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simplified operating circuit appears at end of data sheet. notebook, subnotebook, and tablet computer displays handy terminals general description the max17105 is a high-efficiency driver for white-light- emitting diodes (wleds). it is designed for large liquid- crystal displays (lcds) that employ an array of leds as the light source. an internal switch-current mode step-up controller drives the led array, which can be configured for up to 8 strings in parallel and 10 leds per string. each string is terminated with ballast that achieves 2% current regulation accuracy, ensuring even led brightness. the max17105 has a wide input voltage range from 6v to 28v, and provides adjustable 0 to 30ma full-scale led current. the max17105 can internally generate a digitally adjust - ed pulse-width modulation (dpwm) signal for accurate wled dimming control. the dpwm frequency is resis - tor programmable, while dpwm duty cycle is controlled directly from an external pwm signal or through a control word through the max17105s smbus interface. this dpwm control provides a dimming range with 8-bit reso - lution and supports intel display power-saving technology (dpst) to maximize battery life. the max17105 also has direct pwm control mode, in which the pwmi directly con - trols the led current turn on/off and the smbus interface is disabled. the max17105 has multiple features to protect the con - troller from fault conditions. separate feedback loops limit the output voltage under any circumstance, ensur - ing safe operation. once an open string is detected, the string is disabled while other strings operate normally. the max17105 also features short led detection. the shorted strings are also disabled. the controller features cycle-by-cycle current limit to provide constant operation and soft-start capability. if the max17105 is in current- limit condition, the step-up regulator is latched off after an internal timer expires. a thermal-shutdown circuit provides another level of protection. when the input overcurrent- limit fault, thermal shutdown, or output-voltage short con - dition happens, the input p-channel mosfet is turned off and the step-up regulator output is isolated from the input supply. the max17105 is available in a thermally enhanced, 24-pin, 4mm x 4mm, thin qfn package with exposed pad. applications features 6v to 28v input supply voltage up to eight parallel-string multiple series connected leds 500khz-to-2mhz adjustable switching frequency 0.15? internal hv power mosfet (45v max) low-string feedback voltage: 500mv at 20ma led current full-scale led current adjustable from 0ma to 30ma 2% current regulation accuracy between strings 100:1 dimming ratio at 25khz direct pwm frequency 100hz to 30khz pwmi input range for dpwm mode and 10khz 5% for smbus mode 100hz to 5khz adjustable dpwm frequency full-range dimming with 8-bit resolution open and short led protection output overvoltage protection thermal shutdown fault output to drive p-channel mosfet small 24-pin, 4mm x 4mm, thin qfn package 19-4562; rev 2; 9/14 +denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad. part temp range pin-package max17105etg+ -40c to +85c 24 tqfn-ep* 2324 22 21 87 9 iset osc ovp v cc 10 comp fb8fb6 fb5 pgndfb4 1 2 scl 4 5 6 17 18 16 14 13 sda dfset fb1sgnd fault in pwmo fb7 3 15 en 20 11 fb2 pwmi 19 12 fb3 lx thin qfn 4mm x 4mm top view max17105 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability pin coniguration ordering information evaluation kit available downloaded from: http:///
fault, in to sgnd .............................................. -0.3v to +30v fb_, lx to pgnd .................................................. -0.3v to +45v pgnd to sgnd .................................................... -0.3v to +0.3v v cc , en, pwmi, sda, scl to sgnd .................... -0.3v to +6v comp, iset, osc, ovp, pwmo, dfset to sgnd ......................... -0.3v to (v cc + 0.3v) lx switch maximum continuous rms current ................... 1.6a continuous power dissipation (t a = +70c) thin qfn (derate 20.8mw/c above +70c) ............ 1667mw (single-layer board) thin qfn (derate 27.8mw/c above +70c) ............ 2222mw (multilayer board) operating temperature range ........................... -40c to +85c junction temperature ...................................................... +150c storage temperature range ............................ -65c to +150c lead temperature (soldering, 10s) ................................. +300c (circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k?, r iset = 50k?, r osc = 100k?, r dfset = 250k?, pwmi = sgnd, c pwmo = 1f, t a = 0c to +85c , unless otherwise noted. typical values are at t a = +25c.) parameter conditions min typ max units supply in input voltage range (note 1) v cc = open, en = high 5.5 28 v en = sgnd, smbus mode(note 5) all functions available 6.3 28 smbus interface only 3.9 4.0 in input voltage uvlo threshold (smbus mode only) (note 4) rising edge, hysteresis = 150mv 5.9 6.05 6.2 v falling edge, hysteresis = 150mv 5.75 5.9 6.05 in quiescent current max17105 is enabled at minimum brightness, in smbus mode and no-load, v in = 28v 2.0 3.0 ma max17105 is enabled in smbus mode and is under in uvlo, v in = 5.5v 0.7 1.1 ma max17105 is disabled 30 60 a v cc output voltage max17105 is enabled, 6v < v in < 28v, 0 < i vcc < 10ma 4.7 5.0 5.3 v max17105 is disabled, v in = 12v 3.7 4.6 4.95 v cc current limit v cc is forced to 4.5v 15 40 70 ma v cc uvlo threshold rising edge, typical hysteresis = 85mv 4.00 4.3 4.45 v step-up regulator lx on-resistance 100ma from lx to pgnd 0.15 0.3 w lx leakage current 40v on lx, t a = +25c 1 a operating frequency r osc = 50k w 1.7 2.0 2.3 mhz r osc = 100k w 0.9 1.0 1.1 r osc = 200k w 0.4 0.5 0.6 minimum on-time (note 1) 50 ns maximum duty cycle at f sw = 1mhz 93 95 97 % minimum off-time 50 ns www.maximintegrated.com maxim integrated 2 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability electrical characteristics absolute maximum ratings stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. downloaded from: http:///
(circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k?, r iset = 50k?, r osc = 100k?, r dfset = 250k?, pwmi = sgnd, c pwmo = 1f, t a = 0c to +85c , unless otherwise noted. typical values are at t a = +25c.) parameter conditions min typ max units lx current limit duty cycle = 75% (note 1) 2.4 2.9 3.4 a control input logic-input high level sda, scl, pwmi, en 2.1 v logic-input low level sda, scl, pwmi, en 0.8 v input leakage pwmi leakage current t a = +25c -0.1 +0.1 a logic input bias current t a = +25 c, en, sda, scl -1 +1 a ovp leakage current ta = +25c -0.1 +0.1 a sda output-low sink current v sda = 0.4v 4 ma led currentfull-scale fb_ output current- adjustable range (note 4) pwm-only dimming mode 15 30 ma smbus-enabled dimming modes 15 25 full-scale fb_ output current r iset = 33.3k w 29.1 30.0 30.9 ma r iset = 50.0k w 19.4 20.0 20.6 r iset = 66.6k w 14.45 15.0 15.55 v iset < 0.4v 0.2 0.3 0.4 iset output voltage 1.1 1.2 1.3 v current regulation between strings i fb_ = 30ma -2.0 +2.0 % i fb_ = 20ma -2.0 +2.0 i fb_ = 15ma -2.0 +2.0 minimum fb_ regulation voltage i fb_ = 30ma 770 mv i fb_ = 20ma 480 i fb_ = 15ma 450 fb_ on-resistance v fb_ = 50mv 15 26 w fb_ leakage current v fb_ = 40v, t a = +25c 0.1 5 a fb_ on-time 400 ns fault protection ovp threshold voltage rising edge, hysteresis = 60mv 1.15 1.25 1.35 v ovp shutdown voltage rising edge 1.35 v ovp global fail 48 120 mv fb_ uvlo threshold fb open 140 220 300 mv fb_ overvoltage threshold 7.4 8 8.6 v fb_ check led source current 0.4 ma fb_ check led time 1 ms www.maximintegrated.com maxim integrated 3 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability electrical characteristics (continued) downloaded from: http:///
(circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k?, r iset = 50k?, r osc = 100k?, r dfset = 250k?, pwmi = sgnd, c pwmo = 1f, t a = 0c to +85c , unless otherwise noted. typical values are at t a = +25c.) parameter conditions min typ max units fb_ overvoltage fault timer 88 128 168 s thermal-shutdown threshold (note 1) +150 c overcurrent fault shutdown timer i peak > 2.9a at duty = 75% 72 128 168 s fault high-level leakage current v fault = v in = 28v, t a = +25c 1 a fault low-level clamp voltage v in C 5.5 v in - 5 v in C 4.5 v fault charge current 45 65 a pwm filter pwm output impedance 20 40 60 k w dpwm oscillator frequency r dfset = 500k w 100 hz r dfset = 250k w 190 200 210 r dfset = 25k w 2 khz r dfset = 10k w 4.4 5.0 5.5 dfset short-detection threshold voltage 210 300 mv dfset source current r dfset = 500k w 4.15 a r dfset = 250k w 7.85 r dfset = 25k w 40 r dfset = 10k w 55 pwmi input frequency range dpst/smbus mode 9.5 10.0 10.5 khz direct-pwm mode 0.1 30.0 pwmi brightness setting smbus mode, pwmi duty cycle = 98% 97.5 98 98.5 % smbus mode, pwmi duty cycle = 50% 48 50 52 smbus mode, pwmi duty cycle = 0% 0.2 0.4 0.6 smbus timing specification smbus frequency f smb 10 100 khz bus free time t buf 4.7 s start condition hold time from scl t hd:sta 4 s start condition setup time from scl t su:sta 4.7 s stop condition setup time from scl t su:sto 4 s sda hold time from scl t hd:dat 300 ns sda setup time from scl t su:dat 250 ns scl low period t low 4.7 s scl high period t high 4 s www.maximintegrated.com maxim integrated 4 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability electrical characteristics (continued) downloaded from: http:///
(circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k?, r iset = 50k?, r osc = 100k?, r dfset = 250k?, pwmi = sgnd, c pwmo = 1f, t a = -40c to +85c , unless otherwise noted. typical values are at t a = +25c.) (note 2) parameter conditions min typ max units supply in input voltage range (note 1) v cc = open, en = high (note 5) 5.5 28 v en = sgnd, smbus mode all functions available 6.3 28 smbus interface only 3.9 4.0 in input voltage uvlo threshold (smbus mode only) (note 2) rising edge, hysteresis = 150mv 5.9 6.2 v falling edge, hysteresis = 150mv 5.75 6.05 in quiescent current max17105 is enabled at minimum brightness, in smbus mode and no load, v in = 28v 3.0 ma max17105 is enabled in smbus mode, and is under in uvlo, v in = 5.5v 1.1 max17105 is disabled 60 a v cc output voltage max17105 is enabled, 6v < v in < 28v, 0 < i vcc < 10ma 4.7 5.3 v max17105 is disabled, v in = 12v 3.7 4.95 v cc current limit v cc is forced to 4.5v 15 70 ma v cc uvlo threshold rising edge, typical hysteresis = 85mv 4.00 4.45 v step-up regulator lx on-resistance 100ma from lx to pgnd 0.3 w operating frequency r osc = 50k w 1.7 2.3 mhz r osc = 100k w 0.9 1.1 r osc = 200k w 0.45 0.6 maximum duty cycle at f sw = 1mhz 93 98 % lx current limit duty cycle = 75% 2.4 3.4 a control input logic-input high level sda, scl, pwmi, en 2.1 v logic-input low level sda, scl, pwmi, en 0.8 v sda output-low sink current v sda = 0.4v 4 ma sda, scl input bias current t a = +25nc -1 +1 a led current full-scale fb_ output current adjustable range (note 3) pwm only dimming mode 15 30 ma smbus-enabled dimming modes 15 25 full-scale fb_ output current r iset = 33.3k w 28.8 31.2 ma r iset = 50.0k w 18.9 21 r iset = 66.6k w 14.25 15.75 v iset < 0.4v 0.2 0.4 current regulation between strings i fb _ = 30ma -2.5 +2.5 % i fb _ = 20ma -2.5 +2.5 i fb _ = 15ma -2.5 +2.5 minimum fb_ regulation voltage i fb _ = 30ma 770 mv fb_ on-resistance v fb _ = 50mv 26 w fb_ on-time 400 ns www.maximintegrated.com maxim integrated 5 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability electrical characteristics downloaded from: http:///
(circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k?, r iset = 50k?, r osc = 100k?, r dfset = 250k?, pwmi = sgnd, c pwmo = 1f, t a = -40c to +85c , unless otherwise noted. typical values are at t a = +25c.) (note 2) parameter conditions min typ max units fault protection ovp threshold voltage rising edge, hysteresis = 60mv 1.15 1.35 v ovp shutdown voltage rising edge (tracks with ovp threshold voltage) 1.25 1.45 v ovp global fail 48 120 mv fb_ uvlo threshold fb open 140 300 mv fb_ overvoltage threshold 7.4 8.6 v fb_ check led source current 0.4 ma fb_ overvoltage fault timer 88 168 s overcurrent fault shutdown timer i peak > 2.9a at duty = 75% (typical) 72 168 s fault high-level leakage current v fault = v in = 28v 1 a fault low-level clamp voltage v in C 5.5 v in C 4.5 v fault charge current 45 65 a pwm filter pwm output impedance 20 60 ki dpwm oscillator frequency r dfset = 250k w 190 210 hz r dfset = 10k w 4.4 5.5 khz dfset short-detection threshold voltage 300 mv pwmi input frequency range dpst/smbus mode 9.5 10.5 khz direct-pwm mode 0.1 30.0 pwmi brightness setting smbus mode, pwmi duty cycle = 98% 97.5 98.5 % smbus mode, pwmi duty cycle = 50% 48 52 smbus mode, pwmi duty cycle = 0% 0.2 0.6 smbus timing specification smbus frequency f smb 10 100 khz bus free time t buf 4.7 s start condition hold time from scl t hd:sta 4 s start condition setup time from scl t su:sta 4.7 s stop condition setup time from scl t su:sto 4 s sda hold time from scl t hd:dat 300 ns sda setup time from scl t su:dat 250 ns scl low period t low 4.7 s scl high period t high 4 s www.maximintegrated.com maxim integrated 6 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability electrical characteristics (continued) downloaded from: http:///
(t a = +25c, unless otherwise noted.) note 1: specifications are guaranteed by design, not production tested. note 2: specifications to t a = -40c are guaranteed by design, not production tested. note 3: led full-scale current maximum value is subjected to string number, led number per string, and lx current limit. in smbus modes, if the total load is heavier than 8 strings with 10 wleds per string at 25ma led current, upon step-up regulator input v s removal, an oc fault may occur, resulting in smbus status register ov_curr and fault bits being set to 1 and violating smbus specifications. note 4: minimum voltage drop from v s to in pin, including the forward voltage drop of diode din1 used in figure 1, should be no less than 0.8v. if lower forward voltage drop diode is used, upon step-up regulator input vs removal, an