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mic2870 1.5a synchronous boost flash led driver with i 2 c interface micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 (408) 944 - 0800 ? fax + 1 (408) 474 - 1000 ? http://www.micr el.com september 5, 2013 revisio n 1.0 general description the mic2870 is a high - current, high - efficiency flash led driver for one or two high - brightness camera flash leds. the led driver current is generated by an integrated i nductive boost converter with 2mhz switching frequency which allows the use of a very - small inductor and output capacitor. these features make the mic2870 an idea l solution for high - resolution camera phone led flash light driver applications. mic2870 supports two 750ma w hite - leds (wleds) or a single 1.5a wled configuratio n . when two wleds are connected, their current s are matched automatically. mic2870 operates in either flash or torch modes that can be controlled through either an i 2 c interface or extern al pins. the brightness in the flash and torch mode can be adjusted vi a two external resistors individually. high - speed mode i 2 c interface provides a simple control at a clock speed up to 3.4mhz to support most camera functions such as auto - focus, white b alance , and image c apture (f lash m ode). the mic2870 is available in 16 -p in, 2mm 2mm tqfn 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.micrel.com . features ? up to 1.5a flash led driving curr ent ? 2.7 v to 5. 0v i nput voltage range ? high - efficiency 2mhz v f a daptive b oost d river ? configurable 1 or 2 channel(s) wled d river ? led driving current soft - start ? control through i 2 c interface or external pins ? flash inhibit function for gsm pulse synchronization ? true l oad d isconnect ? flash time - out protection ? 1a shutdown current ? available in 16 - pin 2mm 2mm tqfn package applications ? camera phones/ m obile handsets ? cellular phones/smart phones ? led light for image capture/ a uto focus/white balance ? handset video light (torch light) ? digital cameras ? portable application s typical application
micrel, inc. mic2870 september 5, 2013 2 revision 1.0 ordering information part number marking temperature range package ( 1 ) lead finish mic2 870yft 70h ? 40 c to +125 c 16- pin 2mm 2mm tqfn nipdau note: 1. package is a green, rohs - compliant package. lead finish is nipdau. mold compound is halogen free. 2. thin qfn pin 1 identifier = ? ?. pin configuration 16- pin 2mm 2mm tqfn (top view) pin description pin number pin name pin function 1 scl high -s peed mode (3.4mhz) i2c clock input. 2 vin supply input . connect a low - esr ceramic capacitor of at least 4.7f to pgnd . a small capacitor of 100nf between vin and agnd is highly recommended. 3 fen f lash - mode enable pin. a low - to - high transition initiates the flash mode and flash- mode t imer. if fen is left floating , it is pulled - down internally by a built - in 1a current source when the device is enabled. 4 fi flash inhibit . when fi is pulled high, both led currents are changed from the flash - mode current level to the torch -m ode current level. if fi is left floating , it is pulled - down internally by a built - in 1a current source when the device is enabled. this function is generally used to reduce inst antaneous battery load current by synchronizing with the handset? s gsm p ulse off time. 5 frset flash - mode current level programming. connect a resistor from frset to agnd to set the maximu m current in the flash m ode. for example, a 10k? resistor set s the led sink current to its maximum value of 750ma per channel. frset can be g rounded if the default maximum f lash - m ode current (750ma) is desired. frset , however, cannot be left floating and the maximum resistance is limited to 80k? 6 agnd analog ground. reference ground for frset and trset pins.
micrel, inc. mic2870 september 5, 2013 3 revision 1.0 pin description (continued) pin number pin name pin function 7 , 15 p gnd power ground. pgnd is used for the switching nmos and pmos of boost converter , and power ground for led current sinks . 8 trset torch -mo de current level programming . connect a resistor from trset to agnd to set the maximum current in the torch m ode. for example, a 10k? resistor set s the led sink curren t to its maximum value of 187.5 ma per channel. tr set can be g rounded if the default maximum t orch - m ode current (187.5ma) is desired. trset , however, cannot be left floating and the maximum resistance is limited to 80k? . 9 led2 channel 2 led current sink . connect the led anod e to out and cathode to led2 . 10 led1 channel 1 led current sink . connect the led anode to out and cathode to led1 . 11 ten torch - mode enable. initiates torch m ode when ten is high. if ten is left floating, it is pulled - down internally by a built - in 1a current source when the device is enabled. 12 out boost converter output . 13 en enable (ic). the mic2870 is in standby mode when en is asserted high. if en is driven low for more than 1s, the ic is shut down. alternatively, the i 2 c interface can be used for enabling/disabling the ic through the master control/status register. en is pulled down by an i nternal resistor . 14 sw inductor connection. it is connected to the internal power mosfets. 16 sda high -s peed mode (3.4mhz) i2c data input/ output . ep ep ad exposed heat sink pad. connec t to pgnd ground plane for best thermal performance. this pin is internally connected to pgnd.
