5.5v, i ref = 50a 5 mv reference sink current 10 a electrical characteristics (continued) (figure 1, v in = v ins = +12v, v inc = v cs+ = v cs- = +3.3v, v core = +1.8v, t a = -40 c to +85 c, unless otherwise noted.) (note 2)
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover _______________________________________________________________________________________ 7 parameter symbol conditions min max units cvl, cvh regulators cvl output voltage v cvl i cvl = 50ma, v cs- = 0 2.6 3.1 v cvl switchover threshold v cs- rising, hysteresis = 100mv typical 2.40 2.55 v v in = +4v, i cvh = 25ma v in - 2.8 cvh output voltage v cvh v in = +12v, i cvh = 50ma v in - 3.65 v v cvl rising 2.40 2.57 cvl undervoltage lockout v cvl falling 2.30 2.47 v low-voltage comparators v bkoff rising 0.51 0.59 backup regulator shutdown threshold v bkoff v bkoff falling 0.46 0.54 v bkoff input bias current v bkoff = +5.5v 1 a lbi threshold v lbi v lbi falling, hysteresis = 50mv typical 1.17 1.23 v dbi threshold v dbi v dbi falling, hysteresis = 50mv typical 1.17 1.23 v bkup low-input threshold 0.4 v lbi, dbi input leakage current v lbi , v dbi = +28v 100 na lbo , bkup , aco , mdrv output low i sink = 1ma 0.4 v lbo , bkup , aco , mdrv output leakage current v lbi = +1.3v, v aci = v in = +12v, v aco = v lbo = v bkup = +5.5v, v mdrv = +28v 1.0 a aci threshold v aci - v ins , aci falling 0.5 v aci input leakage current v aci = +1.3v 100 na main input leakage current v ins = +3.3v 10 a logic inputs shdnm , shdnc input low voltage 0.4 v shdnm , shdnc input high voltage 2.0 v shdnm , shdnc input low current shdnm = shdnc = gnd -1 1 a shdnc input high current v shdnc = +5.5v 5 a shdnm input high current v shdnm = +28v 25 a electrical characteristics (continued) (figure 1, v in = v ins = +12v, v inc = v cs+ = v cs- = +3.3v, v core = +1.8v, t a = -40 c to +85 c, unless otherwise noted.) (note 2) note 1: this parameter is guaranteed based on the lxc p-channel current limit and the lxc n-channel valley current. note 2: specifications to -40 c are guaranteed by design and not production tested.
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 8 _______________________________________________________________________________________ typical operating characteristics (circuit of figure 1, v in = +5v, v inc = +3.3v, t a = +25 c, unless otherwise noted.) 100 0 1 10 100 1000 10,000 main efficiency vs. load 20 MAX1774-01 load (ma) efficiency (%) 40 60 80 70 50 30 10 90 v in = +5v v in = +15v v in = +3.3v v in = +18v v in = +12v v main = 3.3v 90 0 1 1000 100 10 core efficiency vs. load 30 10 70 50 100 40 20 80 60 MAX1774-02 load (ma) efficiency (%) v in = +5v v in = +2.7v v in = +3.3v v core = 1.8v 100 0 0.01 0.1 1 10 100 backup efficiency vs. load 20 MAX1774-03 load (ma) efficiency (%) 40 60 80 70 50 30 10 90 v bbatt = +0.8v v bbatt = +1.0v v bbatt = +2.5v v main = 3.3v main switching waveforms (heavy load 1a) MAX1774-07 5 s/div 4v lx 5v/div 0 20mv 0 -20mv 500ma 0 1000ma 1500ma v main (ac-coupled) 20mv/div il1 500ma/div v ref accuracy vs. temperature MAX1774-04 -2.0 -1.5 -0.5 -1.0 1.0 1.5 0.5 0 2.0 v ref accuracy (%) -40 0 20 -20 40 60 80 100 temperature ( c) v ref accuracy (%) -1.6 -1.8 -2.0 -1.2 -1.4 -0.8 -1.0 -0.6 -0.4 -0.2 0 02030 10 40 50 60 70 80 reference load regulation MAX1774-05 i ref ( a) main switching waveforms (light load 100ma) MAX1774-06 5 s/div 5v lx 5v/div 0 40mv 20mv 0 v main (ac-coupled) 20mv/div i li 500ma/div -20mv 500ma 0
