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mic2871 1.2a high - brightness led flash driver with single - wire serial interface micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 (408) 944 - 0800 ? fax + 1 (408) 474 - 1000 ? http://www.micrel.com may 29, 2013 052913 - 1.0 (while in progress) revision 1.0 (final document) general description the mic2871 is a high - current, high - efficiency flash led driver. the led driver current is generated by an integrated inductive boost converter with a 2mhz switching frequen cy which allows the use of very small inductor and output capac itor. these features make the mic2871 an ideal solution for high - resolution camera phone led flash light driver applications. the mic2871 operates in either flash or torch modes that can be controlled through the sing l e - wire serial interface and/or externa l control pins. default flash and torch brightness can be adjusted via an external resistor. a robust single - wire serial interface allows simple control by the host processor to support typical camera functions such as auto - focus, white balance, and image capture (flash mode). the mic2871 is available in a 14 - pin 3mm 2mm ldfn package with a junction temperature range of ? 40 c to +125 c. datasheets and support documentation are available on micrel?s web site at : www.mi crel.com . features ? up to 1.2 a f lash led driving current ? highly - efficient, synchronous boost driver (up to 94%) ? 5% led current accuracy ? control through single - wire serial interface or external control pins ? input voltage range: 2.7v to 5.5v ? true load dis connect ? configurable safety time - out protection ? output over voltage protection (ovp) ? led short detection and protection. ? 1a shutdown current ? available in 14 - pin 3mm 2mm ldfn package applications ? camera phones/mobile handsets ? cell phones/smartphones ? led light for image capture/auto focus/white balance ? handset video light (torch light) ? digital cameras ? portable applications typical application
micrel, inc. mic2871 may 29, 2013 2 0529 13 - 1.0 (while in progress) revision 1.0 (final document) ordering information part number marking code temperature range package ( 1 ) lead finish MIC2871YMK 2871 ? 40 c to +125c 14- pin 3mm 2mm ldfn pb - free note: 1. package is a green rohs - compliant package. lead finish is pb - free. mold compound is halogen free . pin configuration 14- pin 3mm 2mm ldfn (mk) (top view) pin d escription pin number pin name pin function 1 agnd1 analog g round. led current return path. 2 dc single - wire serial interface control input. 3 led led current sink p in. connect the led anode to out and cathode to this pin. 4 fen1 flash mode enable pin. toggling fen1 from low to high enables mic2871 in to the flash mode . fen1 is logic - or with fen2. if this pin is left floating, it is pulled- down internally by a built - in 1a current source when the device is enabled. 5 agnd2 analog g round. reference groun d of the frset pin. 6 vin supply input pin. connect a low - esr ceramic capacitor of at least 2.2f to agnd2 . 7 pgnd1 power ground. inductor current return path. 8 out boost converter output p in . this is connected to the anode of the led. connect a low es r ceramic capacitor of at least 4.7f to pgnd 1 . 9, 12 nc no c onnect. connect this pin to agnd or leave it floating. 10 sw inductor connection p in. it is connected to the internal power mosfets. 11 fen2 additional flash mode enable p in. fen2 is logic - or with fen1. if this pin is left floating, it is pulled - down internally by a built - in 1a current source when the device is enabled. 13 pgnd2 power ground. 14 frset flash mode current - level programming p in. connect a resistor from this pin to agnd 2 to set the maximum current in the flash m ode. this pin may be grounded if the default flash m ode current (1a) is desired. this pin cannot be left floating and the recommended resistance range is from 17k ? to 60k ? . ep epad exposed heat sink p ad. connect to ground for best thermal performance.