micrel, inc. mic2870 september 5, 2013 4 revision 1.0 absolute maximum ratings ( 3 ) supply voltage (v in ) ..................................... ? 0.3v to +6.0v enable input voltage (v en , v fen , v fi , v ten ) ....................... ? 0.3v to v in + 0.3v v out , v led1 , and v led2 .................................... ? 0.3v to 6.0v i 2 c i/o (v scl , v sda ) ................................ ? 0.3v to v in + 0.3v v frset and v trset .................................. ? 0.3v to v in + 0.3v v sw ................................................................. ? 0.3v to 6.0v power dissipation (5) (p diss ) ....................... internally limited lead temp erature (soldering, 10s) .......................... +260c junction temperature (t j ) ........................ ? 40c to +150c storage temperature (t s ) ......................... ? 40c to +150c esd rating ( 6) ................................. 2kv hbm and 15 0v mm operating ratings ( 4 ) supply voltage (v in ) ..................................... +2.7v to +5.0v enable input voltage (v en , v fen , v fi , v ten ) ............ 0v to v in i 2 c i/o (v scl , v sda ) ................................................. 0v to v in junction temperature (t j ) .......................... ? 40c to 125c package thermal resistance (5) 2mm 2mm tqf n ( ja ) .................................... 80c/w electrical characteristics ( 7 ) v in = 3.6v; l = 1h, c out = 2.2f, r frset = 10k?, r trset = 10k?, i led = 100ma; t a = 25 c, bold values indicate ? 40 c t j 125 c, unless otherwise noted. symbol parameter condition min . typ . max . units power supply v in input voltage 2.7 5.0 v i vi n quies cent current v led1 = v led2 > 200 mv, not switching 0.9 ma v led1 = v led2 = 70mv, b oost keeps switching 4.2 i vin(sd) shutdown current v en = 0 v 0.6 a i sw(sd) sw p in shutdown current v en = 0 v 1 5 a uvlo_rise uvlo threshold (r ising) 2.35 2.5 2.65 v uvlo_hyst uvlo hysteresis 300 mv v out output voltage v in v out v in v v out > v in 5.2 v ovp overv oltage protection threshold v out > v in 5.26 5.38 5.6 v overv oltage protection hysteresis 60 mv ovp blanking t ime 24 s d max maximum duty cycle 80 85 90 % d min minimum duty cycle 5.5 % i sw_ oc switch current limit v in = v out = 2.7v 3.35 4.5 5. 65 a notes: 3. exceeding the absolute maximum ratings may damage the device. 4. the device is not guaranteed to function outside its operating ratings. 5. the maximum allowable power dissipation of any t a (ambient temperature) is p diss(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. 6. devices are esd sensitive. handling precautions are recommended. human body model, 1.5k in series with 100pf. 7. specification for packaged product only
micrel, inc. mic2870 september 5, 2013 5 revision 1.0 electrical characteristics ( 7 ) (continued) v in = 3.6v; l = 1h, c out = 2.2f, r frset = 10k?, r trset = 10k?, i led = 100ma; t a = 25 c, bold values indicate ? 40 c t j 125 c, unless otherwise noted. symbol parameter condition min . typ . max . units f sw oscillator frequency 1.8 2.0 2.2 mhz r on(n) switch on - resistance v vin = 2.7 v, i sw = 750 ma 80 m r on(p) v sw = 2.7 v, i out = 750 ma 80 i lk(n) nmos switch leakage cu rrent v en = 0 v , v in = v sw = v out = 5 v 1 5 a i lk( p ) pmos switch leakage current v en = 0 v , v in = v out = 5 v, v sw = 0 v 1 5 a r dchg auto -d ischarge nmos r esistance v en = 0 v, i out = ? 1 ma 160 t sd overt emperature shutdown threshold 160 c t sd _ hyst overt emperature shutdown hysteresis 25 c t flash _ timeout flash safety timeout shutdown maximum timeout setting 1.25 s current sink channels accu led_ch channel current accuracy v led1 = v led2 = 890mv, i led1 = i led2 = 750ma ? 10 10 % match le d_ch channel current matching v led1 = v led2 = 890mv, i led1 = i led2 = 750ma ? 5 5 % v dropout current sink dropout b oost is in regulation 100 mv i lk_led1 led1 leakage current v in = 3.6 v, v en = 0 v, v led1 = 3.6 v 0.0 5 a i lk_led 2 led 2 leakage curren t v in = 3.6 v, v en = 0 v, v led 2 = 3.6 v 0.0 5 a v frset frset pin voltage r frset = 10k, flash m ode 0.970 1.00 1.030 v i frset frset current sourcing f rset pin is shorted to ground , flash m ode 90 100 110 a v t rset trset pin voltage r t rset = 10 k, to rch m ode 0.970 1.00 1.030 v i t rset trset current sourcing t rset pin is shorted to ground , t orc h m ode 90 100 110 a