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover _______________________________________________________________________________________ 9 core switching waveforms (heavy load 500ma) MAX1774-09 2 s/div 4v 2v 0 20mv 0 -20mv 500ma 0 v core (ac-coupled) 20mv/div l2 500ma/div lxc 2v/div core line-transient response MAX1774-11 v inc 2v/div 0 2v 4v v core (ac-coupled) 50mv/div 1 s/div typical operating characteristics (continued) (circuit of figure 1, v in = +5v, v inc = +3.3v, t a = +25 c, unless otherwise noted.) core switching waveforms (light load 50ma) MAX1774-08 1 s/div 3.3v lx 2v/div 0 0 i l2 500ma/div 500ma v core (ac-coupled) 20mv/div main load-transient response MAX1774-12 1000ma 500ma 0 i main 500ma/div -20mv v main (ac-coupled) 20mv/div 20mv 0 100 s/div main load-transient response 50ma to 500ma MAX1774-13 500ma 0 v main (ac-coupled) 10mv/div i main 500ma/div 10mv 0 -10mv 100 s/div main line-transient response MAX1774-10 12v 5v 0 50mv -50mv 10v v in 5v/div v main (ac-coupled) 50mv/div 100 s/div
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 10 ______________________________________________________________________________________ typical operating characteristics (continued) (circuit of figure 1, v in = +5v, v inc = +3.3v, t a = +25 c, unless otherwise noted.) turn-on response MAX1774-14 5v 0 v out 1v/div 2v 1v 100 s/div 3v 0 400 a 200 a 0 input current 200ma/div v shdn 5v/div input current v main v core backup switchover response MAX1774-15 v bkup 5v/div i bbatt 50ma/div v bin 10mv/div v main 10mv/div 5 s/div pin description pin qsop qfn name function 130 shdnm shutdown for main regulator. low voltage on shdnm shuts off the main output. for normal operation, connect shdnm to in. 231 shutdown for core regulator. low voltage on shdnc shuts off the core output. for normal operation, connect shdnc to cvl. 332 bkup open-drain backup input/output. the device is in backup mode when bkup is low. bkup can be externally pulled low to place the device in backup mode. 41 mdrv open-drain drive output. mdrv goes low when the aci voltage drops below the main voltage plus 220mv and device is not in backup. connect mdrv to the gate of the main battery p-channel mosfet to switch the battery to in when the ac adapter voltage is not present. 52 pgndc power ground for the core converter. connect all grounds together close to the ic. 6 3 pgnd power ground. ground for ndrv and core output synchronous rectifier. connect all grounds together close to the ic. 7 4 ndrv n-channel drive output. drives the main output synchronous-rectifier mosfet. ndrv swings between cvl and pgnd. 8 5 cvl low-side bypass. cvl is the output of an internal ldo regulator. this is the internal power supply for the device control circuitry as well as the n-channel driver. bypass cvl with a 1.0f or greater capacitor to gnd. when cs- is above the cvl switchover threshold (2.47v), cvl is powered from the main output. 9 6 in power supply input 10 7 pdrv p - c hannel d r i ve outp ut. d r i ves the m ai n outp ut hi g h- si d e m os fe t sw i tch. p d rv sw i ng s b etw een in and c v h . the vol tag e at c v h i s r eg ul ated at v in - 4.2v unl ess the i np ut vol tag e i s l ess than 5.5v . 11 8 cvh high-side drive bypass. this is the low-side of the p-channel driver output. bypass with a 1.0f capacitor or greater to in. when the input voltage is less than 5.5v, cvh is switched to pgnd. 12 9 lxb backup converter switching node. connect an inductor from lxb to the backup battery and a schottky diode to bin to complete the backup converter. in backup mode, this step-up converter powers the main output from the backup battery through bin.