micrel, inc. mic2871 may 29, 2013 3 0529 13 - 1.0 (while in progress) revision 1.0 (final document) absolute maximum ratings ( 2 ) input voltage (v in ) ........................................ ? 0.3v to + 6.0 v general i/o voltage (v fen 1 , v fen2 ) .................... ? 0.3v to v in v out and v led voltage .................................. ? 0.3v to + 6.0 v sing l e - wire i/o voltage (v dc ) ........................... ? 0.3v to v in v frset voltage ................................................... ? 0.3v to v in v sw voltage .................................................. ? 0.3v to + 6.0 v lea d temperature (soldering, 10s ) .......................... +260c junction temperature ................................... 0c to +150c storage temperature (t s ) ......................... ? 40c to +150c esd ratin g ( 5) ............................... 2kv, hbm and 200v, mm operating ratings ( 3 ) input voltage (v in ) .......................................... 2.7v to + 5.5 v enable input voltage (v fen 1 , v fen2 ) ....................... 0v to v in sing l e - wire i/o voltage (v dc ) ................................ 0v to v in junction temperature (t j ) ........................ ? 40 c to +125 c power dissipation (p d ) ........................... internally limited (4) package thermal resistance 3mm 2mm ldfn ( ja ) (4) ............................ 65.83c/w 3mm 2mm ldf n ( jc ) (4) .............................................. 38.9c/w electrical characteristics ( 6 ) v in = 3.6v, l = 1h, c out = 4.7f , r frset = 20.5k ? , i out = 100ma, t a = 25c, bold values indicate - 40c ? t j ? +125c, unless otherwise noted. . symbol parameter condition min . typ . max . units power supply v in supply voltage range 2.7 5.5 v v sta rt start -u p voltage 2. 6 5 2.95 v v uvlo uvlo threshold (falling) 2.30 2.5 v i stb standby current v dc = high, boost regulator and led current driver both off. 230 a i sd shutdown current v dc = 0v 1 2 a v ovp overv oltage protection (ovp) thresho ld 5.2 5.37 5.55 v ovp hysteresis 60 mv ovp blanking t ime 23 s d max maximum duty cycle 82 86 90 % i sw switch current limit v in = v out = 2.7v 3.5 4. 5 5.5 a d min minimum duty cycle 4 6.4 9 % pmos switch on - resistance i sw = 100ma 100 m ? nmos i sw = 100ma i sw switch leakage current v dc = 0v, v sw = 5.5 v 0.01 1 a f sw oscillator frequency 2 mhz oscillator frequency variation ? 10 10 % notes: 2. exceeding the absolute maximum rating may damage the device. 3. the device is not guar anteed to function outside its operating rating. 4. the maximum allowable power dissipation of any t a (ambient temperature) is p d(max) = (t j(max) ? t a ) / ja . exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. 5. devices are esd sensitive. handling precautions recommended. human body model, 1.5k in series with 100pf. 6. specification for packaged product only.
micrel, inc. mic2871 may 29, 2013 4 0529 13 - 1.0 (while in progress) revision 1.0 (final document) electrical characteristics ( 6 ) v in = 3.6v, l = 1h, c out = 4.7f, r frset = 20.5k ? , i out = 100ma, t a = 25c, bold values indicate - 40c ? t j ? + 125c, unless otherwise noted. symbol parameter conditions min . typ . max . units t sd overt emperature shutdown threshold 155 c overt emperature shutdown hysteresis 15 c t to safety timeout shutdown default timer setting 1.25 s i to safety timer current threshold default current threshold setting 250 ma safety timer current resolution 50 ma safety timer current - threshold accuracy 25 ma v lbvd low - battery voltage detection threshold default lbvd threshold setting 3.6 v low - battery voltage detection threshold accuracy 50 mv v short led short - circuit detection voltage threshold v out ? v led 1.7 v i test led short - circuit detection test current 1 2 3 ma current sink channels channel current accuracy 3.5v < v in <4.2v, i led = 1a ? 5 5 % v led current sink voltage dropout boost regulator on, i led = 1a 160 mv fen1, fen2 control pins fen1/fen2 threshold voltage flash on 1.5 v flash off 0.4 fen1/fen2 pull - down current fen1 = fen2 = 5.5v 1 5 a electrical charact eristics ? singl e -w ire interface (guaranteed by design) v in = 3.6v, l = 1h, c out = 4.7f, i out = 1 0 0ma, t a = 25c, bold values indicate -40c t j +125 c, unless otherwise noted. symbol parameter conditions min . typ . max . units low - level input voltage 0.4 v high - level input voltage 1.5 v dc pull -d own current dc = 5.5v 2.5 5 a t on on time 0.1 72 s t off off time 0.1 72 s t lat latch time 97 324 s t end end time 405 s
micrel, inc. mic2871 may 29, 2013 5 0529 13 - 1.0 (while in progress) revision 1.0 (final document) typical characteristics shutdown current vs. temperature 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -40 -20 0 20 40 60 80 100 120 temperature (c) shutdown current (a) standby current vs. temperature 220 225 230 235 240 245 -40 -20 0 20 40 60 80 100 120 temperature (c) standby current (a) boost switching frequency vs. input voltage 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.5 3.0 3.5 4.0 4.5 input voltage (v) switching frequency (mhz) -40c 125c 75c 25c l = 1 h c out = 1f i led = 1a 50 60 70 80 90 100 2.6 3.0 3.4 3.8 4.2 4.6 5.0 efficiency (%) input voltage (v) wled power efficiency vs. input voltage l = 1 h c out = 4.7f i led = 100ma i led = 250ma i led = 400ma i led = 640ma i led = 1.2a i led = 1.0a 230 235 240 245 250 255 260 265 270 -40 -20 0 20 40 60 80 100 120 torch mode led current (ma) temperature ( c) torch mode led current vs. temperature l = 1 h c out = 4.7f i led = 250ma r frset = 20k 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 -40 -20 0 20 40 60 80 100 120 flash mode led current (a) temperature ( c) flash mode led current vs. temperature l = 1 h c out = 4.7f i led = 1a r frset = 20k flash mode iled(max) vs. frset resistor 0 200 400 600 800 1000 1200 0 10 20 30 40 50 60 frset resistor (k?) flash mode iled(max) (ma) l = 1 h c out = 4.7f torch mode iled(max) vs. frset resistor 0 50 100 150 200 250 300 0 10 20 30 40 50 60 frset resistor ( k? ) l = 1 h c out = 4 . 7 f torch mode iled(max) (ma) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 3.5 3.7 3.9 4.1 4.3 flash mode iled(max) accuracy (%) input voltage (v ) flash mode iled(max) accuracy vs. input voltage r frset = 17k r frset = 20k r frset = 30k r frset = 39k r frset = 62k r frset = 51k