micrel, inc. mic2870 september 5, 2013 6 revision 1.0 electrical characteristics ( 7 ) (continued) v in = 3.6v; l = 1h, c out = 2.2f, r frset = 10k?, r t rset = 10k?, i led = 100ma; t a = 25 c, bold values indicate ? 40 c t j 125 c, unless otherwise noted. symbol parameter condition min . typ . max . units en / fen / ten / fi control pins v en_on en on threshold boost converter and chip logic on 1.5 v v en_ o ff en off threshold boost converter and chip logic on 0.4 v v fe n_on fen on threshold flash on 1.5 v v fe n_o ff fen off threshold flash off 0.4 v v te n_on ten on threshold torch on 1.5 v v te n_o ff ten off threshold torch off 0.4 v v fi _on fi on t hreshold flash inhibit on 1.5 v v fi _o ff fi off threshold flash inhibit off 0.4 v en pin current v en = 5 v 2 5 a fen/ten/fi pin current v fe n = v te n = v fi = 5 v 1 5 a t blank_en_off en off blanking time en pin should be driven low for more than th is time before the ic enters sleep m ode 0.90 1. 10 1.3 0 s i 2 c interface (scl / sda pins) ( guaranteed by d esign ) f scl maximum operating frequency 3.4 mhz v il low -l evel input voltage 0. 4 v v ih high -l evel input voltage 1. 5 v r sda_dn sda pulled -d o wn resistance 20 additional protection features v th_led open led1 / led2 open detect threshold 15 25 40 mv t blank _ open open detect blanking time 65 s t retry _ open open retry timeout 100 ms v th_led short short trigger threshold v out ? max[ v led1 , v led2 ], v out = 3.6 v 400 6 00 800 m v v hyst _led short short trigger hysteresis 200 m v t blank _ short short trigger blanking time 30 s t retry _ short short retry timeout 100 ms
micrel, inc. mic2870 september 5, 2013 7 revision 1.0 typical characteristics 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -40 -20 0 20 40 60 80 100 120 shutdown current (a) temperature ( c) shutdown current vs. temperature 0.87 0.88 0.89 0.90 0.91 0.92 0.93 0.94 -40 -20 0 20 40 60 80 100 120 quiescent current (a) temperature ( c) quiescent current (linear mode) vs. temperature linear mode not switching v led1 = v led2 > 200mv 4.10 4.15 4.20 4.25 4.30 4.35 4.40 4.45 4.50 -40 -20 0 20 40 60 80 100 120 quiescent current (a) temperature ( c) quiescent current (boost mode) vs. temperature boost mode switching v led1 = v led2 = 70mv 185 186 187 188 189 190 -40 -20 0 20 40 60 80 100 120 torch mode led current ( ma ) temperature ( c) torch mode led1 and led2 current vs. temperature torch mode l = 1 h c out = 2.2 f i led = 187.5ma v led = 890mv r trset = 10k 600 650 700 750 800 850 -40 -20 0 20 40 60 80 100 120 flash mode led current ( m a) temperature ( c) flash mode led1 and led2 current vs. temperature flash mode l = 1 h c out = 2.2 f i led = 750ma v led = 890mv r frset = 10k 0 50 100 150 200 250 0 10 20 30 40 50 60 70 80 torch mode i led(max) ( m a) trset resistor (k ?) torch mode i led(max) (dual leds) vs. trset resistor l = 1 h c out = 2.2 f dual leds i led per channel t a = 25 c 0 100 200 300 400 500 600 700 800 0 10 20 30 40 50 60 70 80 flash mode i led(max) ( m a) frset resistor (k ?) flash mode i led(max) (dual leds) vs. frset resistor l = 1 h c out = 2.2 f dual leds i led per channel t a = 25 c 0 50 100 150 200 250 300 350 400 0 10 20 30 40 50 60 70 80 torch mode i led(max) ( m a) trset resistor (k ?) torch mode i led(max) (single led) vs. trset resistor l = 1 h c out = 2.2 f single led i led1 +i led2 t a = 25 c 0 200 400 600 800 1000 1200 1400 1600 0 10 20 30 40 50 60 70 80 flash mode i led(max) ( m a) frset resistor (k ?) flash mode i led(max) (single led) vs. frset resistor l = 1 h c out = 2.2 f single led i led1 +i led2 t a = 25 c
micrel, inc. mic2870 september 5, 2013 8 revision 1.0 typical characteristics (c ontinued) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.5 3.7 3.9 4.1 4.3 torch mode i led(max) accuracy (%) input voltage (v ) torch mode i led(max) accuracy vs. input voltage r trset = 51k r trset = 62k r trset = 39k r trset = 30k r trset = 20k r trset = 10k r trset = 82k r trset = 75k 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 switching frequency (mhz) input voltage (v) boost switching frequency vs. input voltage - 40 c 125 c 75 c 25 c l = 1 h c out = 2.2 f i led1 + i led2 = 1.5a 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 output power efficiency vs. input voltage l = 1 h c out = 2.2f t a = 25 c i led = 150ma i led = 375ma i led = 780ma i led = 1.2a i led = 1.5a