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover _______________________________________________________________________________________ 11 detailed description the MAX1774 dual step-down dc-dc converter is designed to power pda, palmtop, and subnotebook computers. normally, these devices require two sepa- rate power supplies one for the processor and another higher voltage supply for the peripheral circuitry. the MAX1774 provides an adjustable +1.25v to +5.5v main output designed to power the peripheral circuitry of pdas and similar devices. the main output delivers up to 2a output current. the lower voltage core converter has an adjustable +1.0v to +5.0v output, providing up to 1.5a output current. both regulators utilize a propri- etary regulation scheme allowing pwm operation at medium to heavy loads, and automatically switch to pulse skipping at light loads for improved efficiency. under low-battery conditions, the MAX1774 enters backup mode, making use of a low-voltage backup battery and a step-up regulator to power the output. figure 1 is the MAX1774 typical application circuit. operating modes for the step-down converters when delivering low output currents, the MAX1774 oper- ates in discontinuous conduction mode. current through the inductor starts at zero, rises as high as the minimum current limit (i min ), then ramps down to zero during pin qsop qfn name function 10 lxb2 backup converter switching node. connect lxb2 to lxb as close to the ic as possible. 13 11 bin backup batter y inp ut. c onnect bin to the outp ut of the b ackup b oost r eg ul ator . byp ass bin w i th a 10f or g r eater cap aci tor to gn d . w hen the m ax 1774 i s i n b ackup m od e, bin p ow er s the m ai n outp ut. 14 12 bkoff backup d i sab l e inp ut. d r i vi ng bko ff b el ow + 0.5v d i sab l es the b ackup m od e. in b ackup m od e, the d evi ce enter s shutd ow n w hen thi s p i n i s p ul l ed l ow . bko ff can b e d r i ven fr om a d i g i tal si g nal or can b e used as a l ow b atter y d etector to d i sab l e the b ackup conver ter w hen the b ackup b atter y i s l ow . 15 13 aci ac adapter low-voltage detect input. connect to adapter dc input. when the voltage at aci falls below the voltage at ins plus +0.22v, aco asserts. 16 14 dbi d ead batter y inp ut. c onnect d bi to the m ai n b atter y thr oug h a r esi sti ve voltage-divider. w hen d bi d r op s b el ow + 1.20v , no ac ad ap ter i s connected ( aco i s l ow , b ut m ai n outp ut sti l l avai l ab l e) , bku p asser ts. 17 15 lbi low-battery input. connect lbi to the main battery through a r esi sti ve voltage-divider. when the voltage at lbi drops below +1.20v, lbo asserts. 18 16 ref reference voltage output. bypass ref to gnd with a 0.22f or greater capacitor. 17, 25, 29 n.c. no connection. not internally connected. 19 18 fbm main output feedback. connect fbm to a resistive voltage-divider to set main output voltage between +1.25v to +5.5v. 20 19 cs+ main regulator high-side current-sense input. connect the sense resistor between cs+ and cs-. this voltage is used to set the current limit and to turn off the synchronous rectifier when the inductor current approaches zero. 21 20 cs- main regulator low-side current-sense input. connect cs- to the main output. 22 21 fbc c or e outp ut feed b ack. c onnect fbc to a resistive voltage-divider to set cor e outp ut b etw een + 1.0v to + 5.0v . 23 22 gnd analog ground 24 23 inc core supply input 25 24 aco low ac output. open drain aco asserts when aci falls below the main output voltage plus 0.22v. 26 26 lbo open-drain low-battery output. lbo asserts when lbi falls below +1.20v. 27 27 ins power-supply input voltage sense input. connect ins to the power-supply input voltage. 28 28 lxc core converter switching node pin description (continued)
