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mic3205 high-brightness led driver controller with fixed-frequency hysteretic control mlf and micro leadframe are registered trademarks of amkor technology, inc. micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 (408) 944-0800 ? fax + 1 (408) 474-1000 ? http://www.micrel.com general description the mic3205 is a hysteretic , step-down, high-brightness led (hb led) driver with a patent pending frequency regulation scheme that mainta ins a constant operating frequency over input voltage range. it provides an ideal solution for interior/exterio r lighting, architectural and ambient lighting, led bulbs, and other general illumination applications. the mic3205 is well suited for lighting applications requiring a wide input voltage range. the hysteretic control provides good supply rejection and fast response during load transients and pwm dimming. the high-side current sensing and on-chip current-se nse amplifier deliver led current with ? 5% accuracy. an exter nal high-side current- sense resistor is used to set the output current. the mic3205 offers a dedicated pwm input (dim) which enables a wide range of pulsed dimming. a high-frequency switching operation up to 1.5mhz allows the use of smaller external components minimizing space and cost. the mic3205 operates over a junction temperature from ?40c to +125c and is available in a 10-pin 3mm x 3mm mlf ? package. data sheets and support documentation are available on micrel?s web site at: www.micrel.com . features ? 4.5v to 40v input voltage range ? fixed operating frequency over input voltage range ? high efficiency ( ? 90%) ? ? 5% led current accuracy ? high-side current sense ? dedicated dimming control input ? hysteretic control (no compensation!) ? up to 1.5mhz switching frequency ? adjustable constant led current ? over-temperature protection ? ?40? c to ? 125 ? c junction temperature range applications ? architectural, industrial, and ambient lighting ? led bulbs ? indicators and emergency lighting ? street lighting ? channel letters ? 12v lighting systems (mr-16 bulbs, under-cabinet lighting, garden/pathway lighting) _________________________________________________________________________________________________________________________ typical application 0.0 0.5 1.0 1.5 2.0 0 9 18 27 36 45 normalized frequency input voltage (v) normalized switching frequency vs. input voltage 1 led l = 22h i led = 1a r cs = 0.2 ? 4 led l = 47h 6 led l = 68h 10 led l = 33h mic3205 buck led driver october 2012 m9999-102312-a
micrel, inc. mic3205 october 2012 2 m9999-102312-a ordering information part number junction temperature range package (1) MIC3205YML ? 40c to ? 125c 10-pin 3mm x 3mm mlf note: 1. mlf is a green rohs-compliant package. lead finish is nipdau. mold compound is halogen free. pin configuration 10-pin 3mm x 3mm mlf (ml) top view pin description pin number pin name pin function 1 vcc voltage regulator output. the vcc pin is the output of a linear regul ator powered from vin, which supplies power to the internal circ uitry. a 4.7f ceramic capacitor is recommended for bypassing. place it as close as possible to the vcc and agnd pins. do not connect to an external load. 2 cs current sense input. negative input to the curr ent sense comparator. connect an external sense resistor to set the led current. connect the current sense resistor as close as possible to the chip. 3 vin input power supply. vin is the input supply pin to the internal circuitry. due to high frequency switching noise, a 10f ceramic capacitor is recommended for bypassing and should be placed as close as possible to the vin and pgnd pins. see ? pcb layout guidelines .? 4 vins vin sense. positive input to the current sense com parator. connect as close as possible to the current sense resistor. 5 agnd analog ground. ground for all internal low-power circuitry. 6 en enable input. logic high (greater than 2v) powers up t he regulator. a logic low (less than 0.4v) powers down the regulator and reduces the supply current of the device to less than 2a. a logic low pulls down the drv pin turning off the external mosfet. do not drive the en pin above vin. do not leave floating. 7 dim pwm dimming input. a pwm input can be used to contro l the brightness of the led. logic high (greater than 2v) enables the output. logic lo w (less than 0.4v) disables the out put regardless of the en state. do not drive the dim pin above vin. do not leave floating. 8 ctimer timer capacitor. a capacitor is required from ctim er to ground sets the target switching frequency using the equation c timer =2.22*10 -4 / f sw 9 pgnd power ground. ground for the power mosfet gate driver. the current loop for the power ground should be as small as possible and separ ate from the analog ground loop. see ? pcb layout recomme ndations .? 10 drv gate drive output. connect to the gate of an extern al n-channel mosfet. the drain of the external mosfet connects directly to the inductor and prov ides the switching current necessary to operate in hysteretic mode. ep epad exposed pad. must be connected to a gnd plane for best thermal performance.
