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1 ltc1504a 500ma low voltage step-down synchronous switching regulator daisy-chained control outputs features the ltc ? 1504a is a self-contained, high efficiency syn- chronous buck switching regulator. it includes a pair of on-chip 1.3 w power switches, enabling it to supply up to 500ma of load current. efficiency peaks at 92%, minimiz- ing heat and wasted power. the synchronous buck archi- tecture allows the output to source or sink current as required to keep the output voltage in regulation. 100% duty cycle operation minimizes dropout voltage. the ltc1504a is available in adjustable and fixed 3.3v output versions. an adjustable current limit circuit pro- vides protection from overloads. the internal 1% refer- ence combined with a sophisticated voltage feedback loop provides optimum output voltage accuracy and fast load transient response. the ltc1504a is specified to operate with input voltages between 4v and 10v. contact the ltc factory for guaranteed specifications at 2.7v supply. the ltc1504a is a pin-compatible, functional upgrade to the ltc1504. the ltc1504a is available in a plastic so-8 package. descriptio n u n 500ma output current at 3.3v output n up to 92% peak efficiency n 100% maximum duty cycle n internal reference trimmed to 1% n output can source or sink current n requires as few as four external components n input voltage range: 4v to 10v n adjustable current limit n small so-8 package n 200khz switching frequency can typically be synchronized up to 500khz minimum part count 5v to 3.3v regulator 5v to 3.3v efficiency load current (ma) 10 efficiency (%) 100 90 80 70 60 50 40 30 20 10 0 100 500 1504 ?ta02 i max shdn ss comp v cc gnd sw sense + 5v shutdown nc nc 1000pf + 3.3v at 500ma l ext 50 h c in : avx tpsc226m016r0375 c out : avx tajc476m010 l ext : coiltronics ctx50-1p 1504a ?ta01 ltc1504a-3.3 c in 22 f c out 47 f applicatio n s u n small portable digital systems n active termination n auxiliary output voltage supplies n minimum part count/size switchers typical applicatio n u , ltc and lt are registered trademarks of linear technology corporation.
2 ltc1504a absolute m axi m u m ratings w ww u wu u package / o rder i for atio electrical characteristics v cc = 5v, t a = 25 c unless otherwise specified. (note 2) symbol parameter conditions min typ max units v cc minimum supply voltage (note 7) l 4v v fb feedback voltage ltc1504acs8 l 1.25 1.265 1.28 v ltc1504ais8 l 1.23 1.265 1.29 v d v fb feedback voltage psrr figure 1, 4v v cc 10v, ltc1504acs8 l 1.1 1.6 % figure 1, 4v v cc 10v, ltc1504ais8 l 1.1 1.8 % v sense sense pin voltage ltc1504acs8-3.3 l 3.20 3.30 3.40 v d v sense sense voltage psrr figure 1, 4v v cc 10v, ltc1504acs8-3.3 l 1.2 1.8 % figure 1, 4v v cc 10v, ltc1504ais8-3.3 l 1.2 2.0 % i cc supply current figure 1, v shdn = v cc , i out = 0 (note 4) 3 ma figure 1, v shdn = v cc , i out = 0, v fb /v sense = v cc (note 4) l 0.3 0.6 ma v shdn = 0v l 1.0 20 m a f osc internal oscillator frequency commercial l 150 200 250 khz industrial l 130 200 300 khz r sw internal switch resistance commercial l 1.3 2.0 w industrial l 1.3 2.2 w v ih shdn input high voltage l 2.4 v v il shdn input low voltage l 0.8 v i in shdn input current l 0.1 1 m a v oh error amplifier positive swing figure 2 l 4.5 4.95 v v ol error amplifier negative swing figure 2 l 0.05 0.5 v i oh , i ol error amplifier output current figure 2 l 50 100 200 m a g mv error amplifier transconductance commercial (note 5) l 350 600 1100 m mho industrial (note 5) l 300 600 1200 m mho a v error amplifier dc gain (note 5) l 40 48 db g mi i lim amplifier transconductance (note 6) 2000 m mho i max i max sink current v imax = v cc , commercial l 81216 m a v imax = v cc , industrial l 81217 m a order part number s8 part marking consult factory for military grade parts. 1504ai 04ai33 t jmax = 125 c, q ja = 90 c/w 1 2 3 4 8 7 6 5 top view comp ss shdn fb/sense* i max v cc sw gnd s8 package 8-lead plastic so *fb for ltc1504a, sense for ltc1504a-3.3 ltc1504acs8 ltc1504acs8-3.3 ltc1504ais8 ltc1504ais8-3.3 (note 1) supply voltage (v cc to gnd) ................................... 10v peak output current (sw) ....................................... 1a input voltage (all other pins) ...... C 0.3v to (v cc + 0.3v) operating temperature range ltc1504ac ............................................. 0 c to 70 c ltc1504ai ......................................... C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c 1504a 1504a33