each cycle (see typical operating characteristics ). the switch waveform may exhibit ringing, which occurs at the resonant frequency of the inductor and stray capaci- tance, due to the residual energy trapped in the core when the rectifier mosfet turns off. this ringing is nor- mal and does not degrade circuit performance. when delivering medium-to-high output currents, the MAX1774 operates in pwm continuous-conduction mode. in this mode, current always flows through the inductor and never ramps to zero. the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the peak switching current set by the current-sense resistor. 100% duty cycle and dropout the MAX1774 operates with a duty cycle up to 100%, extending the input voltage range by turning the mos- fet on continuously when the supply voltage ap- proaches the output voltage. this services the load when conventional switching regulators with less than MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 12 ______________________________________________________________________________________ pgndc fbc lxc inc fbm cs- cs+ pgnd pdrv ndrv cvh main c5 1 f p2 n1 l1 5 h c main 47 f c6 10 f r cs core c core 22 f r10 r11 r8 r9 l2 5.4 h c7 1 f r5 1m ? r6 1m ? r7 1m ? gnd ref cvl lxb bin in dbi lbi backup battery main battery v in_ac r1 c1 10 f c2 10 f c4 0.22 f c3 1 f d2 ep05q 03l l3 22 h p1 r4 r2 r3 d1 bkoff bkup lbo aco shdnc shdnm mdrv MAX1774 on off on off 1.0v to 5.5v lxb2(qfn only) 0.9v to 5.5v 1m ? aci ins fds8928a 1.25v to 5.5v nds356ap nsd03a10 2.7v to 5.5v 2.7v to 5.5v note: for input voltages to 28v see figure 4 and figure 5 figure 1. typical application circuit for low-input voltage applications
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover ______________________________________________________________________________________ 13 100% duty cycle fail. dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. dropout depends on the mosfet drain-to- source on-resistance, current-sense resistor, and inductor series resistance, and is proportional to the load current: v dropout = i out [r ds(on) + r sense + r l ] regulation control scheme the MAX1774 has a unique operating scheme that allows pwm operation at medium and high current, automatically switching to pulse-skipping mode at lower currents to improve light-load efficiency. figure 2 shows a simplified block diagram. under medium and heavy load operation, the inductor current is continuous and the part operates in pwm mode. in this mode, depending on the duty cycle, either the minimum on-time or the minimum off-time sets the switching frequency. the duty cycle is approxi- mately the output voltage divided by the input voltage. if the duty cycle is less than 50%, the minimum on-time controls the frequency, and the frequency is approxi- mately f 2.5mhz ? d, where d is the duty cycle. if the duty cycle is greater than 50%, the minimum off-time sets the frequency, and the frequency is approximately f 2.5mhz ? (1 - d). in both cases, the error comparator regulates the volt- age. for low duty cycles (<50%), the p-channel mos- fet is turned on for the minimum on-time, causing fixed-on-time operation. during the mosfet on-time, the output voltage rises. once the mosfet is turned off, the voltage drops to the regulation threshold, when another cycle is initiated. for high duty cycles (>50%), the mosfet remains off for the minimum off-time, causing fixed-off-time operation. in this case, the mos- fet remains on until the output voltage rises to the reg- ulation threshold. then the mosfet turns off for the minimum off-time, initiating another cycle. by switching between fixed-on-time and fixed-off-time operation, the MAX1774 can operate at high input-out- put ratios and still operate up to 100% duty cycle for low dropout. when operating from fixed-on-time opera- tion, the minimum output voltage is regulated, but in fixed-off-time operation, the maximum output voltage is regulated. thus, as the input voltage drops below approximately twice the output voltage, a decrease in line regulation can be expected. the drop in voltage is approximately v drop v ripple. at light output loads, the inductor current is discontinuous, causing the MAX1774 to operate at lower frequencies, reducing the mosfet gate drive and switching losses. in discontin- uous mode, under most circumstances, the on-time will be a fixed minimum on-time of 400ns. psw non pon v o nsw nonoverlap protection q s r s r q tonmin toffmin v clm v zero fb ref cs+ cs- v valley v min v in pon figure 2. simplified control system block diagram