micrel, inc. mic3205 october 2012 3 m9999-102312-a absolute maximum ratings (1) v in to pgnd .................................................. ? 0.3v to ? 42v v ins to pgnd......................................... ? 0.3v to (v in +0.3v) v cc to pgnd ................................................ ? 0.3v to ? 6.0v cs to pgnd ........................................ ? 0.3v to (v in ? 0.3v) en to agnd ........................................ ? 0.3v to (v in ? 0.3v) dim to agnd ...................................... ? 0.3v to (v in ? 0.3v) ctimer to agnd .............................. ? 0.3v to (v cc ? 0.3v) drv to pgnd .................................... ? 0.3v to (v cc ? 0.3v) pgnd to agnd .......................................... ? 0.3v to ? 0.3v junction temperature ................................................ 150 ? c storage temperat ure range .................... ? 60c to ? 150 ? c lead temperature (sol dering, 10s ec) ....................... 260 ? c esd ratings (3) hbm ...................................................................... 1.5kv mm .........................................................................200v operating ratings (2) supply voltage (v in ).......................................... 4.5v to 40v enable voltage (v en) .............................................. 0v to v in dimming voltage (v dim ) ................................................................. 0v to v in junction temperature (t j ) ........................ ? 40 ? c to ? 125 ? c junction thermal resistance 10-pin 3x3 mlf ( ? ja ).......................................60.7 ? c/w 10-pin 3x3 mlf ( ? jc ).......................................28.7 ? c/w electrical characteristics (4) v in = v en = v dim = 12v; c vcc = 4.7f; t j = 25 ?c; bold values indicate ? 40?c ? t j ? ? 125 ? c, unless noted. symbol parameter condition min. typ. max. units input supply v in input voltage range (v in ) 4.5 40 v i s supply current drv = open 1.3 3 ma i sd shutdown current v en = 0v 2 a uvlo v in uvlo threshold v in rising 3.2 4 4.5 v uvlo hys v in uvlo hysteresis 600 mv vcc supply v cc v cc output voltage v in = 12v, i cc = 5ma 4.5 5 5.5 v current sense 190 200 210 mv ? v cs average current sense threshold ? v cs =v ins ? v cs 188 200 212 mv v cs rising 50 ns ? t cs current sense response time v cs falling 70 ns i cs cs input current v in = v cs 0.5 10 a ? v hys sense voltage hysteresis (5) v in =12v, v led =3v, l=47h, f sw =250khz, v d = 0.7v, i led = 1a 46 mv frequency i timer ctimer pull-up current 66 a v ctref ctimer threshold 1.189 v (4*i timer )/ v ctref frequency coefficient (6) 1.776 10 -4 2.22 10 -4 2.664 10 -4 a/v
micrel, inc. mic3205 october 2012 4 m9999-102312-a electrical characteristics (4) (continued) v in = v en = v dim = 12v; c vcc = 4.7f; t j = 25 ?c; bold values indicate ? 40?c ? t j ? ? 125 ? c, unless noted. symbol parameter condition min. typ. max. units enable input en hi en logic level high 2.0 v en lo en logic level low 0.4 v v en = 12v 20 60 a i en en bias current v en = 0v 1 a t start start-up time from en pin going high to drv going high 65 s dimming input dim hi dim logic level high 2.0 v dim lo dim logic level low 0.4 v v dim = 12v 20 50 a i dim dim bias current v dim = 0v 1 a t dim dim delay time from dim pin going high to drv going high 450 ns f dim maximum dimming frequency % of switching frequency 2 % external fet driver pull-up, i source = 10ma 4 ? r on drv on-resistance pull-down, i sink = -10ma 1.5 ? rise time, c load = 1000pf 13 ns t drv drv transition time fall time, c load = 1000pf 7 ns thermal protection t lim overtemperature shutdown t j rising 160 ?c t limhys overtemperature shutdown hysteresis 20 ?c notes: 1. exceeding the absolute maximu m rating can damage the device. 2. the device is not guaranteed to function outside its operating rating. 3. devices are esd sensitive. handling prec autions are recommended. human body model, 1.5k ? in series with 100pf. 4. specification for packaged product only. 5. see ? sense voltage hysteresis range ? in the ? application information ? section. 6. see ? frequency of operation ? in t he ? application information ? section.