micrel, inc. mic2870 september 5, 2013 9 revision 1.0 functional characteristics
micrel, inc. mic2870 september 5, 2013 10 revision 1.0 functional characteristics (continued)
micrel, inc. mic2870 september 5, 2013 11 revision 1.0 functional characteristics (continued)
micrel, inc. mic2870 september 5, 2013 12 revision 1.0 functional diagram figure 1 . simplified mic2870 functional block diagram
micrel, inc. mic2870 september 5, 2013 13 revision 1.0 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 .0v. a 4.7 f low - esr ceramic input capacitor should be connected from vin to agnd as close to mic2870 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 mic2870 h as internal low - side and synchronous mosfet switches. the switch node (sw) between the internal mosfet 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. agnd this is the ground path for the internal biasing and control circuitry. the current loop of the a nalog ground should be separate d from that of the power ground (pgnd) . agnd should be connected to pgnd at a single point. pgnd the power ground pin is the ground path for the high current in the boost switch and the ground path of the led current sink s . t he current loop for the power ground should be as small as possible and s eparate from the agnd loop as applicable. out boost converter output pin which is connected to the anode of the led. a low - esr ceramic capacitor of 2.2 f or larger should be connected from out to pgnd as close as possible to the mic2870. the minimum voltage rating of 10v is recommended for the output capacitor. led1/led2 the current sink pins for the led(s). the led anode is connected to the out pin and the led cathode is connected to the led1/led2 pin(s). en this is the enable pin of the mic2870. the mic2870 is in standby mode when the en pin is asserted high . if this pin is driven low for more than 1s, the ic is shutdown. alternatively, the i 2 c interface can be used for enabling/disa bling the ic through the master control/s tatus register . en is pulled down by an i nternal resistor . fen fen is the hardware enable pin for flash mode. a logic low - to - high transition on fen pin initiate s the flash mode. if fen pin 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 fen is pulled low or left floating, and the flash control register is cleared. ten ten is the hardware enable pin for torch mode. a logic lo w - to - high transition on ten pin initiate s the torch mode. if ten pin is left floating, it is pulled down internally by a built - in 1a current source when the device is enabled. torch mode is terminated when ten is pulled low or left floating, and the torch control register is cleared. fi fi is the flash inhibit pin. when this pin is high in flash mode, both led1 and led2 curr ents are changed from the flash - mode current level to the torch - mode current level. when this pin is low, both led1 and led2 currents are changed from torch - mode current l evel back to the original flash - mode current level. frset the flash - mode maximum led current level is programmed through frset. a resistor connected from frset to agnd set s the maximum current in the f lash mode. frset c an be grounded for the default flash - mode current of 0.75a. for best current accuracy, a 0.1% tolerance resistor is recommended. frset cannot be left floating and the maximum resistance is limited to 80k? . trset the torch - mode maximum led current level is programmed through trset . a resistor connected from the trset pin to agnd set s the maximum current in the torch mode. trset can be g rounded for the default torch - mode current of 187.5ma. for best current accuracy, a 0.1% tolerance resistor is recommended. trset cannot be left floating and the maximum resistance is limited to 80k? . scl i 2 c clock input pin provides a reference clock for clocking in the data signal . this is a high - speed mode up to 3.4mhz input pin, and requires a 4.7k? pull - up resistor. sda i 2 c data input/output pin allows for data to be written to and read from the mic2870. this is a high - speed mode up to 3.4mhz i 2 c pin, and requires a 4.7k? pull - up resistor .