the MAX1774 features four separate current-limit threshold detectors and a watchdog timer for each of its step-down converters. in addition to the more common peak-current detector and zero-crossing detector, each converter also provides a valley-current detector, and a minimum-current detector. the valley-current detector is used to force the inductor current to drop to a lower level after hitting peak current before allowing the p- channel mosfet to turn on. this is a safeguard against inductor current significantly overshooting above the peak current when the inductor discharges too slowly when v out /l is small. the minimum-current detector ensures that a minimum current is built up in the induc- tor before turning off the p-channel mosfet. this helps the inductor to charge the output near dropout when the dl/dt is small (because (v in - v out ) / l is small) to avoid multiple pulses and low efficiency. this feature, however, is disabled during dropout and light-load con- ditions where the inductor current may take too long to reach the minimum current value. a watchdog timer overrides the minimum current after the p-channel mos- fet has been on for longer than about 10s. main step-down converter the main step-down converter features adjustable +1.25v to +5.5v output delivering up to 2a from a +2.7v to +28v input (see setting the output voltages ). the use of external mosfets and current-sense resis- tor maximizes design flexibility. the MAX1774 offers a synchronous-rectifier mosfet driver that improves effi- ciency by eliminating losses through a diode. the two mosfet drive outputs, pdrv and ndrv, control these external mosfets. the output swing of these outputs is limited to reduce power consumption by limiting the amount of injected gate charge (see internal linear regulators section for details). current-limit detection for all main converter current limits is sensed through a small-sense resistor at the converters output (see setting the current limit section ). driving the shdnm pin low puts the main converter in a low-power shut- down mode. the core regulator, low-voltage detectors, and backup converter are still functional when the main converter is in shutdown. when the MAX1774 enters backup mode, the main converter and its current sen- sor are shut off. core step-down converter the core step-down converter produces a +1.0v to +5.0v output from a +2.6v to +5.5v input. the low-volt- age input allows the use of internal power mosfets, taking advantage of their low r ds(on) , improving effi- ciency and reducing board space. like the main con- verter, the core regulator makes use of a synchronous- rectifying n-channel mosfet, improving efficiency and eliminating the need for an external schottky diode. current sensing is internal to the device, eliminating the need for an external sense resistor. the maximum and minimum current limits are sensed through the p-chan- nel mosfet, while the valley current and zero-crossing current are sensed through the n-channel mosfet. the core output voltage is measured at fbc through a resistive voltage-divider. this divider can be adjusted to set the output voltage level (see setting the output voltages ). the core input can be supplied from the main regulator or an external supply that does not exceed +5.5v (see high-voltage configuration and low-voltage configuration sections). the core convert- er can be shut down independent of the main converter by driving shdnc low. if the main converter output is supplying power to the core and is shut down, shdnm controls both outputs. in this configuration, the core converter continues to operate when the MAX1774 is in backup mode. voltage monitors and battery switchover the MAX1774 offers voltage monitors aci, lbi, dbi, and bkoff that drive corresponding outputs to indi- cate low-voltage conditions. the ac adapter low-volt- age detect input, aci, is typically connected to the output of an ac-to-dc converter. when the voltage at aci drops below the ins sense input plus 0.22v, the low ac output, aco , is asserted. figure 3 shows a sim- plified block diagram. the low and dead battery monitors (lbi and dbi) moni- tor the voltage at main_batt through a resistive volt- age-divider. when the voltage at lbi falls below +1.20v, the low-battery output flag, lbo, is asserted. when both vin_ac and main_batt are present, the MAX1774 chooses one of the two supplies determined by aci. to facilitate this, the MAX1774 provides an open-drain mosfet driver output ( mdrv ). this drives an external p-channel mosfet used to switch the MAX1774 from the ac