micrel, inc. mic2871 may 29, 2013 6 0529 13 - 1.0 (while in progress) revision 1.0 (final document) typ ical characteristics (continued) -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 3.5 3.7 3.9 4.1 4.3 torch mode iled(max) accuracy (%) input voltage (v ) torch mode iled(max) accuracy vs. input voltage r frset = 51k r frset = 62k r frset = 39k r frset = 30k r frset = 20k r frset = 17k
micrel, inc. mic2871 may 29, 2013 7 0529 13 - 1.0 (while in progress) revision 1.0 (final document) functional characteristics
micrel, inc. mic2871 may 29, 2013 8 0529 13 - 1.0 (while in progress) revision 1.0 (final document) functional diagram figure 1 . simplified mic2871 functional block diagram
micrel, inc. mic2871 may 29, 2013 9 0529 13 - 1.0 (while in progress) revision 1.0 (final document) functional description vin the input supply provides power to the internal mosfets gate drive and controls circuitry for the switch - mode regulator. the operating input voltage range is from 2.7v to 5.5v. a 2 .2 f low - esr ceramic input capacitor should be connected from vin to agnd2 as close to the mic287 1 as possible to ensure a clean supply voltage for the device. the minimum voltage rating of 10v is recommended for the input capacitor. sw the mic2871 has internal low - side and synchronous mosfet switches. the switch node (sw) between the internal mosf et switches connects directly to one end of the inductor and provides the current paths during switching cycles. the other end of the inductor is connected to the input supply voltage. due to the high - speed switching on this pin, the switch node should be routed away from sensitive nodes wherever possible. agnd1 this is the ground path of the led current sink. it should be connected to the agnd2, but not via exposed pad, on the pcb. the current loop of the analog ground should be separate d from that of t he power ground (pgnd1 and pgnd2). agnd1 and agnd2 should be connected to pgnd1 and pgnd2 at a single point. agnd2 this is the ground path for the internal biasing and control circuitry. agnd2 should be connected to the pcb pad for the package exposed pad. agnd2 should be connected to the agnd1 directly without going through the exposed pad. the current loop of the analog ground should be separate d from that of the power ground (pgnd1 and pgnd2). the agnd2 and agnd1 should be connected to pgnd1 and pgnd2 at a single point. pgnd1 and pgnd2 the power ground pins are the ground path for the high current in the boost switch and they are internally connected together. the current loop for the power ground should be as small as possible and separate from the analo g ground (agnd) loop as applicable. out boost converter output pin which is connected to the anode of the led. a low - esr ceramic capacitor of 4.7 f or larger should be connected from out to pgnd1 as close as possible to the mic2871. the minimum voltage r ating of 10v is recommended for the output capacitor. led the current sink pin for the led. the led anode is connected to the out pin and the led cathode is connected to this pin. dc the dc is a single multiplexed device enable and serial data control pin used for functional co ntrol and communication in gpio - limited applications. when the dc pin is used as a hardware device enable pin, a logic high signal on the dc pin enables the device, and a logic low signal on the dc pin disables the device. when the d c pin is used as the single - wire serial interface digital control pin, a combination of bit edges and the period between edges is used to communicate a variable length data word across the single wire. each word is transmit ted as a series of pulses, e ach pulse incrementing an interna l data counter. a stop sequence consisting of an inactive period is used to latch the data word internally. the data word received is then used to set the value of the corresponding register for controlling specific functio n. the mic2871 supports five writeable registers for controlling flash mode, torch mode, safety timer duration, safety timer threshold current, and low - battery threshold. an address/data frame is used to improve protection against erroneous writes where c ommunications are in error. when dc is in a low state and no data is detected for an extended period of time, the mic2871 w ill automatically go into a low - power shutdown state, simultaneously resetting all internal registers to their default states. fen1 and fen2 fen1 and fen2 are hardware enable pins for flash mode. fen1 is logic - or with fen2. a logic low - to - high transition on either fen1 pin or fen2 pin can initiate the mic2871 in flash mode. if fen1 or fen2 is left floating, it is pulled down internally by a built - in 1a current source when the device is enabled. flash mode is terminated when both fen1 and fen2 are pulled low or left floating, and the flash register is cleared.
