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april 2005 1 mic4681 mic4681 micrel mic4681 2a-peak superswitcher sop-8 buck regulator general description the mic4681 superswitcher is an easy-to-use step-down (buck) voltage-mode switching regulator. the 200khz mic4681 achieves over 1a of continuous output current over a 4v to 30v input range in an 8-lead soppackage . the mic4681 features a high 2.1a minimum current limit, making the device ideal for pulsed current applications such as gsm and tdma cell phone battery chargers and power supplies. the mic4681 sustains an output of 4.2v/2a within a typical gsm charging environment. the mic4681 has an input voltage range of 4v to 30v, with excellent line, load, and transient response. the regulator performs cycle-by-cycle current limiting and thermal shut- down for protection under fault conditions. in shutdown mode, the regulator draws less than 6 a of standby current. the mic4681 superswitcher regulator requires a mini- mum number of external components and can operate using a standard series of inductors and capacitors. frequency compensation is provided internally for fast transient re- sponse and ease of use. the mic4681 is available in the 8-lead sop with a ?0 c to +125 c junction temperature range. typical applications sw l1 68 h in fb gnd shdn c2 220 f 10v r1 3.01k r2 2.94k 2.5v/1a mic4681bm c1 22 f 35v +5v to +30v shutdown enable power sop-8 5 8 4 3 2 1 d1 b340a or ss36 adjustable regulator circuit features so-8 package with over 1a continuous output current capable of 2a pulse charging for gsm applications all surface mount solution only 4 external components required fixed 200khz operation output adjustable down to 1.25v internally compensated with fast transient response wide 4v to 30v operating input voltage range less than 6 a typical shutdown-mode current up to 90% efficiency thermal shutdown overcurrent protection applications cellular phone battery charger cellular phone power supply simple 1a continuous high-efficiency step-down (buck) regulator replacement of a to-220 and to-263 designs positive-to-negative converter (inverting buck-boost) negative boost converter higher output current regulator using external fet micrel, inc. 1849 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 http://www.micrel.com superswitcher is a trademark of micrel, inc.
mic4681 micrel mic4681 2 april 2005 pin configuration 1 shdn vin sw fb 8 gnd gnd gnd gnd 7 6 5 2 3 4 sop-8 (m) pin description pin number pin name pin function 1 shdn shutdown (input): logic low enables regulator. logic high (>2v) shuts down regulator. 2 vin supply voltage (input): unregulated +4v to +30v supply voltage. 3 sw switch (output): emitter of npn output switch. connect to external storage inductor and shottky diode. 4 fb feedback (input): connect to 1.23v-tap of voltage-divider network 5 8 gnd ground ordering information part number standard lead-free voltage frequency junction temp. range package mic4681bm MIC4681YM adjustable 400khz 40 c to +125 c 8-lead sop
april 2005 3 mic4681 mic4681 micrel absolute maximum ratings (note 1) supply voltage (v in ), note 3 ...................................... +34v shutdown voltage (v shdn ) .......................... 0.3v to +34v steady-state output switch voltage (v sw ) .................. 1v feedback voltage [adjustable] (v fb ) .......................... +12v storage temperature (t s ) ....................... 65 c to +150 c esd, note 5 operating ratings (note 2) supply voltage (v in ) ....................................... +4v to +30v junction temperature (t j ) ...................................... +125 c package thermal resistance ( ja ), note 6 ............ 63 c/w ( jc ), note 6 ........... 