input to the battery. mdrv goes low when aco is low, the main battery is not dead, and the MAX1774 is not in backup mode. the MAX1774 enters backup mode when the voltage at dbi is below +1.20v and vin_ac is not present to the board. under these conditions, the bkup output is asserted (low), and the device utilizes its boost convert- er and a low-voltage backup battery to supply the main output. the bkup pin can be driven low externally, forcing the MAX1774 to enter backup mode. if the volt- age at bkoff is less than 0.5v, the backup converter is disabled. bkoff can be driven from a digital signal, or can be used as a low-battery detector to disable the backup converter when the backup battery is low. MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 14 ______________________________________________________________________________________
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover ______________________________________________________________________________________ 15 figure 3. simplified block diagram lbo dbo 1.2v lbi dbi aci bin cvl ref shdnm shdnc lxb 1.2v ins 0.5v 0.22v lbo mdrv bkup bkup mode aco cs- (main out) cs+ in pdrv cvh ndrv pgnd inc lxc pgndc gnd fbc rdy cs- main buck on cs+ en fb mdrv bkup noac on core buck pgnd backup boost fb fbm en fb cvh cvl ref soft-start MAX1774 main rdy bkoff lxb2 (qfn only)
place 1m ? pullup resistors from the main output to aco , lbo, and bkup . use a 1m ? pullup resistor from mdrv to in. when not in backup mode, the backup regulator is iso- lated from the main output by an internal switch. when the MAX1774 is in backup mode, the main converter is disabled, and the output of the backup regulator is connected to the main output. the core converter is still operable while in backup mode. the backup step-up converter cannot drive the typical main load current. the load at main must be reduced before entering backup mode. if bkup is de-asserted (goes high), the MAX1774 exits backup mode and resumes operation from the main battery or the ac adapter input. if bkoff goes low, or the backup battery discharges where it cannot sustain the main output load, the backup converter shuts off. to restart the main converter, apply power to v in _ ac or main_batt. the backup converter uses an external schottky diode and internal power nmos switch. since this converter shares the same output as the main buck converter, it shares the same feedback network. this automatically sets the backup converter output voltage to that of the main converter. the backup converter generates an output between +1.25v and +5.5v from a +0.9v to +5.5v input, and provides a load current up to 20ma. when the MAX1774 is in backup mode, the main cur- rent- sense circuit is turned off to conserve power. when the output is out of regulation, the maximum inductor current limit and zero-current detectors regu- late switching. the n-channel mosfet is turned on until the maximum inductor current limit is reached, and shuts off until the inductor current reaches zero. when the output is within regulation, switching is controlled by the maximum pulse width, lxb, switch current limit, zero crossing, and the feedback voltage. internal linear regulators there are two internal linear regulators in the MAX1774. a high-voltage linear regulator accepts inputs up to +28v, reducing it to +2.8v at cvl to provide power to the MAX1774. if the voltage at cs- is greater than +2.47v, cvl is switched to cs-, allowing it to be driven from the main converter, improving efficiency. cvl sup- plies the internal bias to the ic and power for the ndrv gate driver. the cvh regulator output provides the low-side voltage for the main regulator s pdrv output. the voltage at cvh is regulated at 4.2v below v in to limit the voltage swing on pdrv, reducing gate charge and improving efficiency (figure 3). reference the MAX1774 has a trimmed internal +1.25v reference at ref. ref can source no more than 50a. bypass ref to gnd with a 0.22f capacitor. design procedure low-voltage configuration to improve efficiency and conserve board space, the core regulator operates from low input voltages, taking advantage of internal low-voltage, low-on-resistance mosfets. when the input voltage remains below 5.5v, run the core converter directly from the input by con- necting inc to in (figure 