micrel, inc. mic2871 may 29, 2013 10 0529 13 - 1.0 (while in progress) revision 1.0 (final document) frset the flash mode maximum led current level is programmed through the fr set pin. a resistor connected from the frset pin to agnd2 set the maximum current in the flash mode. this pin can be grounded if the default flash mode current of 1a is desired. for the best current accuracy, 0.1% or smaller tolerance resistor for setting the maximum flash mode led current is recommended. this pin cannot be left floating and the minimum resistance is limited to 17k?. the maximum flash mode current to maximum torch mode current ratio is internally fixed as 4 to1.
micrel, inc. mic2871 may 29, 2013 11 0529 13 - 1.0 (while in progress) revision 1.0 (final document) application inform ation the mic2871 can drive a high - current flash wled in either flash mode or torch mode. boost converter the internal boost converter is turned on/off automatically when the led driver is activated/de - activated without any exception. the boost converter is an internally - compensated current - mode pwm boost converter running at 2mhz. it is for stepping up the supply voltage to a high enough value at the out pin to drive the led current. if the supply voltage is high enough, the synchronous switch of the converter is then fully turned on. in this case, all the excessive voltage is dropped over the linear led driver. flash mode the maximum current level in the flash mode is 1.2a. this current level can be adjusted through an external resistor connecting to the frset pin according to equation 1: frset led(max) r 20500 i = eq. 1 alternatively, the default value of 1a is used when the frset pin is grounded. the f lash mode current can be initiated at th is preset frset brightness level by asserting fen1 or fe n2 pin high, or by setting the flash control r egister (address 1) , for the desired f l ash duration, subjected to the safety timeout setting. the f lash mode current is terminated when the fen1 and fen 2 pins are brought low and the flash r egister is cleared. flash mode current can be adjusted to a fraction of the maxim um f lash mode current level by selecting the desired percentage in the flash control r egister through the sing l e - wire serial interface. the f lash curren t is the product of the maximum f lash current setting and the percentage sele cted in the flash r egister. torch mode by default, the maximum t orch mode level is one - fourth (1/4) of the maximum f lash mode current. the t orch mode operati on is activated by setting the torch control r egister (address 2) for the d esired duration. the t orch mode current is termina ted when the torch r egister is cleared or when the configurable safety timer expires. like the flash mode current, the t orch mode current can be tune d to a fraction of the maximum t orch mode level by sel ecting the desired t orch current level percentage i n the torch control r egister (address 2) through the sing l e - wire serial interface. the t orch current is the product of the maximum torch current setting and the percentage selected in the torch r egister. c onfigurable safety timer the flash safety t imeout feature automatically shuts down the led current after the safety timer duration is expired if the programmed led current excee ds a certain current threshold. both the current threshold and the timer dur ati on are programmable via the safety timer r egisters ( a ddress es 3 and 5). low - battery voltage detection (lbvd) when the vin voltage drops below the lbvd threshold (default = 3.6v) in flash or torch mode , the led current driver is disabled . the led driver can be resumed by toggling the corresponding input control signal. the lbvd threshold is adjustable t hru the lbvd control register (a ddress 4) . overv oltage protection when the output voltage rises above the ovp threshold, mic2871 is latched off automatically to avoid permanent damage to the ic. to clear the latched off condition, either power cycle the mic2871 or assert the dc pin low . short - circuit detection each time b efore enabling the led driver, the mic2871 performs the short circuit test by driving the f lash led with a small (2ma typical) current for 200 s . if (v out ? v led ) < 1.7v at the end of the short - circuit test, the led is considered to be shorted and mic2871 will ignore the f lash and/or t orch mo de command. note that the short - circuit test is carrie d out every time prior to flash and t orch mode but the result is not latched. thermal shutdown when the internal die temperature of mic2871 reaches 155 c, the led driver is disabled until the die temperature falls below 140 c.