micrel, inc. mic2870 september 5, 2013 14 revision 1.0 application information the mi c2870 can drive one or two high - current flash wleds in either flash mode or torch m ode. two wleds can be used to optimize the light output and beam shaping through the optical lens/re flector assembly. in this case, the two channels, up to 750ma each, are matched to within 10% for optimal flash illumination. when the two channels are combined to drive a single high - brightness wled, the maximum current is 1.5a. if one of the channels is left floating, mic2870 senses the circuit condition automatically and allows the other channel to operate. flash mode th e maximum current level in the flash mode is 750 ma per channel. this current level can be adjusted through an external resist or connect ing to frset according to the following equa tion: frset led(max) r 7500 i = eq. 1 alternatively, t he default maximum value of 750ma per channel is used when frset is grounded. the f lash - mode current can be initiated at the preset frset brightness level by asserting fen high or by setting the i 2 c f lash c ontrol r egister (address 01h) for the desired flash duration, subjected to the flash safety timeout setting. the f lash - mode current is terminated when fen is brought low and the i 2 c flash r egister is clear ed. the flash i nhibit (fi) pin can be used to synchronize the f lash current to a handset gsm p ulse event to prevent excessive battery droop. when fen and fi pins are both high, the f lash - mode current is limited to the torch - mode current setting. the fi pi n is also functional when the f lash - mode current is enabled through the i 2 c f lash r egister. flash - mode current can be adjusted to a fraction of the maximum f lash - mode level (either default or set by the frset resistor) by selecting the desired f lash curren t level percentage in the f lash control r egister (address 01h) through the i 2 c interface. the f lash current is the product of the maximum f lash current setting and the percentage selected in the f lash r egister. the f lash safety t imeout feature automaticall y shuts down the f lash current if the f lash mode is enabled for an extended period of time. refer to the f lash s afety t imer setting in table 4 . torch mode the maximum t orch - mode current level can be adjusted throu gh an external resistor connecting to the trset pin according to e quation 2 : trset led(max) 4r 7500 i = eq. 2 alternatively, the default maximum value of 187.5 ma per channel is used when the trset pin is grounded. the t orch - mode operation is activated by a sserting ten high or by setting the i 2 c torch register (address 02h) for the desired duration. the t orch - mode current is terminated when ten is brought low and the i 2 c torch r egister is cleared. li ke the flash - mode current, the t orch - mode current can be se t to a fraction of the maximum t orch - mode level (either default or set by the trset resi stor) by selecting the desired t orch c urrent level percentage in the torch r egister (address 02h) through the i 2 c interface. the t orch current is the product of the max imum t orch current setting and the percentage selected in the torch r egister. overv oltage protection when the output voltage rises above the overvoltage protection ( ovp ) threshold, the mic2870 is turned off automatically to avoid permanent damage to the ic . open - circuit detection the open- circuit detector (ocd) is active only when the led current regulator is turned on. when the external led is missing or fails open, the led1/2 pin voltage is pulled to near the ground potential by the internal current sink . if both leds are open or missing, the open - circuit detector would force the boost regulator and led current regulator to turn off. the mic2870 will try to turn on the boost regulator and led current regulator again after a 100ms timeout. however, in most practical cases, the boost output voltage would rise above the ovp threshold, when both led channels have an open fault. the ovp function would cause the mic2870 to shut down. short - circuit detection like the ocd , the short - circuit detector is active onl y when the current regulator is turned on. if either one or both of the external leds fail a short, the short - circuit detector would force the mic2870 to turn off. the mic2870 will try to turn on the boost regulator and led current regulator again after a 100ms timeout. if the short condition persists, the whole cycle repeats again. prolonged operation in short - circuit condition is not recommended as it can damage the device.