3 ltc1504a electrical characteristics the l denotes specifications which apply over the full operating temperature range. note 1: absolute maximum ratings are those values beyond which the life of the device may be impaired. note 2: all currents into device pins are positive; all currents out of device pins are negative. all voltages are referenced to ground unless otherwise specified. note 3: this parameter is guaranteed by correlation and is not tested directly. note 4: ltc1504a quiescent current is dominated by the gate drive current drawn by the onboard power switches. with fb or sense pulled to v cc the output stage will stop switching and the static quiescent current can be observed. with fb or sense hooked up normally, the output stage will be switching and total dynamic supply current can be measured. note 5: fixed output parts will appear to have g mv and a v values 2.6 times lower than the specified values, due to the internal divider resistors. note 6: the i lim amplifier can sink but not source current. under normal (not current limited) operation, the i lim output current will be zero. note 7: contact factory for guaranteed specifications at 2.7v supply. typical perfor m a n ce characteristics u w temperature ( c) ?0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 25 75 1504a ?tpc03 ?5 0 50 100 125 switch on-resistance ( ) v cc = 10v v cc = 5v v cc = 3.3v switch on-resistance vs temperature supply current vs supply voltage supply voltage (v) 2.5 1504a ?tpc01 5 7.5 10 supply current (ma) 14 12 10 8 6 4 2 0 v fb = v out v fb = v cc t a = 25 c i out = 0 current limit threshold vs r imax r imax ( ) 10k current limit threshold (ma) 700 600 500 400 300 200 100 0 100k 1504a ?tpc04 t a = 25 c v cc = 5v temperature ( c) 50 25 0 25 50 75 100 125 supply current (ma) 10 1 0.1 1504a ?tpc02 v fb = v out v fb = v cc v cc = 5v i out = 0 supply current vs temperature shutdown threshold vs supply voltage supply voltage (v) 357 shutdown pin threshold (v) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 10 1504a ?tpc07 current limit threshold vs temperature temperature ( c) ?0 current limit threshold (ma) 25 0 25 50 75 1504a ?tpc05 125 500 450 400 350 300 250 200 150 100 50 0 100 v cc = 5v r imax = 47k r imax = 22k symbol parameter conditions min typ max units i ss soft start source current v ss = 0v, commercial l C8 C12 C16 m a v ss = 0v, industrial l C8 C12 C17 m a t r , t f output switch rise/fall time 10% to 90% ouput swing l 550 ns dc max maximum duty cycle v comp = v cc l 100 % v cc = 5v, t a = 25 c unless otherwise specified. (note 2)
4 ltc1504a pi n fu n ctio n s uuu voltage. the ltc1504a feedback loop will servo the fb pin to 1.265v. sense (ltc1504a-3.3) (pin 5): output voltage sense. connect directly to the output voltage node. the ltc1504a-3.3 feedback loop will servo sense to 3.3v. sense is connected to an internal resistor divider which will load any external dividers. for output voltages other than 3.3v, use the ltc1504a. shdn (pin 6): shutdown, active low. when shdn is at a logic high, the ltc1504a will operate normally. when shdn is low, the ltc1504a ceases all internal operation and supply current drops below 1 m a. in shutdown, the sw pin is pulled low. this ensures that the output is actively shut off when shdn is asserted, but it prevents other supplies from providing power to the output when the ltc1504a is inactive. see the applications information section for more details. ss (pin 7): soft start. connect an external capacitor (usually 0.1 m f) from ss to gnd to limit the output rise time during power-up. c ss also compensates the current limit loop, allowing the ltc1504a to enter and exit current limit cleanly. see the applications information section for more details. comp (pin 8): external compensation. an external rc network should be connected to comp to compensate the feedback loop. comp is connected to the output of the internal error amplifier. i max (pin 1): current limit set. connect a resistor from v cc to i max to set the current limit threshold. an internal 12 m a current source from i max to gnd sets the voltage drop across this resistor. this voltage is compared to the voltage drop across the internal high-side switch (q1) while it is turned on. see the applications information section for more information. to disable current limit, leave i max floating. v cc (pin 2): power supply input. connect to a power supply voltage between 4v and 10v. v cc requires a low impedance bypass capacitor to ground, located as close as possible to the ltc1504a. see the applications infor- mation section for details on capacitor selection and placement. sw (pin 3): power switch output. this is the switched node of the buck circuit. connect sw to one end of the external inductor. the other end of the inductor should be connected to c out and becomes the regulated output voltage. avoid shorting sw to gnd or v cc . gnd (pin 4): ground. connect to a low impedance ground. the input and output bypass capacitors and the feedback resistor divider (adjustable parts only) should be grounded as close to this pin as possible. pin 4 acts as a heat sink in the ltc1504a s0-8 package and should be connected to as large a copper area as possible to improve thermal dissipation. see the thermal considerations section for more information. fb (ltc1504a) (pin 5): feedback. connect fb to a resistor divider from v out to gnd to set the regulated output