20 c/w electrical characteristics v in = 12v; i load = 500ma; t j = 25 c, bold values indicate 40 c t j +125 c, note 7 ; unless noted. parameter condition min typ max units feedback voltage ( 1%) 1.217 1.230 1.243 v ( 2%) 1.205 1.255 v 8v v in 30v, 0.1a i load 1a, v out = 5v 1.193 1.230 1.267 v 1.180 1.280 v maximum duty cycle v fb = 1.0v 93 95 % output leakage current v in = 30v, v shdn = 5v, v sw = 0v 50 500 a v in = 30v, v shdn = 5v, v sw = 1v 4 20 ma quiescent current v fb = 1.5v 7 12 ma frequency fold back 50 110 khz oscillator frequency 180 200 220 khz saturation voltage i out = 1a 1.4 1.8 v short circuit current limit v fb = 0v, see test circuit v in = 30v (note 8) 2.2 3.4 4.5 a standby quiescent current v shdn = 5v (regulator off) 35 100 a v shdn = v in 6 a shutdown input logic level regulator off 2 1.4 v regulator on 1.25 0.8 v shutdown input current v shdn = 5v (regulator off) 10 0.5 1 a v shdn = 0v (regulator on) 10 1.5 1 a thermal shutdown @ t j 160 c note 1. exceeding the absolute maximum rating may damage the device. note 2. the device is not guaranteed to function outside its operating rating. note 3. absolute maximum rating is intended for voltage transients only, prolonged dc operation is not recommended. note 4. v in(min) = v out + 2.5v or 4v whichever is greater. note 5. devices are esd sensitive. handling precautions recommended. note 6. measured on 1" square of 1 oz. copper fr4 printed circuit board connected to the device ground leads. note 7. test at t a = +85 c, guaranteed by design, and characterized to t j = +125 c. note 8. short circuit protection is guaranteed to 30v max.
mic4681 micrel mic4681 4 april 2005 test circuit sw 68 h in fb gnd shdn device under test +30v shutdown enable sop-8 5 8 4 3 2 1 i current limit test circuit shutdown input behavior off on guaranteed off typical off guaranteed on typical on 0.8v 1.25v 0v 1.4v v in(max) 2v shutdown hysteresis
april 2005 5 mic4681 mic4681 micrel 4.95 4.97 4.99 5.01 5.03 5.05 0 0.5 1.0 1.5 2.0 output voltage (v) output current (a) load regulation amb = 25 c v in = 12v typical characteristics 4.95 4.97 4.99 5.01 5.03 5.05 0 5 10 15 20 25 30 output voltage (v) input voltage (v) line regulation amb = 25 c 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 shutdown current ( a) input voltage (v) shutdown current vs. input voltage v shdn = 5v amb = 25 c 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 35 shutdown current ( a) input voltage (v) shutdown current vs. input voltage v shdn = v in amb = 25 c 5.8 6 6.2 6.4 6.6 -40 -20 0 20 40 60 80 100 120 shutdown current ( a) temperature ( c) shutdown current vs. temperature v in = 12v v shdn = v in 0 1 2 3 4 5 6 0 0.5 1 1.5 2 2.5 3 output voltage (v) output current (a) current limit characteristics v in = 12v amb = 25 c 195 197 199 201 203 205 0 5 10 15 20 25 30 35 frequency (khz) input voltage (v) mic4681bm frequency vs. supply voltage i out = 100ma amb = 25 c 185 190 195 200 205 210 215 -40 -20 0 20 40 60 80 100 120 frequency (khz) temperature ( c) frequency vs. temperature v in = 12v 1.222 1.224 1.226 1.228 1.230 1.232 1.234 1.236 1.238 -40 -20 0 20 40 60 80 100 120 feedback voltage (v) temperature ( c) feedback voltage vs. temperature v in = 12v 0 20 40 60 80 0 0.5 1 1.5 2 2.5 efficiency (%) output current (a) mic4681 3.3v output efficiency 24vin 6vin 12vin amb = 25 c 0 20 40 60 80 100 0 0.5 1.0 1.5 2.0 efficiency (%) output current (a) 5v output efficiency 24vin 7vin 12vin amb = 25 c 0 20 40 60 80 100 0 0.5 1 1.5 efficiency (%) output current (a) 12v output efficiency 24vin 15vin amb = 25 c
mic4681 micrel mic4681 6 april 2005 0.0 0.5 1.0 1.5 -40 -20 0 20 40 60 80 100 120 enable threshold voltage (v) temperature ( c) shutdown thresholds vs. temperature re g ulator off regulator on v in = 12v 0 0.5 1 1.5 2 0 0.5 1 1.5 2 saturation voltage (v) output current (a) saturation voltage vs. output current v in = 12v amb = 25 c
april 2005 7 mic4681 mic4681 micrel 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 5 10 15 20 25 30 35 continuous output current (a) input voltage (v) 4681bm soa 5vout v out = 5v t a = 60 c ta = 25 c 4681bm soa 3.3vout 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 5 10 15 20 25 30 35 continuous output current (a) input voltage (v) v out = 3.3v t a = 60 c t a = 25 c 4681bm soa 5vout 4681bm soa 3.3vout
mic4681 micrel mic4681 8 april 2005 switching frequency foldback time normal operation short circuit operation v sw (shorted) 12v in, 0v out v sw (normal) 12v in, 5v/1a out 200khz 60khz load transient time (100 s/div) v out (100mv/div) i out (1a/div) v in = 12v v out = 5v 0a 100mv 1.5a functional characteristics frequency foldback the mic4681 folds the switching frequency back during a hard short-circuit condition to reduce the energy per cycle and protect the device.