micrel, inc. mic2870 september 5, 2013 15 revision 1.0 i 2 c interface figure 2 shows the communications required for write and read operations via the i 2 c interface. the black lines show master communications and the red lines show the slave communications. during a write operation the master must drive sda and scl for all stages except the acknowl edgement (a) shown in red, which are provided by the slave (mic2870): figure 2 . i 2 c timing example the read operation begins firstly with a data - less write to select the register address from which to read. then a restart sequence is issued, and then a read command followed by the data read. the mic2870 responds to a slave address of hex 0xb4 and 0xb5 for write and read operations respectively, or binary 1011010x (where x is the read/write bit). the register address is eight bits wide and carries the address of the mic2870 register to be operated upon. only the lower three bits are used. i2c registers mic2870 contains three 8 - bit read/write registers having an address from 00h to 02h for operation control as shown in table 1 . these registers are reset to their default values in power - on -r eset ( por ) event. in other words, they hold their previous contents when the chip is shutdown as long as supply voltage is above 1.5v (typical). tab le 1 . mic2870 register map register address register name description 00h master control/ status chip e nable control and s tatus register 01h flash control f lash - mode current , flash - mode e nable , and flash timeout control register 02h torch control torch - mode current and torch - mode e nable control register master control / status register [00h] the master control / status register allows the mic2870 to be enabled by the i 2 c interface -- setting the on [ ] bit high has the same effect as asserting en pin. the led short bit, led_sht[ ] is set if any or both of the led is shorted to out, while the led open bit, led_op[ ] is asserted only when both led are open circuit. the thermal shutdown bit, tsd[ ] is set when the junction temperat ure of the mic2870 is higher than 160 c. flash control register [01h] t he flash safety timer and flash - mode current are configurable via the flash control register. refer to flash tim eout duration setting and flash - mode current setting in table 4 and table 5. torch control register [02h] the torch - mode current is configurable via the torch control register. refer to torch - mode current setting in table 7 . the fi[ ] bit has the same function as the fi pin. when the fi[ ] bit is set, the flash - mode current is reduced to the torch - mode current setting.
micrel, inc. mic2870 september 5, 2013 16 revision 1.0 table 2 . master control register [00h] bit d7 d6 d5 d4 d3 d 2 d1 d0 name reserved on led_sht led_op tsd access r r/w r default value 0 table 3 . flash control register [01h] bit d7 d6 d5 d4 d3 d2 d1 d0 name ftmr fen fcur access r/w default value 111 0 0000 table 4 . flash safety timer setting (ftmr) register value [d7:d5] of 01h flash timeout duration (ms) 111 1250 110 1093.75 101 937.5 100 781.25 011 625 010 468.75 001 312.5 000 156.25
micrel, inc. mic2870 september 5, 2013 17 revision 1.0 table 5 . flash - mode current setting ( fcur) percentage of maximum current / % register value [d3:d0] of 01h current p er channel (ma) (r frset = 0 ? ) combined current (ma) (r frset = 0 ? ) 100 0000 750.0 1500.0 90 0001 675.0 1350.0 80 0010 600.0 1200.0 70 0011 525.0 1050.0 63 0100 472.5 945.0 56 0101 420.0 840.0 50 0110 375.0 750.0 44.7 0111 335.3 670.5 39.8 1000 298.5 597.0 35.5 1001 266.3 532.5 31.6 1010 237.0 474.0 28.2 1011 211.5 423.0 25.1 1100 188.3 376.5 22.4 1101 168.0 336.0 20 1110 150.0 300.0 18 1111 135.0 270.0 table 6 . torch control register [02h] bit d7 d6 d5 d4 d3 d2 d1 d0 name reserved fi ten tcur access ro r/w default value 0 0000
micrel, inc. mic2870 september 5, 2013 18 revision 1.0 table 7 . torch - mode current setting (tcur) percentage of maximum current ( % ) re gister value [d3:d0] of 02h current per channel (ma ) (r trset = 0 ? ) combined current ( ma ) (r trset = 0 ? ) 100 0000 187.5 375.0 90 0001 168.8 337.5 80 0010 150.0 300.0 70 0011 131.3 262.5 63 0100 118.1 236.3 56 0101 105.0 210.0 50 0110 93.8 187.5 44.7 0111 83.8 167.6 39.8 1000 74.6 149.3 35.5 1001 66.6 133.1 31.6 1010 59.3 118.5 28.2 1011 52.9 105.8 25.1 1100 47.1 94.1 22.4 1101 42.0 84.0 20 1110 37.5 75.0 18 1111 33.8 67.5