micrel, inc. mic3205 october 2012 5 m9999-102312-a typical characteristics 60 65 70 75 80 85 90 95 100 0 9 18 27 36 45 efficiency (%) input voltage (v) efficiency (i led = 1a) vs. input voltage 1 led l = 22h 4 led l = 47h 6 led l = 68h 10 led l = 33h 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 9 18 27 36 45 v in supply current (ma) input voltage (v) v in supply current vs. input voltage i led = 0a t a = 25c 0.0 0.2 0.4 0.6 0.8 1.0 0 9 18 27 36 45 v in shutdown current (a) input voltage (v) v in shutdown current vs. input voltage v en = 0v i led = 0a t a = 25c 4.0 4.5 5.0 5.5 6.0 0 9 18 27 36 45 v cc output voltage (v) input voltage (v) v cc output voltage vs. input voltage t a = 25c i led = 1a 0.0 0.5 1.0 1.5 2.0 0 9 18 27 36 45 normalized frequency input voltage (v) normalized switching frequency vs. input voltage 1 led l = 22h i led = 1a r cs = 0.2 ? 4 led l = 47h 6 led l = 68h 10 led l = 33h 0.90 0.95 1.00 1.05 1.10 0 9 18 27 36 45 i led output current (a) input voltage (v) i led output current vs. input voltage t a = 25c r cs = 0.2 ? 1 led l = 22h 4 led l = 47h 6 led l = 68h 60 62 64 66 68 70 0 9 18 27 36 45 ctimer current (a) input voltage (v) ctimer current vs. input voltage v en = v in t a = 25c 0.0 0.3 0.6 0.9 1.2 1.5 0 9 18 27 36 45 enable threshold (v) input voltage (v) enable threshold vs. input voltage hyst i led = 1a t a = 25c rising falling 0 20 40 60 80 100 0 9 18 27 36 45 enable bias current (a) input voltage (v) enable bias current vs. input voltage v en = v in t a = 25c i led = 0a
micrel, inc. mic3205 october 2012 6 m9999-102312-a typical characteristics (continued) 0 20 40 60 80 100 0 9 18 27 36 45 enable bias current (a) enable voltage (v) enable bias current vs. enable voltage v en v in t a = 25c i led = 0a v in = 42v 0 40 80 120 160 200 0 9 18 27 36 45 thermal shutdown (c) input voltage (v) thermal shutdown vs. input voltage hyst i led = 1a rising falling 1.0 1.2 1.4 1.6 1.8 2.0 -50 -25 0 25 50 75 100 125 supply current (ma) temperature (c) v in supply current vs. temperature v in = 12v i led = 0a 0.0 0.4 0.8 1.2 1.6 2.0 -50-25 0 255075100125 supply current (a) temperature (c) v in shutdown current vs. temperature v in = 12v i led = 0a v en = 0v 0.98 0.99 1.00 1.01 1.02 1.03 -50 -25 0 25 50 75 100 125 i led output current (a) temperature (c) i led output current vs. temperature v in = 12v v led = 3.5v r cs = 0.2 ? 430 450 470 490 510 530 -50-25 0 255075100125 frequency (khz) temperature (c) switching frequency vs. temperature v in = 12v v led = 3.5v l = 22h c t = 470pf r cs = 0.2 ? 4.0 4.5 5.0 5.5 6.0 -50 -25 0 25 50 75 100 125 v cc (v) temperature (c) v cc vs. temperature v in = 12v i led = 1a 0.0 0.4 0.8 1.2 1.6 -50 -25 0 25 50 75 100 125 enable threshold (v) temperature (c) enable threshold vs. temperature v in = 12v i led = 1a rising falling hyst 10 15 20 25 30 -50 -25 0 25 50 75 100 125 en bias current (a) temperature (c) enable bias current vs. temperature v in = 12v i led = 0a v en = 12v
micrel, inc. mic3205 october 2012 7 m9999-102312-a typical characteristics (continued) 0 1 2 3 4 5 -50 -25 0 25 50 75 100 125 v in uvlo threshold (v) temperature (c) v in uvlo threshold vs. temperature rising falling hyst
micrel, inc. mic3205 functional characteristics october 2012 8 m9999-102312-a
micrel, inc. mic3205 functional characteristics (continued) october 2012 9 m9999-102312-a
micrel, inc. mic3205 october 2012 10 m9999-102312-a functional diagram figure 1. mic3205 block diagram