oc fault may occur, resulting in smbus status register ov_curr and fault bits being set to 1 and violating smbus specifications. note 5: dimming modes not to be done after the driver starts up or on-the-fly. circuit of figure 1. v in = 12v, c comp = 33nf, r comp = 1k w , r iset = 50k w , rosc = 100k w , r dfset = 250k w , pwmi = sgnd, c pwmo = 1f, t a = -40 c to +85 c , unless otherwise noted. typical values are at t a = +25c.) (note 2) led current vs. brightness setting pwm duty cycle (%) led current (ma) 40 60 80 100 20 5 1510 20 0 0 max17105 toc03 direct-pwm mode boost converter efficiency vs. brightness (v s = 12v, v out = 32v/i out = 160ma at 100%) brightness (%) efficiency (%) 40 60 80 100 20 60 8070 9050 0 max17105 toc02 directpwm mode without external p-channel mosfet boost converter efficiency vs. input voltage (v s ) (v out = 32v/i out = 160ma, brightness = 100%) input voltage (v) efficiency (%) 13 16 19 22 10 8786 8988 9190 85 84 7 max17105 toc01 without external p-channel mosfet maxim integrated 7 www.maximintegrated.com max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability typical operating characteristics electrical characteristics (continued) downloaded from: http:///
(t a = +25c, unless otherwise noted.) switching waveforms (v s = 7v, brightness = 100%) 1s/divw v lx 20v/divinductor current 500ma/div 0a 0v max17105 toc09 in shutdown current vs. in voltage in voltage (v) shutdown current (a) 13 16 19 22 10 3226 23 29 3520 7 max17105 toc08 direct-pwm mode smbus mode in quiescent current vs. in voltage in voltage (v) quiescent current (ma) 13 16 19 22 10 1 53 2 4 60 7 max17105 toc07 smbus = 0 smbus = 255 led current vs. input voltage input voltage (v) led current (ma) 13 16 19 22 10 5 2010 15 25 0 7 max17105 toc06 brightness = 10% direct-pwm mode brightness = 100% led current vs. pwmi duty cycle pwmi duty cycle led current (ma) 40 60 80 100 20 5 1510 20 0 0 max17105 toc05 smbus = 128 dpst mode smbus = 255 led current vs. smbus brightness setting smbus brightness setting led current (ma) 100 150 200 250 50 5 1510 20 0 0 max17105 toc04 smbus mode maxim integrated 8 www.maximintegrated.com max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability typical operating characteristics (continued) downloaded from: http:///
(t a = +25c, unless otherwise noted.) led-open fault protection (brightness = 100%, led open on fb1) 400s/div v out 20v/div v fb1 1v/div0v 0v0v v fb2 10v/divi fb2 20ma/div0a max17105 toc15 led current waveforms (brightness = 1%) 2ms/div inductor current 1a/divv out 20v/div v fb1 10v/div0v 0vi fb1 20ma/div0a 0a max17105 toc14 direct-pwm mode led current waveforms (brightness = 50%) 2ms/div inductor current500ma/div v out 20v/div v fb1 10v/div0v 0vi led 20ma/div0a 0a max17105 toc13 startup waveforms (brightness = 20%) 2ms/div v lx 20v/divv out 20v/div v en 5v/div0v 0vv fault 10v/div0v 0v max17105 toc12 direct-pwm mode startup waveforms (brightness = 100%) 2ms/div v lx 20v/divv out 20v/div v en 5v/div0v 0vv fault 10v/div0v 0v max17105 toc11 direct-pwm mode switching waveforms (v s = 24v, brightness = 100%) 1s/div v lx 20v/div0v inductor current 500ma/div 0a max17105 toc10 maxim integrated 9 www.maximintegrated.com max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability typical operating characteristics (continued) downloaded from: http:///
(t a = +25c, unless otherwise noted.) led current balance vs. input voltage input voltage (v) led current balance (%) 13 19 16 22 10 0.90.7 0.6 0.8 1.00.5 7 max17105 toc20 led current balance vs. brightness brightness (%) led current balance (%) 30 50 40 70 60 90 100 80 20 1.20.4 0.8 1.6 0 10 max17105 toc19 direct-pwm mode line-transient response (v s = 21v ? 9v, brightness = 100%) 200s/div v out 20v/div0a 0v0v v s 10v/divi fb1 20ma/div inductor current1a/div 0a max17105 toc18 line-transient response (v s = 9v ? 21v, brightness = 100%) 200s/div inductor current1a/div v out 20v/div0a 0a 0v0v v s 10v/divi fb1 20ma/div max17105 toc17 led-short fault protection (brightness = 100%, 3leds short on fb1) 400s/div i fb1 50ma/div v fb1 10v/div0a 0v0v v fb2 5v/divi fb2 50ma/div 0a max17105 toc16 maxim integrated 10 www.maximintegrated.com max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability typical operating characteristics (continued) downloaded from: http:///