5 ltc1504a test circuits i max shdn ss comp v cc gnd sw fb/sense + c in 100 f v cc nc 0.01 f + c out 220 f v out l ext 47 h c in : avx tpse107m016r0125 c out : sanyo 16cv220gx l ext : coilcraft d03316-473 1504a ?tc01 ltc1504a 1 f 0.1 f 220pf 7.5k figure 1 + v ref ltc1504a comp fb/sense a b a: test v ol , i ol b: test v oh , i oh 1504a ?tc02 figure 2 block diagra m w figure 3. block diagram + + fb ? i lim + 1.265v 12 a 12 a pwm 12.6k 20.4k q1 q2 sw fb (adj only) sense (3.3v only) saw i max ss comp shdn 1504a ?bd v ref v cc to internal blocks applicatio n s i n for m atio n wu u u overview the ltc1504a is a complete synchronous switching regulator controller (see block diagram). it includes two on-chip 1.3 w power mosfets, eliminating the need for external power devices and minimizing external parts count. the internal switches are set up as a synchronous buck converter with a p-channel device (q1) from the input supply to the switching node and an n-channel device (q2) as the synchronous rectifier device from the switching node to ground. an external inductor, input and
6 ltc1504a applicatio n s i n for m atio n wu u u 3 w , depending on supply voltage. this high power pulse train is filtered by the external inductor and capacitor, providing a steady dc value at the output node. this node returns to fb or sense, closing the loop. the ltc1504a includes a second feedback loop that controls operation in current limit. the i lim amplifier monitors the voltage at the sw pin while q1 is on. it compares this voltage to the voltage at the i max pin. as the peak current through q1 rises, the voltage drop across it due to its r on increases proportionally. when sw drops below i max , indicating the current through q1 has in- creased beyond the desired value, i lim starts pulling a controlled amount of current out of ss, the external soft start pin. as ss falls, it pulls comp down with it, limiting the duty cycle and reducing the output voltage to control the current. the speed at which the current limit circuit reacts is set by the value of the external soft start capacitor. external component selection external components required by the ltc1504a fall into three categories: input bypass, output filtering and com- pensation. additional components to set up soft start and current limit are usually included as well. a minimum ltc1504a circuit can be constructed with as few as four external components; a circuit that utilizes all of the ltc1504as functionality usually includes eight or nine external components, with two additional feedback resis- tors required for adjustable parts. see the typical applica- tions section for examples of external component hookup. input bypass the input bypass capacitor is critical to proper ltc1504a operation. the ltc1504a includes a precision reference and a pair of high power switches feeding from the same v cc pin. if v cc does not have adequate bypassing, the switch pulses introduce enough ripple at v cc to corrupt the reference voltage and the ltc1504a will not regulate accurately. symptoms of inadequate bypassing include poor load regulation and/or erratic waveforms at the sw pin. if an oscilloscope wont trigger cleanly when looking at the sw pin, the ltc1504a doesnt have adequate input bypass. output bypass capacitors and a compensation network complete the control loop. the ltc1504a adjustable output parts require an additional pair of resistors to set the output voltage. the ltc1504a-3.3 parts include an onboard resistor divider preset to a 3.3v output voltage. a functional 3.3v output regulator can be constructed with an ltc1504a-3.3 and as few as four external components. the ltc1504a feedback loop includes a precision refer- ence trimmed to 1% (v ref ), a wide bandwidth transconductance feedback amplifier (fb) and an onboard pwm generator (saw and pwm). the pwm generator is capable of generating pulse widths from 0% to 100%, minimizing dropout and maximizing transient response. the internal sawtooth oscillator typically runs at 200khz. q1 and q2 are capable of carrying peak currents in excess of 500ma, with the continuous output power level limited primarily by the thermal dissipation of the so-8 package. with a 5v input and a 3.3v output, the ltc1504a can supply 500ma of continuous output current with an appropriate layout. an on-chip current limit circuit, set with a single external resistor, can be used to help limit power dissipation. see the thermal considerations sec- tion for more information. theory of operation the ltc1504a primary feedback loop consists of the main error amplifier fb, the pwm generator, the output drive logic and the power switches. the loop is closed with the external inductor and the output bypass capacitor. the feedback amplifier senses the output voltage directly at the sense pin for fixed output versions or through an external resistor divider in the adjustable output version. this feedback voltage is compared to the 1.265v internal reference voltage by fb and an error signal is generated at the comp pin. comp is a high impedance node that is brought out to an external pin for optimizing the loop compensation. comp is compared to a 200khz sawtooth wave by com- parator pwm. the output stage takes the pwm signal and generates nonoverlapping drive for the onboard p- and n- channel power mosfets, which drive the sw pin with a low impedance image of the pwm waveform. typical open-loop output impedance at sw is between 1 w and