micrel, inc. mic3205 october 2012 11 m9999-102312-a functional description the mic3205 is a hysteretic step-down driver that regulates the led current with a patent pending frequency regulation scheme. this scheme maintains a fixed operating frequency over a wide input voltage range. theory of operation the device operates from a 4.5v to 40v input mosfet voltage. at turn-on, after the vin input voltage crosses 4.5v, the drv pin is pulled high to turn on an external mosfet. the inductor and series led current builds up linearly. this rising current results in a rising differential voltage across the current sense resistor (r cs ). when this differential voltage reaches an upper threshold, the drv pin is pulled low, the mosfet turns off, and the schottky diode takes over and returns the series leds and inductor current to v in . then, the current through the inductor and series leds starts to decrease. this decreasing current results in a decreasing differential voltage across r cs . when this differential voltage reaches a lower threshold, the drv pin is pulled high, the mosfet is turned on, and the cycle repeats. the average of the cs pin voltage is 200mv below v in voltage. this is the average current sense threshold ( ? v cs ). thus, the cs pin voltage switches about v in ? 200mv with a peak-to-peak hysteresis that is the product of the peak-to-peak inductor current times the current sense resistor (r cs ). the average led current is set by r cs , as explained in the ? application information ? se ction. mic3205 dynamically adjusts hysteresis to accommodate fixed-frequen cy operation. average frequency is programmed using an external capacitor connected to the ctimer pin, as explained in the ? frequency of operation ? sub section in the ? application information ? section. the internal frequency regulator dynamically adjusts the induct or current hysteresis every eight switching cycles to make the average switching frequency a constant. if the instantaneous frequency is higher than the programmed average value, the hysteresis is increased to lower the frequency and vice versa. in other hysteretic c ontrol systems, current sense hysteresis is constant and frequency can change with input voltage, inductor value, series leds voltage drop, or led current. however, with this patent pending frequency regulation scheme, the mic3205 changes inductor current hysteresis and keeps the frequency fixed even upon changing input voltage, inductor value, series leds voltage drop, or led current. the mic3205 has an on-board 5v regulator, which is for internal use only. connect a 4.7f capacitor on vcc pin to analog ground. the mic3205 has an en pin t hat gives the flexibility to enable and disable the output with logic high and low signals. the maximum en voltage is v in . figure 2. theory of operation led dimming the mic3205 led driver can c ontrol the brightness of the led string through the use of pulse width modulated (pwm) dimming. a dim pin is provided, which can turn on and off the leds if en is in an active-high state. this dim pin controls the brightness of the led by varying the duty cycle of dim pin from 1% to 99%. an input signal from dc up to 20khz can be applied to the dim pin (see ? typical application ?) to pulse the led string on and off. a logic si gnal can be applied on the dim pin for dimming, independent of input voltage (v in ). using pwm dimming signals above 120hz is recommended to avoid any recognizable flicker by the human eye. maximum allowable dimming frequency is 2% of operating frequency that is set by the external capacitor on the ctimer pin (see ? frequency of operation ? ). see ? functional characteristics ? on page 9 for pwm dimming waveforms. maximum dim voltage is v in . pwm dimming is the preferred way to dim an led to prevent color/wavelength shifting. color/wavelength shifting occurs with analog dimming. by using pwm dimming, the output current level remains constant during each dim pulse. the hysteretic buck converter switches only when the dim pin is high. when the dim pin is low, no led current flows and the drv pin is low turning the mosfet off.