pin name function 1 comp step-up regulator compensation pin. connect a 0.033f ceramic capacitor and 1k w resistor from comp to sgnd and an additional 220pf capacitor from comp to sgnd. when the max17105 shuts down, comp is discharged to 0v through an internal 20k resistor. 2 iset full-scale led current adjustment pin. the resistance from iset to sgnd controls the full-scale current in each led string: i led_max = 20ma x 50k/r iset the acceptable resistance range is 33.3k < r iset < open, which corresponds to full-scale led current of 30ma > i led_max > 0ma. connecting iset to sgnd sets the test mode for 0.3ma (typ) full-scale led current. 3 pwmo filtered pwm signal output. connect a capacitor between pwmo and sgnd. the capacitor forms a lowpass ilter with an internal 40k (typ) resistor to ilter the pwm signal into an analog signal whose level represents the duty cycle information of the input pwm signal. 4 osc oscillator frequency adjustment pin. the resistance from osc to sgnd sets the step-up regulators oscillator frequency: f sw = 1mhz x 100k/r osc the acceptable resistance range is 50k < r osc < 200k, which corresponds to the switching frequency of 2mhz > f s w > 500khz . 5 ovp overvoltage sense. connect ovp to the center tap of a resistive voltage-divider from the output of the step-up regulator to ground. 6 v cc 5v linear-regulator output. v cc provides power to the max17105. bypass v cc to sgnd with a ceramic capacitor of 1f or greater. 7 in power-supply input. v in biases the internal 5v linear regulator that powers the device. bypass in to sgnd directly at the pin with a 0.1f ceramic capacitor or greater. 8 fault external p-channel mosfet gate drive-output. external pullup resistor is connected between fault and in when p-channel mosfet is used. if the p-channel mosfet is not used, leave fault unconnected. 9 sgnd analog ground 10 fb1 led string 1 cathode connection. fb1 is the open-drain output of an internal regulator, which controls current through fb1. fb1 can sink up to 30ma. if unused, connect fb1 to sgnd. 11 fb2 led string 2 cathode connection. fb2 is the open-drain output of an internal regulator, which controls current through fb2. fb2 can sink up to 30ma. if unused, connect fb2 to sgnd. 12 fb3 led string 3 cathode connection. fb3 is the open-drain output of an internal regulator, which controls current through fb3. fb3 can sink up to 30ma. if unused, connect fb3 to sgnd. 13 fb4 led string 4 cathode connection. fb4 is the open-drain output of an internal regulator, which controls current through fb4. fb4 can sink up to 30ma. if unused, connect fb4 to sgnd. 14 fb5 led string 5 cathode connection. fb5 is the open-drain output of an internal regulator, which controls current through fb5. fb5 can sink up to 30ma. if unused, connect fb5 to sgnd. 15 fb6 led string 6 cathode connection. fb6 is the open-drain output of an internal regulator, which controls current through fb6. fb6 can sink up to 30ma. if unused, connect fb6 to sgnd. 16 fb7 led string 7 cathode connection. fb7 is the open-drain output of an internal regulator, which controls current through fb7. fb7 can sink up to 30ma. if unused, connect fb7 to sgnd. 17 fb8 led string 8 cathode connection. fb8 is the open-drain output of an internal regulator, which controls current through fb8. fb8 can sink up to 30ma. if unused, connect fb8 to sgnd. www.maximintegrated.com maxim integrated 11 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability pin description downloaded from: http:///
pin name function 18 pgnd step-up regulator power ground 19 lx step-up regulator switching node. drain of the internal n-channel mosfet between lx and pgnd. connect the inductor and catch diode here and minimize trace area for lowest emi. 20 pwmi pwm signal input. this pwm signal is used for brightness control in direct-pwm mode or dpst mode of smbus mode. in direct-pwm mode, the dpwm duty cycle is equal to the input pwm duty cycle. in dpst mode, the dpwm duty cycle is the input pwm duty cycle multiplied by the smbus brightness command. pwmi directly controls the led current source on or off in direct-pwm mode. 21 en direct-pwm mode enable pin. when direct-pwm mode is selected through the dfset pin, the max17105 can start up with direct-pwm mode by en = high. 22 scl smbus serial clock input 23 sda smbus serial data input 24 dfset dpwm frequency adjustment pin. connect a resistor from dfset to sgnd to set the internal dpwm frequency in smbus mode. in direct-pwm mode, dfset pin is connected to sgnd: f dpwm = 200hz x 250k/r dfset the acceptable resistance range is 10k < r dfset < 500k, which corresponds to the dpwm frequency of 5khz > f dpwm > 100hz. ep exposed backside pad. solder to the circuit board ground plane with suficient copper connection toensure low thermal resistance. see the pcb layout guidelines section. www.maximintegrated.com maxim integrated 12 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability pin description (continued) downloaded from: http:///