7 ltc1504a ideally, the ltc1504a requires a low impedance bypass right at the chip and a larger reservoir capacitor that can be located somewhat farther away. this requirement usually can be met with a ceramic capacitor right next to the ltc1504a and an electrolytic capacitor (usually 10 m f to 100 m f, depending on expected load current) located some- where nearby. in certain cases, the bulk capacitance requirement can be met by the output bypass of the input supply. applications running at very high load currents or at input supply voltages greater than 6v may require the local ceramic capacitor to be 1 m f or greater. in some cases, both the low impedance and bulk capacitance requirements can be met by a single capacitor, mounted very close to the ltc1504a. low esr organic semicon- ductor (os-con) electrolytic capacitors or surge tested surface mount tantalum capacitors can have low enough impedance to keep the ltc1504a happy in some circuits. often the rms current capacity of the input bypass capaci- tors is more important to capacitor selection than value. buck converters like the ltc1504a are hard on input capacitors, since the current flow alternates between the full load current and near zero during every clock cycle. in the worst case (50% duty cycle or v out = 0.5v in ) the rms current flow in the input capacitor is half of the total load current plus half the ripple current in the inductor perhaps 300ma in a typical 500ma load current applica- tion. this current flows through the esr of the input bypass capacitor, heating it up and shortening its life, sometimes dramatically. many ordinary electrolytic ca- pacitors that look ok at first glance are not rated to withstand such currentscheck the rms current rating before you specify a device! if the rms current rating isnt specified, it should not be used as an input bypass capaci- tor. again, low esr electrolytic and surge tested tantalums usually do well in ltc1504a applications and have high rms current ratings. the local ceramic bypass capacitor usually has negligible esr, allowing it to withstand large rms currents without trouble. table 1 shows typical surface mount capacitors that make acceptable input bypass capacitors in ltc1504a applications. inductor the ltc1504a requires an external inductor to be con- nected from the switching node sw to the output node applicatio n s i n for m atio n wu u u table 1. representative surface mount input bypass capacitors part value esr max rms type height avx tpsc226m016r0375 22 m f 0.38 w 0.54a tantalum 2.6mm tpsd476m016r0150 47 m f 0.15 w 0.86a tantalum 2.9mm tpse107m016r0125 100 m f 0.13 w 1.15a tantalum 4.1mm 1206yc105m 1 m f low >1a x7r ceramic 1.5mm 1210yg106z 10 m f low >1a y5v ceramic 1.7mm sanyo 16sn33m 33 m f 0.15 w 1.24a os-con 7mm 16sn68m 68 m f 0.1 w 1.65a os-con 7mm 16cv100gx 100 m f 0.44 w 0.23a* electrolytic 6mm 16cv220gx 220 m f 0.34 w 0.28a* electrolytic 7.7mm sprague 593d476x0016d2w 47 m f 0.17 w 0.93a tantalum 2.8mm 593d107x0016e2w 100 m 0.15 w 1.05a tantalum 4mm *note: use multiple devices in parallel or limit output current to prevent capacitor overload. where the load is connected. inductor requirements are fairly straightforward; it must be rated to handle continu- ous dc current equal to the maximum load current plus half the ripple current and its value should be chosen based on the desired ripple current and/or the output current transient requirements. large value inductors lower ripple current and decrease the required output capacitance, but limit the speed that the ltc1504a can change the output current, limiting output transient re- sponse. small value inductors result in higher ripple currents and increase the demands on the output capaci- tor, but allow faster output current slew rates and are often smaller and cheaper for the same dc current rating. a typical inductor used in an ltc1504a application might have a maximum current rating between 500ma and 1a and an inductance between 33 m h and 220 m h. different core materials and shapes will change the size/ current and price/current relationship of an inductor. toroid or shielded pot cores in ferrite or permalloy mate- rials are small and dont radiate much energy, but gener- ally cost more than powdered iron rod core inductors with similar electrical characteristics. the choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/emi requirements than on what the ltc1504a requires to operate. table 2 shows some typical surface mount inductors that work well in ltc1504a applications.