april 2005 9 mic4681 mic4681 micrel bode plots the following bode plots show that the mic4681 is stable over all conditions using a 68 f inductor (l) and a 220 f output capacitor (c out ). to assure stability, it is a good practice to maintain a phase margin of greater than 35 . no-load stability phase margin = 94 v in = 7v v out = 5.0v i out = 0a amb = 22 c l = 68 f c out = 220 f full-load stability phase margin = 74 v in = 7v v out = 5.0v i out = 1.1a amb = 22 c l = 68 f c out = 220 f no-load stability phase margin = 102 v in = 12v v out = 5.0v i out = 0a amb = 22 c l = 68 f c out = 220 f full-load stability phase margin = 59 v in = 30v v out = 5.0v i out = 1.1a amb = 22 c l = 68 f c out = 220 f no-load stability phase margin = 118 v in = 30v v out = 5.0v i out = 0a amb = 22 c l = 68 f c out = 220 f full-load stability phase margin = 53 v in = 12v v out = 5.0v i out = 1.4a amb = 22 c l = 68 f c out = 220 f
mic4681 micrel mic4681 10 april 2005 block diagrams sw fb r1 r2 c out v in in v out mic4681 [adj.] internal regulator shdn 200khz oscillator thermal shutdown reset current limit com- parator error amp 3a switch driver 1.23v bandgap reference vv r1 r2 1 r1 r2 v v 1 v 1.23v out ref out ref ref =+ ? ? ? ? ? ? =? ? ? ? ? ? ? = adjustable regulator
april 2005 11 mic4681 mic4681 micrel functional description the mic4681 is a variable duty cycle switch-mode regulator with an internal power switch. refer to the block diagrams. supply voltage the mic4681 operates from a +4v to +30v unregulated input. highest efficiency operation is from a supply voltage around +12v. see the efficiency curves on page 6. enable/shutdown the shutdown ( shdn ) input is ttl compatible. ground the input if unused. a logic-low enables the regulator. a logic- high shuts down the internal regulator which reduces the current to typically 35 a when v shdn = v in = 12v and 6 a when v shdn = 5v. see shutdown input behavior: shutdown hysteresis. feedback require an external resistive voltage divider from the output voltage to ground, center tapped to the fb pin. see figure 1b for recommended resistor values. duty cycle control a fixed-gain error amplifier compares the feedback signal with a 1.23v bandgap voltage reference. the resulting error amplifier output voltage is compared to a 200khz sawtooth waveform to produce a voltage controlled variable duty cycle output. a higher feedback voltage increases the error amplifier output voltage. a higher error amplifier voltage (comparator inverting input) causes the comparator to detect only the peaks of the sawtooth, reducing the duty cycle of the com- parator output. a lower feedback voltage increases the duty cycle. the mic4681 uses a voltage-mode control architec- ture. output switching when the internal switch is on, an increasing current flows from the supply v in, through external storage inductor l1, to output capacitor c out and the load. energy is stored in the inductor as the current increases with time. when the internal switch is turned off, the collapse of the magnetic field in l1 forces current to flow through fast recovery diode d1, charging c out . output capacitor external output capacitor c out provides stabilization and reduces ripple. see bode plots for additional information. return paths during the on portion of the cycle, the output capacitor and load currents return to the supply ground. during the off portion of the cycle, current is being supplied to the output capacitor and load by storage inductor l1, which