micrel, inc. mic2871 may 29, 2013 12 0529 13 - 1.0 (while in progress) revision 1.0 (final document) sing le - wire interface the single - wire interface allows the use of a single multiplexed enable and data pin (dc) for control and communication in gpio limited applications. the interface is implemented using a simple mechanism allowing any open drain or directly driven gpio to contr ol the mic2871. the mic2871 uses the single - wire interface for simple command and control functions. the interface provides fast access to write only registers with protection features to avoid potentially erroneous data writes and improve robustness. when dc is in a low state and no data is detected for an extended period of time, the mic2871 w ill automatically go into a low - power shutdown state, simultaneously resetting internal registers to default states. overview the single - wire interface relies on a combination of bit edges and the period between edges in order to com municate across a single wire. each word is transmitted as a series of pulses, with each pulse incremen ting an internal data counter. a stop sequence consisting of an inactive period is u sed to latch the data word internally. an address and data fram ing format is used to improve protection against erroneous writes by enforcing address and data field lengths as well as the timing duration between them. timing is designed such that when comm unicating with a device using a low cost on chip oscillator, the worst case minimum and maximum conditions can be easily met within the wide operating range of the oscillator. using this method guarantees that the device can always detect the delay introd uced by the communication master. idle states and error conditions in shutdown mode, the mic2871 is in a reset condition with all functions off while consuming minimal power . r egister settings are reset to default state when coming out of shutdown state. in idle mode, all register settings persist and all mic2871 functions co ntinue in their current state. table 1 summarises the difference between the two idle modes: table 1 . differences be tween idle modes dc shutdown idle low high i supply (all functions off) 1a 230a register state default persist start - up time 1s 100ns idle mode is entered automatically at the end of a communication frame by holding dc high for t end , by enabling the device by bringing dc high when in shutdown mode, or when an error i s detected by the single - wire interface logic. shutdown mode can be entered at any time by pulling down dc for t end , discarding any current communication and res etting the internal registers. if a communication is received before the shutdown period but a fter the t lat period, the communication is discarded. this state is also used to create an internal error state to avoid erroneously latching data where the communication proc ess cannot be serviced in time. additionally, each register has a max imum value associated with it. if the number of bits clocked in exceeds the maximum value for the register, the data is assumed to be in error and the data is discarded. figure 2 . abort, shutdown, and idle timing waveforms communication d etails the serial interface requires delimiters to indicate the start of frame, data as a series of pulses, and end of frame indicated by a lack of activity. the start of frame is the first high to low transition of dc when in idle mode. the first rising edge resets the internal data counter to 0.
micrel, inc. mic2871 may 29, 2013 13 0529 13 - 1.0 (while in progress) revision 1.0 (final document) figure 3 . data word pulse timing a pulse is delimited by the signal first going below v l and then above v h within the latch timeout t lat . during this transition, minimum on (t on ) an d off (t off ) periods are observed to improve tolerance to glitches . each rising edge increments the internal data register. data is automatically latched into internal shadow address and data registers after an inactivity period of >t lat . to send registe r write commands, the address and data are entered in series as two data words using the above pattern, with the second word starting after the first latch period has expired. after the second word is entered, the idle command should be issued by leaving t he dc pins high for t end . after receiving the stop sequence, the internal register s decode and update cycle is started, with the shadow register values being transferred to the decoder. figure 4 shows an exampl e of entering a write of data 5 to address 3. figure 4 . communication timing example of entering write for data 5 to address 3 only correctly formatted address/data combination will be treated as a valid frame and processed by the mic2871. any other input, such as a single data word followed by t end , or three successive data words will be discarded by the target hardware as an erroneous entry. additionally, any register write to either an invalid register or with invalid regis ter data will also be discarded. mic2871 registers the mic2871 supports five writeable registers for controlling the torch and the flash modes of operation as shown in table 2 . note that register addressing starts at 1. writing any value above the maximum value shown for each registers will cause an invalid data error and the frame will be discarded. table 2 . five writable registers of mic2871 address name max. value description 1 fen/fcur 31 flash enable/current 2 ten/tcur 31 torch enable/current 3 stdur 7 safety timer duration 4 lb_th 9 low battery voltage detection threshold 5 st_th 5 safety timer threshold flash current register (fen/fcur: default 0) the flash current register enab les and sets the flash mode current level. valid values are 0 to 31; values 0 ? 15 will set the flash current without enabling the flash (such that it can be triggered externally), values 16 ? 31 will set the flash current and enable the flash. the flash c urrent register maps into the internal fen and fcur registers as shown in the table below . table 3 describes the relationship between the flash current as a percentage of maximum current, and the fcur register sett ing.
micrel, inc. mic2871 may 29, 2013 14 0529 13 - 1.0 (while in progress) revision 1.0 (final document) table 3 . flash current register mapping into internal fen and fcur registers, and relationship between flash current as % of maximum current and the fcur register setting value fen/fcur[4:0] dec. binary fen[4] fcur[3:0] % of i max 0 00000 0 100.00 1 00001 0 88.96 2 00010 0 79.04 3 00011 0 70.72 4 00100 0 63.04 5 00101 0 56.00 6 00110 0 49.92 7 00111 0 44.64 8 01000 0 39.68 9 01001 0 35.52 10 01010 0 31.68 11 01011 0 28.16 12 01100 0 25.12 13 01101 0 22.40 14 01110 0 20.00 15 01111 0 17.92 16 10000 1 100.00 17 10001 1 88.96 18 10010 1 79.04 19 10011 1 70.72 20 10100 1 63.04 21 10101 1 56.00 22 10110 1 49.92 23 10111 1 44.64 24 11000 1 39.68 25 11001 1 35.52 26 11010 1 31.68 27 11011 1 28.16 28 11100 1 25.12 29 11101 1 22.40 30 11110 1 20.00 31 11111 1 17.92 torch current register (ten/tcur: default 0) the torch current register enables and sets the torch mode current level. valid values are 0 to 31; values 0 ? 15 will set the torc h current w ithout enabling the torc h (such that it can be triggered by setting the internal ten register value to 1 ), values 16 ? 31 will set the torch current and enable the torch . a valu e of 0 at the internal ten register will disable the torch. the torch current register maps into the internal ten and tcur registers as shown in table 4 . the table also describes the relationship between the torch current as a percentage of maximum current, and the tcur register setting.