8 ltc1504a applicatio n s i n for m atio n wu u u table 2. representative surface mount inductors core core part value max dc type material height coilcraft dt3316-473 47 m h 1a shielded ferrite 5.1mm dt3316-104 100 m h 0.8a shielded ferrite 5.1mm do1608-473 47 m h 0.5a open ferrite 3.2mm do3316-224 220 m h 0.8a open ferrite 5.5mm coiltronics ctx50-1 50 m h 0.65a toroid koolm m ? 4.2mm ctx100-2 100 m h 0.63a toroid koolm m 6mm ctx50-1p 50 m h 0.66a toroid type 52 4.2mm ctx100-2p 100 m h 0.55a toroid type 52 6mm tp3-470 47 m h 0.55a toroid ferrite 2.2mm tp3-470 47 m h 0.72a toroid ferrite 3mm sumida cdrh62-470 47 m h 0.54a shielded ferrite 3mm cdrh73-101 100 m h 0.50a shielded ferrite 3.4mm cd43-470 47 m h 0.54a open ferrite 3.2mm CD54-101 100 m h 0.52a open ferrite 4.5mm output capacitor the output capacitor affects the performance of the ltc1504a in a couple of ways: it provides the first line of defense during a transient load step and it has a large effect on the compensation required to keep the ltc1504a feedback loop stable. transient load response of an ltc1504a circuit is controlled almost entirely by the output capacitor and the inductor. in steady load opera- tion, the average current in the inductor will match the load current. when the load current changes suddenly, the inductor is suddenly carrying the wrong current and requires a finite amount of time to correct itselfat least several switch cycles with typical ltc1504a inductor values. even if the ltc1504a had psychic abilities and could instantly assume the correct duty cycle, the rate of change of current in the inductor is still related to its value and cannot change instantaneously. until the inductor current adjusts to match the load cur- rent, the output capacitor has to make up the difference. applications that require exceptional transient response (2% or better for instantaneous full-load steps) will re- quire relatively large value, low esr output capacitors. applications with more moderate transient load require- ments can often get away with traditional standard esr electrolytic capacitors at the output and can use larger valued inductors to minimize the required output capaci- tor value. note that the rms current in the output capacitor is slightly more than half of the inductor ripple current much smaller than the rms current in the input bypass capacitor. output capacitor lifetime is usually not a factor in typical ltc1504a applications. large value ceramic capacitors used as output bypass capacitors provide excellent esr characteristics but can cause loop compensation difficulties. see the loop com- pensation section. loop compensation loop compensation is strongly affected by the output capacitor. from a loop stability point of view, the output inductor and capacitor form a series rlc resonant circuit, with the l set by the inductor value, the c by the value of the output capacitor and the r dominated by the output capacitors esr. the amplitude response and phase shift due to these components is compensated by a network of rs and cs at the comp pin to (hopefully) close the feedback loop in a stable manner. qualitatively, the l and c of the output stage form a 2nd order roll-off with 180 of phase shift; the r due to esr forms a single zero at a somewhat higher frequency that reduces the roll-off to first order and reduces the phase shift to 90 . if the output capacitor has a relatively high esr, the zero comes in well before the initial phase shift gets all the way to 180 and the loop only requires a single small capacitor from comp to gnd to remain stable (figure 4a). if, on the other hand, the output capacitor is a low esr type to maximize transient response, the esr zero can increase in frequency by a decade or more and the output stage phase shift can get awfully close to 180 before it turns around and comes back to 90 . large value ceramic, os-con electrolytic and low impedance tantalum capacitors fall into this category. these loops require an additional zero to be inserted at the comp pin; a series rc in parallel with a smaller c to ground will usually ensure stability. figure 4b shows a typical compensation network which will opti- mize transient response with most output capacitors. adjustable output parts can add a feedforward capacitor across the feedback resistor divider to further improve kool m m is a registered trademark of magnetics, inc..