means that d1 is part of the high-current return path. applications information adjustable regulators adjustable regulators require a 1.23v feedback signal. rec- ommended voltage-divider resistor values for common out- put voltages are included in figure 1b. for other voltages, the resistor values can be determined using the following formulas: vv r1 r2 1 r1 r2 v v 1 v 1.23v out ref out ref ref =+ ? ? ? ? ? ? =? ? ? ? ? ? ? = sw l1 in fb gnd shdn c out r1 r2 d1 v out mic4681bm c in v in shutdown enable 5 8 4 3 2 1 figure 1a. adjustable regulator circuit v t u o * 1 r* 2 rc n i 1 d1 lc t u o v 8 . 1 k 1 0 . 3k 9 4 . 6 v 5 3 f 2 2 e l a d y a h s i v t 2 e 5 3 0 x 6 2 2 d 3 9 5 y k t t o h c s v 0 4 a 3 * * * . c n i , e d o i d - y a h s i v a 0 4 3 b r o r o t c u d n o c i m e s l a r e n e g 6 3 s s a 0 . 2 h 8 6 0 8 6 - b 3 p u s c i n o r t l i o c r o * * c m 0 8 6 - 7 2 1 h r d c a d i m u s v 0 1 f 0 2 2 e l a d y a h s i v 2 d 0 1 0 0 x 7 2 2 d 4 9 5 v 5 . 2 k 1 0 . 3k 4 9 . 2 v 3 . 3 k 1 0 . 3k 8 7 . 1 v 0 . 5 k 1 0 . 36 7 9 ? v 0 . 6 k 1 0 . 37 8 7 ? % 1 s r o t s i s e r l l a * s n o i t a c i l p p a i f r w o l r o f s c i t e n g a m d e d l e i h s * * 0 0 8 4 - 6 4 4 ) 5 0 8 ( . c n i , e d o i d - y a h s i v * * * figure 1b. recommended components for common ouput voltages
mic4681 micrel mic4681 12 april 2005 ja jc ca am bient printed circuit board ground plane heat sink area sop-8 figure 2. power sop-8 cross section thermal considerations the mic4681 superswitcher features the power-sop-8. this package has a standard 8-lead small-outline package profile, but with much higher power dissipation than a stan- dard sop-8. micrel's mic4681 superswitcher family are the first dc-to-dc converters to take full advantage of this package. the reason that the power sop-8 has higher power dissipa- tion (lower thermal resistance) is that pins 5 through 8 and the die-attach paddle are a single piece of metal. the die is attached to the paddle with thermally conductive adhesive. this provides a low thermal resistance path from the junction of the die to the ground pins. this design significantly im- proves package power dissipation by allowing excellent heat transfer through the ground leads to the printed circuit board. one limitation of the maximum output current on any mic4681 design is the junction-to-ambient thermal resistance ( ja ) of the design (package and ground plane). examining ja in more detail: ja = ( jc + ca ) where: jc = junction-to-case thermal resistance ca = case-to-ambient thermal resistance jc is a relatively constant 20 c/w for a power sop-8. ca is dependent on layout and is primarily governed by the connection of pins 5 though 8 to the ground plane. the purpose of the ground plane is to function as a heat sink. ja is ideally 63 c/w, but will vary depending on the size of the ground plane to which the power sop-8 is attached. determining ground-plane heat-sink area there are two methods of determining the minimum ground plane area required by the mic4681. quick method make sure that mic4681 pins 5 though 8 are connected to a ground plane with a minimum area of 6cm 2 . this ground plane should be as close to the mic4681 as possible. the area may be distributed in any shape around the package or on any pcb layer as long as there is good thermal contact to pins 5 though 8 . this ground plane area is more than sufficient for most designs. minimum copper/maximum current method using figure 3, for a given input voltage range, determine the minimum ground-plane heat-sink area required for the application s maximum continuous output current. figure 3 assumes a constant die temperature of 75 c above ambient. 