micrel, inc. mic3205 october 2012 12 m9999-102312-a application information the internal block diagram of the mic3205 is shown in figure 1 . the mic3205 is composed of a current-sense comp arator, voltage reference, frequency regulator, 5v regulator, and mosfet driver. hysteretic mode control, also called bang-bang control, is a topology that does not use an error amplifier, instead using an error comparator. the frequency regulator dynamically adjusts hysteresis for the current sense comparator to regulate frequency. the inductor current is sensed by an external sense resistor (r cs ) and controlled within a hysteretic window. it is a simple control schem e with no oscillator and no loop compensation. the control scheme does not need loop compensation. this makes design easy, and avoids instability problems. transient response to load and line variation is very fast and depends only on propagation delay. this makes the control scheme very popular for certain applications. led current and r cs the main feature in mic3205 is that it controls the led current accurately within ? 5% of set current. choosing a high-side r cs resistor is helpful for setting constant led current regardless of wide input voltage range. the following equation and table 1 give the r cs value for required led current: led cs i 200mv r ? eq. 1 r cs ( ? ) i led (a) i 2 r (w) size (smd) 1.33 0.15 0.03 0603 0.56 0.35 0.07 0805 0.4 0.5 0.1 0805 0.28 0.7 0.137 0805 0.2 1.0 0.2 1206 0.13 1.5 0.3 1206 0.1 2.0 0.4 2010 0.08 2.5 0.5 2010 0.068 3.0 0.6 2010 table 1. r cs for led current frequency of operation the patent pending frequency regulation scheme allows for operating frequency to be programmed by an external capacitor from the ctimer pin to agnd. the frequency co-efficient (typically 2.22 10 -4 a/f) divided by the value of this external capacitor connected to the ctimer pin, gives the average frequency of operation, as seen in the following equation: t -4 sw c 1022.2 f ? ? eq. 2 the actual average frequency can vary depending on the variation of the frequency co-efficient and the parasitic board capacitances in parallel to the external capacitor c t . as shown in the electrical characteristics table, pa rt to part variation for the frequency co-efficient is 20% over temperature, from the target frequency co-efficient of 2.22 10 -4 . switching frequency selection is based on the trade-off between efficiency and system size. higher frequencies result in smaller, but less efficient, systems and vice versa. the operating frequency is in dependent of input voltage, inductor value, series leds voltage drop, or led current, as long as 40mv ? v hys ? 100mv is maintained as explained in the next sections. sense voltage hysteresis range the frequency regulation scheme requires that the hysteresis remain in a controlled window. components and operating conditions must be such that the hysteresis on the cs pin is between 40mv and 100mv. hysteresis less than 40mv or more than 100mv can result in loss of frequency regulation. after average led current (i led ) has been set by r cs and operating frequency has been set by external capacitor c t , the hysteresis v hys is calculated as follows: as seen in figure 2 , for the inductor, cs hys l r v i ? ?? eq. 3 where: i l = inductor ripple current v hys = hysteresis on cs pin for rising inductor current (mosfet is on): l_rise l r v il t ?? ? eq. 4 where: v l_rise = v in ? i led r cs ? v led v led is the total voltage drop of the led string v in is the input voltage r cs is the current sense resistor i led is the average led current
micrel, inc. mic3205 october 2012 13 m9999-102312-a t r is the mosfet on-time l is the inductor for falling inductor current (mosfet is off): l_fall l f v il t ?? ? eq. 5 where: v l_fall = v d + i led r cs ? v led v d is the freewheeling diode forward drop t f is the mosfet off-time operating frequency and time period are given by: t 1 f sw ? eq. 6 eq. 7 fr ttt ?? usi ng equations 3, 4, 5, 6, and 7: sw din cs led cs ledd led cs ledin hys f l ) v v( r) v ri (v) v- ri - (v v ??? ????? ? ?? eq. 8 the value of ? v hys calculated in this way must be between 40mv and 100mv to ensure frequency regulation. inductor according to the above equations, the inductor value can be calculated once average led current, operating frequency and an appropriate hysteresis ? v hys value have been chosen. thus, inductor