9 ltc1504a applicatio n s i n for m atio n wu u u phase margin. the typical applications in this data sheet show compensation values that work with several combi- nations of external componentsuse them as a starting point. for complex cases or stubborn oscillations, contact the ltc applications department. external schottky diode an external schottky diode can be included across the internal n-channel switch (q2) to improve efficiency at heavy loads. the diode carries the inductor current during the nonoverlap time while the ltc1504a turns q1 off and q2 on and prevents current from flowing in the intrinsic body diode in parallel with q2. this diode will improve efficiency by a percentage point or two as output current approaches 500ma and can help minimize erratic behav- ior at very high peak current levels caused by excessive parasitic current flow through q2. a motorola mbrs0530l is usually adequate, with the cathode connected to sw and the anode connected to gnd. note that this diode is not required for normal operation and has a negligible effect on efficiency at low (< 250ma) output currents. comp fb c c *adjustable parts only 1504a ?f04a ltc1504a r fb2 * r fb1 * v out comp fb c c *adjustable parts only 1504a ?f04b ltc1504a c f c ff * r c r fb2 * r fb1 * v out figure 4a. minimum compensation network figure 4b. optimum compensation network soft start and current limit soft start and current limit are linked in the ltc1504a. soft start works in a straightforward manner. an internal 12 m a current source connected to the ss pin will pull up an external capacitor connected from ss to gnd at a rate determined by the capacitor value. comp is clamped to a voltage one diode drop above ss; as ss rises, comp will rise at the same rate. when comp reaches roughly 2v below v cc , the duty cycle will slowly begin to increase until the output comes into regulation. as ss continues to rise, the feedback amplifier takes over at comp, the clamp releases and ss rises to v cc . current limit operates by pulling down on the soft start pin when it senses an overload condition at the output. the current limit amplifier (i lim ) compares the voltage drop across the internal p-channel switch (q1) during its on time to the voltage at the i max pin. i max includes an internal 12 m a pull-down, allowing the voltage to be set by a single resistor between v cc and i max . when the ir drop across q1 exceeds the drop across the i max resistor, i lim pulls current out of the external soft start capacitor, reducing the voltage at ss. a soft start capacitor should always be used if current limit is enabled. ss, in turn, pulls down on comp, limiting the output duty cycle and controlling the output current. when the current overload is removed, the i lim amplifier lets go of ss and allows it to rise again as if it were completing a soft start cycle. the size of the external soft start capacitor controls both how fast the current limit responds once an overload is detected and how fast the output recovers once the overload is re- moved. the soft start capacitor also compensates the feedback loop created by the i lim amplifier. because the i lim loop is a current feedback loop, the additional phase shift due to the output inductor and capacitor do not come into play and the loop can be adequately compensated with a single capacitor. usually a 0.1 m f ceramic capacitor from ss to gnd provides adequate soft start behavior and acceptable current limit response. this type of current limit circuit works well with mild current overloads and eliminates the need for an external current sensing resistor, making it attractive for ltc1504a applications. these same features also handicap the cur- rent limit circuit under severe short circuits when the
10 ltc1504a applicatio n s i n for m atio n wu u u output voltage is very close to ground. under this condi- tion, the ltc1504a must run at extremely narrow duty cycles (< 5%) to keep the current under control. when the on-time falls below the time required to sense the current in q1, the ltc1504a responds by reducing the oscillator frequency, increasing the off-time to decrease the duty cycle and allow it to maintain some control of the output current. the oscillator frequency may drop by as much as a factor of 10 under severe current overloads. under extreme short circuits (e.g., screwdriver to ground) the on-time will reduce to the point where the ltc1504a will lose control of the output current. at this point, output current will rise until the inductor saturates, and the current will be limited by the parasitic esl of the inductor and the r on of q2 inside the ltc1504a. this current is usually nondestructive and dissipates a limited amount of power since the output voltage is very low. a typical ltc1504a circuit can withstand such a short for many seconds without damage. the test circuit in figure 1 will typically withstand a direct output short for more than 30 seconds without damage to the ltc1504a. eventually, however, a continuous short may cause the die