micrel, inc. mic2870 september 5, 2013 19 revision 1.0 component selection inductor inductor selection is a balance betwe en efficiency, stability, cost, size, and rated current. since the boost converter is compensated internally, the recommended 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 led 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 required 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 same inductance usually has a larger size. the saturation current rating of the selected inductor must be higher than the maximum peak induc tor current to be encountered and should be at least 20% to 30% higher than the average inductor current at maximum output current. input capacitor a ceramic capacitor of 4.7 f or larger with low esr is recommended to reduce the input voltage ripple to ens ure a clean supply voltage for the device. the input capacitor should be placed as close as possible to the mic2870 vin pin with short trace for good noise performance. x5r or x7r type ceramic capacitors are recommended for better tolerance over temperatur e. the y5v and z5u type temperature rating ceramic capacitors are not recommended due to their large reduction in capacitance 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 operating temperature range. output capacitor output capacitor selection is also a trade - off between performance, size, and cost. increasin g output capacitor will lead to an improved transient response, however, the size and cost also increase. th e output capacitor is preferred in the range of 2.2f to 10f with esr from 10m? to 50m?. x5r or x7r type ceramic capacitors are recommended for better tolerance over temperature. the y5v and z5u type ceramic capacitors are not recommended due to their wide variation in capacitance over temperature and increased 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/trse t resistor since frset/trset resistor is used for setting the maximum led current in flash mode and torch mode respectively, resistor type with 0.1% tolerance is recommended for more accurate led current setting.
micrel, inc. mic2870 september 5, 2013 20 revision 1.0 power dissipation consideration as with a ll power devices, the ultimate current rating of the output is limited by the thermal properties of the device package and the pcb on which the device is mounted. there is a simple, ohm? s law type relationship between thermal resistance, power dissipation and temperature which are analogous to an electrical circuit: figure 3 . series electrical 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 equatio n 3: z yz xy source x v ) r (r i v + + = eq. 3 thermal circuits can be considered using this same rule and can be drawn similarly by replacing current sources with power dissipation (in watts), resistance with thermal resistance (in c/w) and voltage sources with tem perature (in c). figure 4 . series thermal resistance circuit now replacing the variables in the equation for v x , 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. 4 as can be seen in the diagram, total thermal resistance ja = jc + ca . hence this can also be written as in equation 5: a ja diss j t ) ( p t + = eq. 5 since effectively all of the power losses (minus the inductor losses) in the converter are dissipated within the mic2870 package, p diss can be calculated thus: linear mode: dcr 2 i ] 1 1 [p p out out diss ? ? = ? ? ? ? ? ? eq. 6 boost mo de: dcr 2 d 1 i ] 1 1 [p p out out diss ? ? ? = ? ? ? ? ? ? ? ? ? ? ? ? eq. 7 duty cycle in boost mode: out in out v v v d ? = eq. 8 where: = efficiency taken from efficiency curves and dcr = i nductor dcr. jc and ja are found in the operating ratings section of the data sheet. 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 5 . figure 5 is taken from designing with low dropout voltage regulators available from the micrel w ebsite .