l is given by: sw hys din cs led cs ledd led cs ledin f v ) v v( r) v ri (v) v- ri - (v l ???? ????? ? ? eq. 9 table 2 , table 3 , and table 4 give reference inductor values for an operating frequency of 400 khz, for a given led current, freewheeling diode forward drop, and number of leds. by selecting ? v hys in the 55mv to 75mv range, we get the following inductor values: r cs ( ? ) i led (a) v in (v) l (h) v hys (mv) 0.56 0.35 5 22 64.1 0.56 0.35 12 68 57.7 0.28 0.7 5 10 70.5 0.28 0.7 12 33 59.4 0.2 1.0 5 6.8 72.6 0.2 1.0 12 22 62.4 0.1 2.0 5 3.6 68.5 0.1 2.0 12 10 68.6 table 2. inductor for f sw = 400 khz, v d = 0.4v, 1 led r cs ( ? ) i led (a) v in (v) l (h) v hys (mv) 0.56 0.35 24 150 55.8 0.56 0.35 36 220 56.8 0.28 0.7 24 68 61.6 0.28 0.7 36 100 62.5 0.2 1.0 24 47 62.4 0.2 1.0 36 68 64.3 0.1 2.0 24 22 66.6 0.1 2.0 36 33 66.2 table 3. inductor for f sw = 400 khz, v d = 0.4v, 4 led r cs ( ? ) i led (a) v in (v) l (h) v hys (mv) 0.56 0.35 36 150 58.4 0.56 0.35 40 220 54.3 0.28 0.7 36 68 64.4 0.28 0.7 40 100 59.6 0.2 1.0 36 47 65.2 0.2 1.0 40 68 61.4 0.1 2.0 36 22 69.6 0.1 2.0 40 33 63.3 table 4. inductor for f sw = 400 khz, v d = 0.4v, 8 led given an inductor value, the size of the inductor can be determined by its rms and peak current rating. because leds are in series with the inductor, ledl ii ? eq. 10 from equations 1, 3, and 10: 200m v i i hys l l ? ? ? eq. 11
micrel, inc. mic3205 october 2012 14 m9999-102312-a with 40mv ? v hys 100mv: l 2 l 2 l)rms(l ii 12 1 ii ???? eq. 12 ) 400m v (1ii hys l l(pk) ? ?? eq. 13 where: i l is the average inductor current i l(pk) is the peak inductor current select an inductor with a saturation current rating at least 30% higher than the peak current. for space-sensitive applicati ons, smaller inductors with higher switching frequency could be used but regulator efficiency will be reduced. mosfet n-channel mosfet selection depends on the maximum input voltage, output led current, and switching frequency. the selected n-channel mosfet should have 30% margin on maximum voltage rating for high reliability requirements. the mosfet channel resistance (r dson ) is selected such that it helps to get t he required efficiency at the required led currents and meets the cost requirement. logic level mosfets are preferred as the drive voltage is limited to 5v. the mosfet power loss has to be calculated for proper operation. the power loss consists of conduction loss and switching loss. the conduction loss can be found by: in led led rms(fet) dson 2 rms(fet) loss(con) v v d di i r i p ? ?? ? ? the switching loss occurs during the mosfet turn-on and turn-off transition and can be found by: gate drv drv gd2gs drv sw ledin )tran(loss r v =i )q+q( i fiv = p where: r gate is total mosfet gate resistance; q gs2 and q gd can be found in a mosfet manufacturer data sheet. a gate resistor can be connected between the mosfet gate and the drv pin to slow down mosfet switching edges. a 2 ? resistor is usually sufficient. the total power loss is: )tran(loss )con(loss )tot(loss p+ p= p the mosfet junction temperature is given by: aja )tot(loss j t+r p=t t j must not exceed maximum junction temperature under any conditions. freewheeling diode the freewheeling diode should have a reverse voltage rating that is at least 20% higher than the maximum input supply voltage. the forward voltage drop should be small to get the lowest conduction dissipation for high efficiency. the forward current rating should be at least equal to the led current. schottky diodes with low forward voltage drop and fast reverse recovery are ideal choices and give the highest efficiency. the freewheeling diode average current (i d ) is given by: led d i)d1(i ? ? ? diode power dissipation (p d ) is given by: ddd ivp ? ? typically, higher current rating diodes have a lower v d and have better thermal performance, improving efficiency. input capacitor the ceramic input capacitor is selected by voltage rating and ripple current rating. a 10f ceramic capacitor is usually sufficient. select a voltage rating that is at least 30% larger than the maximum input voltage. led ripple current the led current is the same as inductor current ? i l . a ceramic capacitor should be placed across the series leds to pass the ripple current. a 4.7f capacitor is usually sufficient for most applications. voltage rating should be the same as the input capacitor.