detailed description the max17105 typical operating circuit is shown in figure 1. table 1 lists some recommended components, and table 2 lists the contact information of component suppliers. table 2. component suppliers table 1. component list figure 1. typical operating circuit supplier phone website fairchildsemiconductor 888-522-5372 www.fairchildsemi.com murata 770-436-1300 www.murata-northamerica.com nichia corp. 248-352-6575 www.nichia.com tdk corp. 847-803-6100 www.component.tdk.com toshiba america electronic components, inc. 949-623-2900 www.toshiba.com/taec vishay 402-563-6866 www.vishay.com designation description white led nichia nssw008c 3.2v (typ), 3.5v (max) at 20ma l1 10h, 1.5a, h = 1.2mm tdk vlp6812t-100m1r5 q1 (optional) 30v, 65m w p-channel mosfet si3481dvfdc658ap c in 4.7f 10%, 25v x5r ceramiccapacitor (1206) murata grm319r61e475ka12d c1, c2 2.2f 20%, 50v x7r ceramiccapacitors (1206) murata grm31cr71h225k d1 2a, 40v schottky diode (m-flat) toshiba cms11 fb1 pgnd in fault r comp 5.1k w d1 q1 p optional l1 10h lx r1 2.21m w r ext 200k w c in 4.7f din1 0.1f vcca 5v v s 7v to 24v c pwmo 1f r osc 100k w r iset 50k w r dfset 250k w c comp2 0.022f c comp1 220f 1f smbus interface c2 2.2f r2 71.5k w ovp fb2fb3 fb4 fb5 fb6 fb7 fb8 iset v cc pwmipwmo 10khz max17105 dfseten scl sda osccomp ep c1 2.2f sgnd www.maximintegrated.com maxim integrated 13 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
the max17105 is a high-efficiency driver for arrays of white leds. it contains a fixed-frequency, current-mode pwm step-up controller, a 5v linear regulator, dimming control cir - cuit, smbus interface, internal power mosfet, eight regu - lated current sources, and an external p-channel mosfet drive circuit. figure 2 shows the max17105 functional diagram. when enabled, the step-up controller boosts the output voltage to provide sufficient headroom for the cur - rent sources to regulate their respective string currents. the max17105 features a resistor-adjustable switching fre- quency (500khz to 2mhz), which allows trade-offs between external component size and operating efficiency. figure 2. functional diagram 5v linear regulator oscillator slope compensation overvoltage comparator error comparator output overvoltage comparator ovplx n n pgnd error comparator in fb1sgnd fb2 fault control to fault control control and driver logic current sense hvc fb6fb5 fb4 fb3 fb2 lvc gm fb7 fb8 en v cc osc fault iset 8v 1.25v 1.35v iset en pwmo pwmi sda smbus control and dpwm generator vsat dpwm control signal current source scl dfset comp v cc fb3 current source fb4 current source fb5 current source fb6 current source fb7 current source fb8 current source max17105 www.maximintegrated.com maxim integrated 14 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
wled brightness is controlled by turning the wleds on and off with a dpwm signal. the dpwm frequency can be accurately adjusted with a resistor or directly controlled by the pwmi signal. the brightness of the leds is pro - portional to the duty cycle of the dpwm signal, which is controlled externally through either a pwm or 2-wire smbus-compatible interface, or both. when both inter - faces are used at the same time, the product of the pwm duty cycle and smbus command value is used for the dimming control. this dpwm control provides a dimming range with 8-bit resolution. the max17105 has multiple features to protect the controller from fault conditions. separate feedback loops limit the output voltage in all circumstances. the max17105 checks each fb_ voltage during the opera - tion. if one or more strings are open, the corresponding fb_ voltages are pulled below 220mv (typ), and an open-circuit fault is detected. as a result, the respective current sources are disabled. when one or more leds are shorted and the fb_ voltage exceeds 8v, a short fault is detected and the respective current source is disabled. in either led open or short conditions, the fault strings are disabled while other strings can still operate normally. the controller features cycle-by-cycle current limit to provide consistent operation and soft-start pro - tection. in a current-limit condition, the controller shuts down after a 128s overcurrent fault timer expires. a thermal-shutdown circuit provides yet another level of protection. the max17105 includes a 5v linear regulator that pro - vides the internal bias and gate driver for the step-up controller. fixed-frequency step-up controller the max17105s fixed-frequency, current mode, step-up controller automatically chooses the lowest active fb_ voltage to regulate the feedback voltage. specifically, the difference between the lowest fb_ voltage and the current source control signal plus an offset (vsat) is integrated at the comp output. the resulting error signal is compared to the external switch current plus slope compensation to determine the switch on-time. as