micrel, inc. mic2871 may 29, 2013 15 0529 13 - 1.0 (while in progress) revision 1.0 (final document) ta ble 4 . torch current register mapping into internal ten and tcur registers, and relationship between torch current as % of maximum current and the tcur register setting value ten/tcur[4:0] dec. binary ten[4] tcur[3:0] % of i max 0 00000 0 100.00 1 00001 0 88.96 2 00010 0 79.04 3 00011 0 70.72 4 00100 0 63.04 5 00101 0 56.00 6 00110 0 49.92 7 00111 0 44.64 8 01000 0 39.68 9 01001 0 35.52 10 01010 0 31.68 11 01011 0 28.16 12 01100 0 25.12 13 01101 0 22.40 14 01110 0 2 0.00 15 01111 0 17.92 16 10000 1 100.00 17 10001 1 88.96 18 10010 1 79.04 19 10011 1 70.72 20 10100 1 63.04 21 10101 1 56.00 22 10110 1 49.92 23 10111 1 44.64 24 11000 1 39.68 25 11001 1 35.52 26 11010 1 31.68 27 11011 1 28.16 28 11100 1 25.1 2 29 11101 1 22.40 30 11110 1 20.00 31 11111 1 17.92 safety timer duration register (stdur : default 7) the safety timer duration register sets the duration of the flash and t orch mode when the led current exceeds the programmed threshold current. val id values are 0 for the minimum timer duration to 7 for the maximum duration. table 5 . safety timer duration register setting and safety timer duration value fdur[2:0] (binary) timeout (ms) dec. binary 0 000 000 156.25 1 001 001 312.5 2 010 010 468.75 3 011 011 625 4 100 100 781.25 5 101 101 937.5 6 110 110 1093.75 7 111 111 1250 low - battery threshold register (lb_th: default 7) the lb_th register sets the supply threshold voltage below which the internal low battery fl ag is asserted and flash functions are inhibited. table 6 shows the threshold values that correspond to the register settings. setting 0 is reserved for disabling the function, and settings between 1 and 9 inclusi vely enable and set the lb_th value between 3.0v and 3.8v with 100mv resolution . table 6 . low - battery threshold register setting and supply threshold voltage value lb_th[3:0] v bat threshold (v) dec. binary 0 0000 0000 disabled 1 0001 0001 3.0 2 0010 0010 3.1 3 0011 0011 3.2 4 0100 0100 3.3 5 0101 0101 3.4 6 0110 0110 3.5 7 0111 0111 3.6 8 1000 1000 3.7 9 1001 1001 3.8
micrel, inc. mic2871 may 29, 2013 16 0529 13 - 1.0 (while in progress) revision 1.0 (final document) safety timer threshold current register (st_th: default 4) safety timer threshold current determines the amount of led current flowing through the external led before the internal led safety timer is act ivated. setting st_th to 0 disables the safety timer function, and setting the register to value s 1 to 5 set the safety time threshold current 100ma to 3 00ma in 50ma steps. table 7 . safety timer threshold current register setting and safety timer threshold current value st_th[2:0] safety timer t hreshold c urrent (ma) dec. binary 0 000 000 disabled 1 001 001 100ma 2 010 010 150m a 3 011 011 200ma 4 100 100 250ma 5 101 101 300ma
micrel, inc. mic2871 may 29, 2013 17 0529 13 - 1.0 (while in progress) revision 1.0 (final document) component selection inductor inductor selection is a balance between efficiency, stability, cost, size, and rated current. since the boost converter is compensated internally, the reco mmended inductance of l is limited from 1 h to 2.2 h to ensure system stability. it is usually a good balance between these considerations. a large inductance value reduce s the peak - to - peak inductor ripple current hence the output ripple voltage and the le d ripple current. this also reduces both the dc loss and the transition loss at the same inductor?s dc resistance (dcr) . however, the dcr of an inductor usually increases with the inductance in the same package size. this is due to the longer windings requ ired for an increase in inductance. since the majority of the input current pass es through the inductor, the higher the dcr the lower the efficiency is , and more significantly at higher load currents. on the other hand, inductor with smaller dcr but the sa me inductance usually has a larger size. the saturation current rating of the selected inductor must be higher than the maximum peak inductor current to be encountered and should be at least 20% to 30% higher than the average inductor current at maximum ou tput current. input capacitor a ceramic capacitor of 2.2 f or larger with low esr is recommended to reduce the input voltage ripple to ensure a clean supply voltage for the device. the input capacitor should be placed as