micrel, inc. mic3205 october 2012 15 m9999-102312-a pcb layout guidelines note: to minimize emi and output noise, follow these layout recommendations. pcb layout is critical to achieve reliable, stable, and efficient performance. a ground plane is required to control emi and minimize the inductance in power, signal, and return paths. follow these guidelines to ensure proper operation of the mic3205. ic ? use thick traces to route the input and output power lines. ? keep signal and power grounds separate and connect them at only one location. input capacitor ? place the input capacitors on the same side of the board and as close to the ic as possible. ? keep both the vin and pgnd traces as short as possible. ? if the application requires vias to the ground plane, place them close to the input capacitor ground terminal, but not between the input capacitors and ic pins. ? use either x7r or x5r dielectric input capacitors. do not use y5v or z5u type capacitors. ? do not replace the ceramic input capacitor with any other type of capacitor. any type of capacitor can be placed in parallel with the ceramic input capacitor. ? if a tantalum input capacitor is placed in parallel with the ceramic input capacitor, it must be recom- mended for switching regulator applications and the operating voltage must be derated by 50%. ? in ?hot-plug? applications, place a tantalum or electrolytic bypass capacitor in parallel to the ceramic capacitor to limit the overvoltage spike seen on the input supply when power is suddenly applied. in this case, an additional tantalum or electrolytic bypass input capacitor of 22f or higher is required at the input power connection. inductor ? keep the inductor connection to the switch node (mosfet drain) short. ? do not route any digital lines underneath or close to the inductor. ? to minimize noise, place a ground plane underneath the inductor. led ripple current carrying capacitor ? place this ceramic capacitor as close to the leds as possible. ? use either x7r or x5r dielectric capacitors. do not use y5v or z5u type capacitors. mosfet ? to avoid trace inductance, place the n-channel mosfet as close as possible to the mic3205. ? provide sufficient copper area on mosfet ground to dissipate the heat. freewheeling diode ? place the schottky diode on the same side of the board as the ic and input capacitor. ? keep the connection from the schottky diode?s anode to the switching node as short as possible. ? keep the diode?s cathode connection to the r cs as short as possible. rc snubber ? if an rc snubber is needed, place the rc snubber on the same side of the board and as close to the schottky diode as possible. a 1.2 ? resistor in series with a 1nf capacitor is usually a good choice. r cs (current-sense resistor) ? vins pin and cs pin must be as close as possible to r cs. ? make a kelvin connection to the vins and cs pin, respectively, for current se nsing. for low values of ? v hys (around 40mv) the switching noise could cause faulty switching on the drv pin. if this occurs, place two 30 ? resistors and a 1nf capacitor, as shown in figure 3 , to filter out switching noise for low values of ? v hys . alternatively, as seen in equation 8, a smaller inductor value can be used to increase ? v hys and make the system more noise tolerant.
micrel, inc. mic3205 october 2012 16 m9999-102312-a for f sw = 400 khz c t = 550pf the actual frequency may vary as explained in ? frequency of operation ? in the ? application information ? section. 3. inductor selection from equation 9: sw hys din cs led cs ledd led cs ledin f v ) v v( r) v ri (v) v- ri - (v l ???? ????? ? ? given v supply = 24v rectified ac the peak voltage = 2 x v supply thus for mic3205, v in 34v v led = 3.5 x 4 = 14, v d = 0.4v figure 3. input filter for low values of v hys selec t ? v hys = 60mv thus, l = 70h trace routing recommendation keep the power traces as short and wide as possible. there is one current flowing loop during the mosfet on-time; the traces connect the input capacitor (c in ), r cs , the leds, the inductor, the mosfet, and back to c in . there is another current flowing loop during the mosfet off-time; the traces for this loop connect r cs , the led, the inductor, the freewheeling diode, and back to r cs . these two loop areas should kept as small as possible to minimize noise interference cho se l = 68h as closest available value. as a side note, for this example, l = 68h can be used even if v supply = 24v dc. this is because ? v hys calculates to around 44mv (with v in = v supply = 24v) which is acceptable. from equations 12 and 13: i l(pk) = 1.15a thus, we choose l = 68h with an rms saturation current of 1.5a or higher. keep all analog signal traces away from the switching node and its connecting traces. 4. mosfet selection for this example, v in = 34v, a 50v rating or greater n- channel mosfet is required. a high current rating mosfet is a good choice because it has lower r dson . design example a 60v, 12a mosfet wit h 10m ? r dson is a good choice. specifications: 5. capacitor selection f sw = 400 khz use a 10f/50v x7r type ceramic capacitor for the input cap acitor. v supply = 24v rectified ac i led = 1a use a 4.7f/50v x5r type ceramic capacitor for the led ripple current carrying capacitor connected across the series connection of 4 leds voltage drop per led = 3.5v number of leds = 4 schottky diode drop at 1a = 0.4v 6. freewheeling diode selection 1. current sense resistor from equation 1: led cs i 200mv r ? with v in = 34v, choose a 2a, 60v schottky diode with a forward drop voltage of 0.4v at 1a forward current. for i led = 1a r cs = 0.2? 2. switching frequency from equation 2: t -4 sw c 1022.2 f ? ?