the load changes, the error amplifier sources or sinks current to the comp output to deliver the required peak inductor current. the slope-compensation signal is added to the current-sense signal in order to improve stability at high duty cycles. internal 5v linear regulator and uvlo the max17105 includes an internal low-dropout (ldo) linear regulator (v cc ). when v in is higher than 5.5v, this linear regulator generates a 5v supply to power the internal pwm controller, control logic, and mosfet driver. the v cc voltage drops to 4.5v in standby. if v in is less than or equal to 5.5v, v cc and in can be connected together and powered from an external 5v supply. there is a body diode from v cc to in, so v in must be greater than v cc . (see figure 2.) the max17105 includes power-on reset (por) and undervoltage lockout (uvlo) features. por resets the fault latch and sets all the smbus resisters to their por values. por occurs when v cc rises above 2.8v (typ). the controller is disabled until v cc exceeds the uvlo threshold of 4.3v (typ). the hysteresis on v cc uvlo is approximately 85mv. in standby mode, the internal ldo is low-power mode with 60a (max) input current and regulated at 4.5v (typ). when en is high in direct-pwm mode or an enable command through the smbus interface, the internal ldo is enabled and regulated at 5.0v (typ). in addition to v cc uvlo, the max17105 has also implemented an in uvlo function. this in uvlo function is not activated in direct- pwm mode. however, when the controller is enabled with smbus, the in uvlo feature is enabled. if the in voltage is below its uvlo threshold, the step-up converter is shut off. the v cc pin should be bypassed to sgnd with a mini - mum 1f ceramic capacitor. startup the max17105 has two operating modes: direct-pwm mode and smbus mode. the two modes are identified by the dfset pin. at the first enable signal, either en = high or an smbus enable command, the dfset pin sources a current to check whether the dfset pin is connected to sgnd. when the dfset voltage is lower than 210mv (typ), the max17105 is set for direct-pwm mode. otherwise, the max17105 is set for smbus mode. once the mode is set, the mode is not cleared until the vcc is lower than the por voltage. when the max17105 is set for smbus mode, the en pin signal is ignored and the max17105 is woken by the smbus enable command. when the max17105 is set for direct- pwm mode, the smbus command is ignored and the max17105is woken by the en signal. table 3 summa - rizes the operating mode. www.maximintegrated.com maxim integrated 15 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
at startup, there are three phases. the first phase is p-channel mosfet soft-start time, the second is check led time (1ms), and the last phase is boost soft-start time (4ms). in the first phase, fault sink current is increased from 0a to 50a to avoid high inrush current caused by the p-channel mosfet turn-on. in check led time, the max17105 performs a diagnostic test of the led array. in the test phase, all fb_ are pulled up by 0.8ma (typ, 0.4ma min) current source during 1ms (typ). if some fb_ voltage is lower than 1.2v (max), the string is considered to be unused. therefore, when a string is not in use, it should be connected to sgnd. all other strings with fb_ higher than 1.2v (max) are detected as in use. after the led string diagnostic phases are finished, the step-up regulator starts. the total startup time is less than 10ms. figure 3 shows the sequence. standby and fault shutdown the max17105 can be placed into standby by en = low in direct-pwm mode or clearing bit 0 of the device control register (0x01) in smbus mode. when a critical failure is detected, the ic enters fault shutdown mode. in standby or fault shutdown mode, all functions of the ic are turned off, including the 5v linear regulator. only a crude linear regulator remains on providing a 4.5v (typ) output voltage to v cc , with 1a current sourcing capability. the fault/ status register is not reset in fault shutdown. when bit 0 of the device control register (0x01) is set to 1 in smbus mode or cycling the en pin, the max17105 exits fault shutdown mode and starts. the fault/status register is reset at startup. figure 3. startup timing sequence x = dont care table 3. operating mode en dfset scl, sda direct pwm (standby) low sgnd x direct pwm (startup) high sgnd x smbus (standby) x r dfset clock smbus (startup) x r dfset enable command v vcc v fault vout i fault 0ua pfet soft-start step-up regulator soft-start check led v in 0v0v 0v 0v www.maximintegrated.com maxim integrated 16 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