tempera- ture to rise to destructive levels. note that the current limit is primarily designed to protect the ltc1504a from damage and is not intended to be used to generate an accurate constant-current output. as the die temperature varies in a current limited condition, the r on of the internal switches will change and the current limit threshold will move around. r on will also vary from part to part due to manufacturing tolerance. the external i max resistor should be chosen to allow enough room to account for these variations without allowing the current limit to engage at the maximum expected load current. a current limit setting roughly double the expected load is often a good compromise, eliminating unintended current limit operation while preventing circuit destruction under actual fault conditions. if desired, current limit can be disabled by floating the i max pin; the internal current source will pull i max to gnd and the i lim amplifier will be disabled. shutdown the ltc1504a includes a micropower shutdown mode controlled by the logic level at shdn. a logic high at shdn allows the part to operate normally. a logic low at shdn stops all internal switching, pulls comp, ss and sw to gnd and drops quiescent current below 1 m a typically. note that the internal n-channel power mosfet from sw to gnd turns on when shdn is asserted. this ensures that the output voltage drops to zero when the ltc1504a is shut down, but prevents other devices from powering the output when the ltc1504a is disabled. external clock synchronization the ltc1504a shdn pin can double as an external clock input for applications that require a synchronized clock or a faster switching speed. the shdn pin terminates the internal sawtooth wave and resets the oscillator immedi- ately when it goes low, but waits 50 m s before shutting down the rest of the internal circuitry. a clock signal applied directly to the shdn pin will force the ltc1504a internal oscillator to lock to its frequency as long as the external clock runs faster than the internal oscillator frequency. attempting to synchronize to a frequency lower than the 250khz maximum internal frequency may result in inconsistent pulse widths and is not recom- mended. because the sawtooth waveform rises at a fixed rate internally, terminating it early by synchronizing to a fast external clock will reduce the amplitude of the sawtooth wave that the pwm comparator sees, effectively raising the gain from comp to sw. 500khz is the maximum recommended synchronization frequency; higher frequen- cies will reduce the sawtooth amplitude to the point that the ltc1504a may run erratically. thermal considerations each of the ltc1504a internal power switches has ap- proximately 1.3 w of resistance at room temperature and will happily carry more than the rated maximum current if the current limit is set very high or is not connected. since the inductor current is always flowing through one or the other of the internal switches, a typical application supply- ing 500ma of load current will cause a continuous dissi- pation of approximately 325mw. the so-8 package has a thermal resistance of approximately 90 c/w, meaning that the die will begin to rise toward 30 c above ambient
11 ltc1504a at this power level. the r on of the internal power switches increases as the die temperature rises, increasing the power dissipation as the feedback loop continues to keep the output current at 500ma. at high ambient tempera- tures, this cycle may continue until the chip melts, since the ltc1504a does not include any form of thermal shutdown. applications can safely draw peak currents above the 500ma level, but the average power dissipation should be carefully calculated so that the maximum 125 c die temperature is not exceeded. the ltc1504a dissipates the majority of its heat through its pins, especially gnd (pin 4). thermal resistance to ambient can be optimized by connecting gnd to a large copper region on the pcb, which will serve as a heat sink. applications which will operate the ltc1504a near maxi- mum power levels or which must withstand short circuits of extended duration should maximize the copper area at all pins and ensure that there is some airflow over the part to carry away excess heat. for layout assistance in situa- tions where power dissipation may be a concern, contact the ltc applications department. the current limit circuit can be used to limit the power under mild overloads to a safe level, but severe overloads where the output is shorted to ground may still cause the die temperature to rise dangerously. for more information on current limit behavior, see the current limit section. layout considerations like all precision switching regulators, the ltc1504a requires special care in layout to ensure optimum perfor- mance. the large peak currents coupled with significant dc current flow will conspire to keep the output from regulating properly if the layout is not carefully planned. a poorly laid out op amp or data converter circuit will fail to give