0 0.5 1.0 1.5 0 5 10 15 20 25 continuous output current (a) area (cm 2 ) 12v 8v v in = 30v 24v t a = 50 c figure 3. output current vs. ground plane area when designing with the mic4681, it is a good practice to connect pins 5 through 8 to the largest ground plane that is practical for the specific design. checking the maximum junction temperature: for this example, with an output power (p out ) of 5w, (5v output at 1a maximum with v in = 12v) and 65 c maximum ambient temperature, what is the maximum junction tem- perature? referring to the typical characteristics: 5v output effi- ciency graph, read the efficiency ( ) for 1a output current at v in = 12v or perform you own measurement. = 79% the efficiency is used to determine how much of the output power (p out ) is dissipated in the regulator circuit (p d ). p= p p d out out ? p= 5w 0.79 5w d ? p d = 1.33w a worst-case rule of thumb is to assume that 80% of the total output power dissipation is in the mic4681 (p d(ic) ) and 20% is in the diode-inductor-capacitor circuit. p d(ic) = 0.8 p d p d(ic) = 0.8 1.33w p d(ic) = 1.064w calculate the worst-case junction temperature: t j = p d(ic) jc + (t c t a ) + t a(max) where: t j = mic4681 junction temperature p d(ic) = mic4681 power dissipation jc = junction-to-case thermal resistance. the jc for the mic4681 s power-sop-8 is approximately 20 c/w. t c = pin temperature measurement taken at the entry point of pins 6 or 7
april 2005 13 mic4681 mic4681 micrel load sw l1 68 h in fb gnd shdn c out r1 r2 d1 v out mic4681bm gnd c in v in +4v to +30v power sop-8 5678 4 3 2 1 figure 5. critical traces for layout sw l1 68 h in fb gnd shdn d1 b340a or ss36 j2 v out 1a j4 gnd u1 mic4681bm c2 0.1 f 50v c1 22 f 35v j1 v in 4v to +30v j3 gnd sop-8 5 8 4 3 2 1 jp1 on off c4 220 f 10v c3* 1800pf / 50v optional c5 0.1 f 50v r1 3.01k r2 6.49k jp2a 1.8v r6 optional r3 2.94k r4 1.78k r5 976 ? jp2b 2.5v jp2c 3.3v jp2d 5.0v 1 2 3 4 5 6 7 8 * c3 can be used to provide additional stability and improved transient response. figure 6a. evaluation board schematic diagram t a = ambient temperature t a(max) = maximum ambient operating temperature for the specific design. calculating the maximum junction temperature given a maximum ambient temperature of 65 c: t j = 1.064 20 c/w + (45 c 25 c) + 65 c t j = 106.3 c this value is within the allowable maximum operating junc- tion temperature of 125 c as listed in operating ratings. typical thermal shutdown is 160 c and is listed in electrical characteristics. layout considerations layout is very important when designing any switching regu- lator. rapidly changing currents through the printed circuit board traces and stray inductance can generate voltage transients which can cause problems. to minimize stray inductance and ground loops, keep trace lengths, indicated by the heavy lines in figure 5, as short as possible. for example, keep d1 close to pin 3 and pins 5 through 8, keep l1 away from sensitive node fb, and keep c in close to pin 2 and pins 5 though 8. see applications information: thermal considerations for ground plane lay- out. the feedback pin should be kept as far way from the switching elements (usually l1 and d1) as possible. a circuit with sample layouts are provided. see figures 6a though 6e. gerber files are available upon request.