close as possible to the mic2871 vi n pin with short trace for good noise performance. x5r or x7r type ceramic capacitors are recommended for better tolerance over temperature. the y5v and z5u type temperature rating ceramic capacitors are not recommended due to their large reduction in capa citance over temperature and increased resistance at high frequencies. these reduce their ability to filter out high - frequency noise. the rated voltage of the input capacitor should be at least 20% higher than the maximum operating input voltage over the o perating temperature range. output capacitor output capacitor selection is also a trade - off between performance, size, and cost. increasing output capacitor will lead to an improved transient response, however, the size and cost also increase. the output c apacitor is preferred in the range of 2.2 f to 10 f with esr from 10m? to 50m? . x5r or x7r type ceramic capacitors are recommended for bett er tolerance over temperature. the y5v and z5u type ceramic capacitors are not recommended due to their wide variation in capacitance over temperature and increase d resistance at high frequencies. the rated voltage of the output capacitor should be at least 20% higher than the maximum operating output voltage over the operating temperature range. frset resistor since frset resistor is used for setting the maximum l ed current, resistor type with 0.1% tolerance is recommended for more accurate maximum led current setting.
micrel, inc. mic2871 may 29, 2013 18 0529 13 - 1.0 (while in progress) revision 1.0 (final document) power dissipation consideration as with all power devices, the ultimate current rating of the output is limited by the thermal properties of the de vice package and the pcb on which the device is mounted. there is a simple, ?s law type relationship between thermal resistance, power dissipation and temperature which are analogous to an electrical circuit: figure 5 . series e lectrical resistance circuit from this simple circuit we can calculate v x if we know i source , v z and the resistor values, r xy and r yz using equation 2: z yz xy source x v ) r (r i v + + = eq. 2 thermal circuits can be considered using this same rule and can be dra wn similarly by replacing current sources with power dissipation (in watts), resistance with thermal resistance (in c/w) and voltage sources with temperature (in c). figure 6 . series thermal resistance circuit now replacing the variables in equation 2, we can find the junction temperature (t j ) from the power dissipation, ambient temperature and the known thermal resistance of the pcb ( ca ) and the package ( jc ). a ca jc diss j t ) ( p t + + = eq. 3 as can be seen in the diagram, t otal thermal resistance ja = jc + ca . hence this can also be written as in equation 4: a ja diss j t ) ( p t + = eq. 4 since effectively all of the power losses (minus the inductor losses) in the converter are dissipated within the mic2871 package, p di ss can be calculated thus: linear mode: dcr 2 i ] 1 1 [p p out out diss ? ? = ? ? ? ? ? ? eq. 5 boost mode: dcr 2 d 1 i ] 1 1 [p p out out diss ? ? ? = ? ? ? ? ? ? ? ? ? ? ? ? eq. 6 duty c ycle in b oost m ode: out in out v v v d ? = eq. 7 w here: = efficiency taken from efficiency curves and dcr = inductor dcr. jc and ja are found in the operating ratings section of the datasheet.
micrel, inc. mic2871 may 29, 2013 19 0529 13 - 1.0 (while in progress) revision 1.0 (final document) where the real board area differs from 1? square, ca (the pcb thermal resistance) values for various pcb copper areas can be taken from figure 7 . figure 7 is taken from designing with low dropout voltage regulators available from the micrel website (?ldo application hints?). figure 7 . graph to determine pc board area for a given pcb thermal resistance figure 7 shows the total area of a round or square pad, centered on the device. the solid trace represen ts the area of a square, single - sided, hor izontal, solder - masked, copper pc board trace heat sink, measured in square millimeters. no airflow is assumed. the dashed line shows pc boards trace heat sink covered in black oil - based paint and with 1.3m/sec (250 feet per minute) airflow. this approache s a ?best case? pad heat sink. conservative design dictates using the solid trace data, which indicates a maximum pad size of 5000 mm 2 is needed. this is a pad 71mm by 71mm (2.8 inches per side).