the desired performance, but will usually still act like an op amp or data converter. a poorly laid out ltc1504a circuit may look nothing at all like a regulator. wire-wrap or plug-in prototyping boards are not useful for bread- boarding ltc1504a circuits! open-core inductors lo- cated close to the ltc1504a can cause erratic regulation due to stray flux coupled into pc board traces or the ltc1504a itself. changing the orientation of the inductor or switching to a shielded type will solve the problem. perhaps most critical to proper ltc1504a performance is the layout of the ground node and the location of the input and output capacitors. the negative terminals of both the input and output bypass capacitors should come together at the same point, as close as possible to the ltc1504a ground pin. the compensation network and soft start capacitor can be connected together with their own trace, which should come directly back to this same common ground point. the input supply ground and the load return should also connect to this common point. each ground line should come to a star connection with pin 4 at the center of the star. this node should be a fairly large copper region to act as a heat sink if required. second in importance is the proximity of the low esr (usually ceramic) input bypass capacitor. it should be located as close to the ltc1504a v cc and gnd pins as physically possible. ideally, the capacitor should be located right next to the package, straddling the sw pin. high peak current applica- tions or applications with v cc greater than 6v may require a 1 m f or larger ceramic capacitor in this position. one node that isnt quite so critical is sw. extra lead length or narrow traces at this pin will only add parasitic induc- tance in series with the external inductor, slightly raising its value. the sw trace need only be wide enough to support the maximum peak current under short circuit conditionsperhaps 1a. if a trace needs to be compro- mised to make the layout work, this is the one. note that long traces at the sw node may aggravate emi consider- ationsdont get carried away. if a schottky diode is used at the sw node, it should be located at the ltc1504a end of the trace, close to the device pins. the ltc applications department has constructed liter- ally hundreds of layouts for the ltc1504a and related parts, many of which worked and some of which are now archived in the bad layout hall of fame. if you need layout assistance or you think you have a candidate layout for the hall of fame, give applications a call at (408) 954-8400. demo boards with properly designed layouts are available and specialized layouts can be designed if required. the applications team is also experienced in external compo- nent selection for a wide variety of applications, and they have a never-ending selection of tall tales to tell as well. when in doubt, give them a call. applicatio n s i n for m atio n wu u u information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
12 ltc1504a ? linear technology corporation 1997 1504afs, sn1504a lt/tp 1098 4k ? printed in usa typical applicatio n s u high efficiency 5v to 2.5v converter with current limit scsi-2 active terminator i max shdn ss comp v cc gnd sw fb + 1 f 0.1 f r imax * c in v cc 5v shdn 7.5k 0.01 f 220pf 11.8k 12.1k mbrs0530l + c out v out 2.5v l ext c in : avx tpse107m016r0125 c out : sanyo 16cv220gx l ext : coilcraft do3316-473 *select r imax value using current limit threshold graph on page 3 1504a ?ta03 ltc1504a i max shdn ss comp v cc gnd sw fb 4.7 f ceramic termpwr 7.5k 0.01 f 220pf 15k 12k + c out l ext c out : avx tpsc107m006r0150 l ext : sumida cd54-470 1504a ?ta04 nc nc ltc1504a 110 110 110 110 110 18 to 27 lines part number description comments ltc1174 600ma, high efficiency step-down converter nonsynchronous, better low load efficiency ltc1430 high power step-down dc/dc controller 5v to 1.xv C 3.xv voltage conversion for high end processors ltc1433/ltc1434 450ma, low noise current mode step-down converters nonsynchronous, better low load efficiency ltc1474 low quiescent current, high efficiency step-down converter 10 m a standy current, 92% efficiency, msop package lt1507 1.5a, 500khz monolithic buck regulator nonsynchronous, 1.5a max current ltc1627 monolithic synchronous step-down switching regulator 2.65v to 8.5v input range, 95% efficiency, so-8 package related parts package descriptio n u dimensions is inches (millimeters) unless otherwise noted. 1 2 3 4 0.150 ?0.157** (3.810 ?3.988) 8 7 6 5 0.189 ?0.197* (4.801 ?5.004) 0.228 ?0.244 (5.791 ?6.197) 0.016 ?0.050 0.406 ?1.270 0.010 ?0.020 (0.254 ?0.508) 45 0 ?8 typ 0.008 ?0.010 (0.203 ?0.254) so8 0996 0.053 ?0.069 (1.346 ?1.752) 0.014 ?0.019 (0.355 ?0.483) 0.004 ?0.010 (0.101 ?0.254) 0.050 (1.270) typ dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side * ** s8 package 8-lead plastic small outline (narrow 0.150) (ltc dwg # 05-08-1610) linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com

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