mic4681 micrel mic4681 14 april 2005 printed circuit board layouts figure 6b. top-side silk screen figure 6c. top-side copper figure 6d. bottom-side silk screen figure 6e. bottom-side copper abbreviated bill of material (critical components) reference part number manufacturer description qty c1 593d226x035e2t vishay dale 1 22 f / 35v 1 c4 594d227x0010d2 vishay dale 1 220 f / 10v 1 c2,c5 vj0805y104kxxmb vishay dale 1 0.1 / 50v 1 d1 340a diodes inc. 2 schottky diode 3a, 40v 1 l1 cdrh127-680mc sumida 3 68 h, i sat 2.1a, shielded 1 u1 mic4681bm micrel semiconductor 4 200khz super switcher soic 8 pin 1 1 vishay dale, inc., tel: 1 877-847-4291, http://www.vishay.com 2 diodes inc, tel: (805) 446-4800, http://www.diodes.com 3 sumida, tel: (408) 982-9960, http://www.sumida.com 4 micrel, tel: (408) 944-0800, http://www.micrel.com
april 2005 15 mic4681 mic4681 micrel applications circuits* for continuously updated circuits using the mic4681, see application hint 37 at www.micrel.com. sw l1 100 h in fb gnd shdn c3 220 f 10v r2 3.01k r3 976 ? d2 1n4148 r4 16.2k r5 221k r6 10k u2 r1 0.100 ? c4 10nf d1 mmbr140lt3 d3 1n4148 c5 220nf j2 5v 2% 800ma 5% j4 gnd u1 mic4681 c2 100nf c1 22 f 35v j1 +30v max. j3 gnd sop-8 5 8 4 3 2 1 s1 nkk g12ap on off lm4041dim3-1.2 u3 mic6211bm5 4 5 2 3 r7 4.99k to cellular telephone figure 7. constant current and constant voltage battery charger sw l1 33 h in fb gnd shdn c1 68 f 20v d1 es1b 1a 100v j3 gnd u1 mic4681 j4 12v/150ma c2 0.1 f c4 68 f 20v c5 33 f 35v j1 +12v j2 gnd sop-8 5 8 4 3 2 1 r1 8.87k r2 1k c3 0.022 f 50v figure 8. +12v to 12v/150ma buck-boost converter sw l1* 50 h in fb gnd shdn c2 470 f 6.3v c3 470 f 6.3v r7 3.01k 1% c5 0.1 f d2 1n4148 r5 16k 1% r6 16k 1% c4 1000pf r2 20m ? r3 1k 1% r4 1k 1% r8 1.78k 1% d1 5a r1 1k 1% 3.3v/5a gnd u1 mic4681 u2 mic4417bm4 si4425dy c1 100 f 20v c6 0.1 f 50v +4.5v to +17v shutdown enable sop-8 5 8 4 3 2 1 * i sat = 7.5a u3 mic6211bm5 q1 figure 9. 5v to 3.3v/5a power supply * see application hint 37 at www.micrel.com for bills of material.
mic4681 micrel mic4681 16 april 2005 package information 45 0 8 0.244 (6.20) 0.228 (5.79) 0.197 (5.0) 0.189 (4.8) seating plane 0.026 (0.65) max ) 0.010 (0.25) 0.007 (0.18) 0.064 (1.63) 0.045 (1.14) 0.0098 (0.249) 0.0040 (0.102) 0.020 (0.51) 0.013 (0.33) 0.157 (3.99) 0.150 (3.81) 0.050 (1.27) typ pin 1 dimensions: inches (mm) 0.050 (1.27) 0.016 (0.40) 8-lead sop (m) micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 web http://www.micrel.com the information furnished by micrel in this data sheet is believed to be accurate and reliable. however, no responsibility is a ssumed by micrel for its use. micrel reserves the right to change circuitry and specifications at any time without notification to the customer. micrel products are not designed or authorized for use as components in life support appliances, devices or systems where malfu nction of a product can reasonably be expected to result in personal injury. life support devices or systems are devices or systems that (a) are intend ed for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant inj ury to the user. a purchaser s use or sale of micrel products for use in life support appliances, devices or systems is at purchaser s own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2005 micrel, incorporated.

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