1). this configuration takes advantage of the core s low-voltage design and improves efficiency. high-voltage configuration for input voltages greater than 5.5v, cascade the main and core converters by connecting inc to the main out- put voltage (figure 4). in this configuration, the core converter is powered from the main output. ensure that the main output can simultaneously supply its load and the core input current. backup converter configuration the MAX1774 provides a backup step-up converter to power the device and provide the main output voltage when other power fails. the backup converter operates from a +0.9v to +5.5v battery. for most rechargeable batteries, such as nicd or nimh, the simple circuit of figure 5 can be used to recharge the backup battery. in this circuit, the backup battery is charged through r1 and d10. consult the battery manufacturer for charging requirements. to prevent the backup battery from overdischarging, connect a resistive voltage- divider from the backup battery to bkoff . resistor val- ues can be calculated through the following equation: r12 = r13 ? [(v bu / v bkoff ) - 1] where v bkoff = 0.5v, and v bu is the minimum accept- able backup battery voltage. choose r13 to be less than 150k ? . setting the output voltages the main output voltage is set from +1.25v and +5.5v with two external resistors connected as a voltage- divider to fbm (figure 1). resistor values can be calcu- lated by the following equation: r10 = r11 ? [(v outm / v fbm ) - 1] where v fbm = +1.25v. choose r11 to be 40k ? or less. the core regulator output is adjustable from +1.0v to +5.0v through two external resistors connected as a MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 16 ______________________________________________________________________________________
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover ______________________________________________________________________________________ 17 voltage-divider to fbc (figure 1). resistor values can be calculated with the following equation: r8 = r9 ? [(v outc / v fbc ) - 1] where v fbc = +1.0v. choose r9 to be 30k ? or less. setting the current limit the main regulator current limit is set externally through a small current-sense resistor, r cs (figure 1). the value of r cs can be calculated with the following equa- tion: r cs = v clm / (1.3 ? i out ) where v clm = 80mv is the current-sense threshold, and i out is the current delivered to the output. the core and backup converter current limits are set inter- nally and cannot be modified. careful layout of the current-sense signal traces is imperative. place r cs as close to the MAX1774 as pos- sible. the two traces should have matching length and width, be as far as possible from noisy switching sig- pgndc fbc lxc inc fbm cs- cs+ pgnd aci pdrv ndrv cvh main c5 1 f p2 n1 l1 5 h c main 47 f c6 10 f r cs core c core 22 f r10 r11 r8 r9 l2 5.4 h c7 1 f r5 1m ? r6 1m ? r7 1m ? gnd ref cvl lxb bin in dbi lbi backup battery main battery v in_ac r1 c1 10 f c2 10 f c4 0.22 f c3 1 f d2 l3 22 h p1 r4 r2 r3 d1 bkoff bkup lbo aco shdnc shdnm mdrv MAX1774 on off on off ins nds356ap nsd03a10 2.7v to 20v ep05 q03l lxb2 (qfn only) 0.9v to 5.5v 1.0v to 5.5v 2.6v to 5.5v fds8928a 2.7v to 28v 1m ? figure 4. typical application circuit (cascaded)
nals, and be close together to improve noise rejection. these traces should be used for current-sense signal routing only and should not carry any load current. refer to the MAX1774 evaluation kit for layout exam- ples. setting the voltage monitor levels the low battery and dead battery detector trip points can be set by adjusting the resistor values of the divider string (r1, r2, and r3) in figure 1 according to the following equations: r1 = (r2 + r3) ? [(v bd / v th ) - 1] r2 = r3 ? [(v bl / v bd ) - 1] where v bl is the low battery voltage, v bd is the dead battery voltage, and v th = +1.20v. choose r3 to be less than 250k ? . MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 18 ______________________________________________________________________________________ figure 5. typical application circuit (with recharge) pgndc fbc lxc inc fbm cs- cs+ pgnd pdrv ndrv cvh main c5 l1 10 h c main 47 f r cs core c core 22 f r10 r11 r8 r9 l2 c7 1 f r5 1m ? r6 1m ? r7 1m ? gnd ref cvl lxb bin in dbi lbi backup battery main battery r1 r13 c2 10 f c3 c4 0.22 f c1 10 f p1 r4 r2 r3 r12 d1 bkoff bkup lbo aco shdnm mdrv MAX1774 l3 22 h v in_ac aci shdnc c6 10 f p2 n1 on off on off 1.0v to 5.5v 2.6v to 5.5v fds8928a ins 1m ? 2.7v to 20v 2.7v to 28v nds356ap nsd03a10 0.9v to 5.5v lxb2 (qfn only) d2 ep05 q03l