micrel, inc. mic2871 may 29, 2013 20 0529 13 - 1.0 (while in progress) revision 1.0 (final document) pcb layout guidelines pcb layout is critical to achieve r eliable, stable and efficient performance. a ground plane is required to control emi and minimize the inductance in power, signal and return paths. the following guidelines should be followed to ensure proper operation of the device: ic (integrated circuit ) ? place the ic close to the point - of - load (in this case, the flash led). ? use fat traces to route the input and output power lines. ? analog grounds (agnd1 and agnd2) and power grounds (pgnd1 and pgnd2) should be kept separate and connected at a single locati on. ? the exposed pad (epad) on the bottom of the ic must be connected to the analog grounds agnd2 of the ic. ? 8 to 12 thermal vias must be placed on the pcb pad for exposed pad and connected it to the ground plane to ensure a good pcb thermal resistance can be achieved. vin decoupling capacito r ? the vin decoupling capacitor must be placed close to the vin pin of the ic and preferably connected directly to the pin and not through any via. the capacitor must be located right at the ic. ? the vin decoupling capaci tor should be connected to analog ground (agnd2). ? the vin terminal is noise sensitive and the placement of capacitor is very critical. inductor ? keep both the inductor connections to the switch node (sw) and input power line short and wide enough to handle the switching current. keep the areas of the switching current loops small to minimize the emi problem. ? do not route any digital lines underneath or close to the inductor. ? keep the switch node (sw) away from the noise sensitive pins. ? to minimize noise, p lace a ground plane underneath the inductor. output capacitor ? use wide and short traces to connect the output capacitor to the out and pgnd1 pins. ? place several vias to the ground plane close to the output capacitor ground terminal. ? use either x5r or x7 r temperature rating ceramic capacitors. do not use y5v or z5u type ceramic capacitors. flash led ? use wide and short trace to connect the led anode to the out pin. ? use wide and short trace to connect the led cathode to the led pin. ? make sure that the led?s pcb land pattern can provide sufficient pcb pad heat sink to the flash led. frset resistor the frset resistor should be placed close to the frset pin and connected to agnd2.
micrel, inc. mic2871 may 29, 2013 21 0529 13 - 1.0 (while in progress) revision 1.0 (final document) typical application schematic bill of materials item part number man ufacturer description qty. c1 grm188r61a225ke34d murata ( 7 ) 2.2f, 10v, 10%, x5r, 0603 capacitor 1 c4 lmk107bj475ka -t taiyo yuden ( 8 ) 4.7f, 10v, 10%, x5r, 0603 capac itor 1 l1 pife25201b - 1r0ms -39 cyntec ( 9 ) 1.0h, 3.55a, 2.5mm 2.0mm 1.2mm inductor 1 led1 slsw6r007 samsung ( 10 ) 4mm 4mm 2.2mm high - power flash led 1 lxcl - mn 06-3002 philips ( 11) luxeon flash 6 module, 4mm 4mm 2.2mm, 180lux @ i led = 1a led r4 era3aeb2052v panasonic ( 12 ) 20.5 k , 1/10w, 0.1%, 0603 resistor 1 u1 MIC2871YMK micrel, inc. ( 13) 1.2a high - brightness led flash driver with single - wire serial interface 1 notes: 7. murata: www.murata.com . 8. tai yo yuden: www.t - yuden.com . 9. cyntec: www.cyntec.com . 10. samsung : www.samsung.com . 11. philips: www.philipslumileds.com . 12. panasonic: www.panasonic.com . 13. micrel, inc.: www.micrel.com .
micrel, inc. mic2871 may 29, 2013 22 0529 13 - 1.0 (while in progress) revision 1.0 (final document) pcb layout recommendations top layer bottom layer
micrel, inc. mic2871 may 29, 2013 23 0529 13 - 1.0 (while in progress) revision 1.0 (final document) package information and recommended landing pattern ( 14, 15) 14- pin 3mm 2mm ldfn (mk) note s : 14. pack age information is correct as of the publication date. for updates and most current information, go to www.micrel.com . 15. disclaimer : this is only a recommendation based on information availabl e to micrel from its suppliers. actual land pattern may have to be significantly different due to various materials and processes used in pcb assembly. micrel makes no representation or warran ty of performance based on the recommended land pattern.
micrel, inc. mic2871 may 29, 2013 24 0529 13 - 1.0 (while in progress) revision 1.0 (final document) micrel, inc. 2180 fortune drive san jose, ca 95131 usa te l +1 (408) 944 - 0800 fax +1 (408) 474- 1000 web http://www.micrel.com micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in th is data sheet. this i nformation is not intended as a warranty and micrel does not assume responsibility for its use. micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. no license, whether express, implied, arising by es toppel or otherwise, to any intellectual property rights is granted by this document. except as provided in micrel?s terms and conditions of sale for such products, micrel assumes no liability whatsoever, and micrel disclaims any express or implied warran ty relating to the sale and/or use of micrel products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right . micrel products are not d esigned or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. life support devices or systems are devices or systems that (a) are intended 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 injury to the user. a purchaser?s use or sale of micrel products for use in life support appliances, devic es or systems is a purchaser?s own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 20 13 micrel, incorporated.

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