micrel, inc. mic3205 october 2012 17 m9999-102312-a evaluation board schematic
micrel, inc. mic3205 october 2012 18 m9999-102312-a bill of materials item part number manufacturer description qty. 12105c475kaz2a avx (1) grm32er71h475ka88l murata (2) c1, c2,c3,c4,c11 cga6p3x7r1h475k tdk (3) 4.7f/50v, ceramic capacitor, x7r, size 1210 5 grm21br71h105ka12l murata c5 cga4j3x7r1h105k tdk 1f/50v, ceramic capacitor, x7r, size 0805 1 06035c471k4t2a avx grm188r71h471ka01d murata c10 c1608x7r1h471k tdk 470pf/50v, ceramic capacitor, x7r, size 0603 1 06036d475kat2a avx grm188r60j475ke19j murata c8 cga3e1x5r0j475k tdk 4.7f/6.3v, ceramic capaci tor, x5r, size 0603 1 06035c102kat2a avx grm188r71h102ka01d murata c7,c9 c1608x7r1h102k tdk 1nf/50v, ceramic capacitor, x7r, size 0603 2 sk36-tp mcc (4) sk36 fairchild (5) d1 sk36-7-f diodes, inc. (6) 60v, 3a, smc, schottky diode 1 l1 slf10145t-220m1r9-pf tdk 22h, 2.1a, 0.0591 ? , smt, power inductor 1 m1 fds5672 fairchild mosfet, n-ch, 60v, 12a, so-8 1 r cs csr1206fkr200 stackpole electronics, inc. (7) 0.2 ? resistor, 1/2w, 1%, size 1206 1 r5, r8 crcw0603100kfkea vishay dale (8) 100k ? resistor, 1%, size 0603 2 r2, r3 crcw060330r0fkea vishay dale 30? resistor, 1%, size 0603 2 r1, r9 crcw06032r00fkea vishay dale 2 ? resistor, 1%, size 0603 2 r4 crcw060310k0fkea vishay dale 10k ? resistor, 1%, size 0603 1 r6 crcw060351r0fkea vishay dale 51? resistor, 1%, size 0603 1 r7 crcw06030000z0ea vishay dale 0 ? resistor, size 0603 1 u1 MIC3205YML micrel, inc. (9) high-brightness led driver controller with fixed frequency hysteretic control 1 notes: 1. avx: www.avx.com . 2. murata: www.murata.com . 3. tdk: www.tdk.com . 4. mcc: www.mccsemi.com . 5. fairchild: www.fairchildsemi.com . 6. diodes inc.: www.diodes.com . 7. stackpole electronics: www.seielect.com . 8. vishay dale: www.vishay.com . 9. micrel, inc.: www.micrel.com .
micrel, inc. mic3205 october 2012 19 m9999-102312-a pcb layout recommendations top assembly top layer
micrel, inc. mic3205 october 2012 20 m9999-102312-a pcb layout recommendations (continued) bottom layer
micrel, inc. mic3205 october 2012 21 m9999-102312-a package information 10-pin 3mm x 3mm mlf (ml)
micrel, inc. mic3205 october 2012 22 m9999-102312-a recommended landing pattern 10-pin 3mm x 3mm mlf (ml) land pattern
micrel, inc. mic3205 october 2012 23 m9999-102312-a 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 micrel makes no representations or warranties with respect to t he accuracy or completeness of the information furnished in this data sheet. this information is not intended as a warranty and micrel does not assume responsibility for it s use. micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. no license, whether expre ss, implied, arising by estoppel or other wise, to any intellectual property rights is granted by this document. except as provided in micrel?s terms and conditions of sale for such products, mi crel assumes no liability whatsoever, and micrel disclaims any express or implied warranty relating to the sale and/or use of micrel products including l iability or warranties relating to fitness for a particular purpose, merchantability, or infringement of an y patent, copyright or other intellectual p roperty right. micrel products are not designed or authori zed for use as components in life support app liances, devices or systems where malfu nction of a product can reasonably be expected to result in pers onal injury. life support devices or system s are devices or systems that (a) are in tended 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 significan t injury to the user. a purchaser?s use or sale of micrel produc ts for use in life support app liances, devices or systems is a purchaser?s own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2012 micrel, incorporated.

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