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover ______________________________________________________________________________________ 19 inductor selection the essential parameters for inductor selection are inductance and current rating. the MAX1774 operates with a wide range of inductance values. calculate the inductance value for either core or main, l min : l (min) = (v in - v out ) ? (t on(min) / l ripple ) where t onmin is typically 400ns, and l ripple is the con- tinuous conduction peak-to-peak l ripple current. in continuous conduction, l ripple should be chosen to be 30% of the maximum load current. with high induc- tor values, the MAX1774 begins continuous-conduction operation at a lower fraction of full load (see detailed description ). the inductor s saturation current must be greater than the peak switching current to prevent core saturation. saturation occurs when the inductor s magnetic flux density reaches the maximum level the core can sup- port and inductance starts to fall. the inductor heating current rating must be greater than the maximum load current to prevent overheating. for optimum efficiency, the inductor series resistance should be less than the current-sense resistance. capacitor selection choose the output filter capacitors to service input and output ripple current with acceptable voltage ripple. esr in the output capacitor is a major contributor to output ripple. for the main converter, low-esr capaci- tors such as polymer or ceramic capacitors are recom- mended. for the core converter, choosing a low-esr tantalum capacitor with enough esr to generate about 1% ripple voltage across the output is helpful in ensur- ing stability. voltage ripple is the sum of contributions from esr and the capacitor value: v ripple v ripple,esr + v ripple,c for tantalum capacitors, the ripple is determined mostly by the esr. voltage ripple due to esr is: v ripple,esr (r esr ) ? i ripple for ceramic capacitors, the ripple is mostly due to the capacitance. the ripple due to the capacitance is approximately: v ripple,c l i ripple 2 c out v out where v out is the average output voltage. these equations are suitable for initial capacitor selec- tion. final values should be set by testing a prototype or evaluation kit. when using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their esr. also, ensure that the tantalum capacitors surge-current ratings exceed the startup inrush and peak switching currents. the input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple at in, caused by the circuit s switching. use a low-esr capacitor. two smaller value low-esr capaci- tors can be connected in parallel if necessary. choose input capacitors with working voltage ratings higher than the maximum input voltage. mosfet selection the MAX1774 drives an external enhancement-mode p- channel mosfet and a synchronous-rectifier n-channel mosfet. when selecting the mosfets, important para- meters to consider are on-resistance (r ds(on) ), maxi- mum drain-to-source voltage (v ds(max) ), maximum gate-to-source voltage (v gs(max) ), and minimum threshold voltage (v th(min) ). chip information transistor count: 4545 process: bicmos 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 lxc ins lbo aco inc gnd aci fbc cs- cs+ fbm ref lbi dbi bkoff bin lxb cvh pdrv in cvl ndrv pgnd pgndc mdrv bkup shdnc shdnm 28 qsop top view MAX1774 pin configurations (continued)
MAX1774 dual, high-efficiency, step-down converter with backup battery switchover 20 ______________________________________________________________________________________ package information qfn 28, 32,44, 48l.eps
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. 21 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ? 2002 maxim integrated products printed usa is a registered trademark of maxim integrated products. MAX1774 dual, high-efficiency, step-down converter with backup battery switchover package information (continued) qsop.eps

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