micrel, inc. mic2870 september 5, 2013 21 revision 1.0 figure 5 . graph to determine pc board area for a given pcb thermal resistance figure 5 shows the total area of a round or square pad, centered on the device. the solid trace represents the area of a square, single sided, horizontal, 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 approaches a ?best case? pad heat sink. conservative design dictates using the solid trace data, which indicates that a maximum pad size of 5000 mm 2 is needed. this is a pad 71mm 71mm (2.8 inches per side).
micrel, inc. mic2870 september 5, 2013 22 revision 1.0 pcb layout guidelines 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. the following guidelines shou ld 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 ground (agnd) and power ground (pgnd) s hould be kept separate and connected at a single location. ? the exposed pad ( epad ) on the bottom of the ic must be connected to the pgnd ground plane of the pcb . ? 4 to 6 thermal vias must be placed on the pcb pad for exposed pad and connected it to the pgnd ground plane to ensure a good pcb thermal resistance can be achieved. vin decoupling capacitor ? 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 mu st be located right at the ic. ? the vin decoupling capacitor should be connected to analog ground (agnd). ? 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, place a ground plane underneath the inductor. output capacitor ? use wide and short traces to connect the output capacitor to the out and pgnd pins. ? place several vias to the ground plane close to the output capacitor ground terminal. ? use either x5r or x7r 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 th e led cathode to the led1/led2 pins. ? make sure that the led?s pcb land pattern can provide sufficient pcb pad heat sink to the flash led. frset/trset resistor ? the frset/trset resistor should be placed close to the frset/trset pin and connected to agnd.
micrel, inc. mic2870 september 5, 2013 23 revision 1.0 typical application schematic bill of materials item part number manufacturer description qty. c3 0603z d475kat2a avx ( 8 ) ceramic capacitor 4.7f, 10 v, x 5 r, 0603 1 c1608x5r1a475k080ac tdk ( 9 ) c4 0603zd225kat2a avx ceramic capacitor 2.2 f, 10 v, x 5 r, 0603 1 grm188r61a225ke34d murata ( 10 ) c1608x5r1a225k08 0ac tdk c5 06033 d104ka j 2a avx ceramic capacitor 0.1 f, 25 v, x 5 r, 0603 1 grm188r 61c 104ka01d murata c1608x5r1 e104k tdk c6 0603z d105kat2a avx ceramic capacitor 1 f, 10 v, x 5 r, 0603 1 grm188r61a105ka 6 1d murata c1608x5r1a105k080ac tdk l1 cdrh4d28cldnp - 1r0p c sumida ( 11 ) inductor 1 h, 3.0a, 14m?, l5.1mm w5.1mm h3.0mm 1 lqh44pn1r0n p0l murata inductor 1 h, 2. 45 a, 36 m?, l4.0mm w4.0mm h1. 65 mm r1, r2 crcw0 60 34k7 0fkea vishay/dale ( 12 ) resistor 4.7k?, 1%, 1/1 0w, 0603 2 r3, r4 era3aeb103v panasonic ( 13 ) resistor 10k?, 0. 1%, 1/10w, 0603 2 d1, d2 sml - lxl99uwc - tr/5 lumex ( 14) led sq 5w cool wht 6000k smd , 190lm 2 u1 mic2870 yft micrel, inc. ( 15 ) 1.5a synchronous boost flash led driver with i 2 c inte r face 1 notes: 8. avx: www.avx.com . 9. tdk: www.tdk.com . 10. murata: www.murata.com . 11. sumida: www.sumida.com . 12. vishay: www.vishay.com . 13. panasonic: www.panasonic.com . 14. lumex: www.lumex.com . 15. micrel, inc.: www.micrel.com .
micrel, inc. mic2870 september 5, 2013 24 revision 1.0 pcb layout recommendations top layer bottom layer
micrel, inc. mic2870 september 5, 2013 25 revision 1.0 package information ( 16) 16 -pin 2mm 2mm tqfn ( ft ) note: 16. package information is correct as of the publication date. for updates and most current information, go to www.micrel.com . micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel +1 (408) 944 - 0800 fax +1 (408) 474- 1000 we b 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 information 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 estoppel or otherwise, to any intellectual prope rty 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 warranty 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 designed or authorized for use as components in lif e 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, devices or systems is a purchaser?s own risk and purcha ser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 20 13 micrel, incorporated.

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