in standby or fault shutdown, the fault pin is pulled up to in with an internal resistor of 1m w (typ), and the p-chan - nel mosfet is turned off. an external resistor could be used as well (r ext > 200k w ). frequency selectionthe step-up regulator switching frequency can be adjust - ed through the external resistor-connected osc pin. the switching frequency adjustable range is 500khz to 2mhz. high-frequency (2mhz) operation optimizes the regulator for the smallest component size, at the expense of effi - ciency due to increased switching losses. low-frequency (500khz) operation offers the best overall efficiency, but requires larger components and pcb area. the maximum step-up regulator duty is limited by the 50ns (typ) mini - mum on-time.overvoltage protection to protect the step-up regulator when the load is open, or the output voltage becomes excessive for any reason, the max17105 features a dedicated overvoltage feedback input (ovp). the ovp pin is connected to the center tap of a resistive voltage-divider from the high-voltage output. the ovp pin has two levels of threshold. when the ovp pin voltage, v ovp , exceeds 1.25v (typ), a compara - tor turns off the internal power mosfet and prevents excessive voltage from damage. this step-up regulator switch is reenabled after the v ovp drops 60mv (typ) hys - teresis below the protection threshold. this overvoltage- protection feature ensures the step-up regulator fail-safe operation when the led strings are disconnected from the output. when the ovp pin voltage exceeds 1.35v (typ), the ic is in fault shutdown. led current sources maintaining uniform led brightness and dimming capabil - ity are critical for backlight applications. the max17105 is equipped with a bank of eight matched current sources. these specialized current sources are accurate to within 2%, and can be switched on- and off-pwm frequencies of up to 30khz in direct-pwm mode and 5khz in smbus mode. all led full-scale currents are identical and are set through the iset pin (0ma < i led < 30ma). the led cur - rent source has high-speed capability and allows 400ns minimum on-time within 400ns rise and fall time. the minimum voltage drop across each current source is 500mv (max) when the led current is 20ma. the low voltage drop helps reduce dissipation while maintaining sufficient compliance to control the led current within the required tolerances. the led current sources can be disabled by connecting the respective fb_ pin to sgnd at startup. when the ic is enabled, the controller scans settings for all fb_ pins. if a fb_ pin is not connected to sgnd, an internal circuit pulls this pin high, and the controller enables the corre - sponding current source to regulate the string current. if the fb_ pin is connected to sgnd, the controller disables the corresponding current regulator. the current regulator cannot be disabled by connecting the respective fb_ pin to sgnd after the ic is enabled. all fb_ pins in use are combined to extract a lowest fb_ voltage (lvc) (see figure 2). lvc is fed into the step-up regulators error amplifier and is used to set the output voltage. current source fault protection led fault open/short is detected after startup. when one or more strings fail after startup, the corresponding cur - rent source is disabled. the remaining led strings are still operated normally. when a fault is detected, bit 4 or/ and bit 5 of the fault/status resister are set in smbus mode. (see the fault status register description in the dimming control register descriptions section.) led short and string mismatch protection the max17105 can tolerate slight mismatch between led strings. when severe mismatches or wled shorts occur, the fb_ voltages are uneven because of mis - matched voltage drops across strings. at each led turn- on, the fb_ voltage is brought down to the regulation voltage quickly. when fb_ voltage is higher than 8v (typ) after the led turns on, the led short is detected. when the led short condition is continued for 2ms, the strings are disabled. the remaining led strings can still oper - ate normally. if only one string is used, the output volt - age decreases and regulates the needed current during the operation of led short. the led short protection is not triggered since fb_ voltage is never higher than the threshold. the led short protection is disabled during the soft-start phase of the step-up regulator. open-current source protection the max17105 step-up regulator output voltage is regu - lated according to the minimum fb_ voltages on all of the strings in use. if one or more strings are open, the respective fb_ pins are pulled to ground. for any fb_ lower than 220mv, the corresponding current source is disabled. the remaining led strings can still operate nor - mally. if all strings in use are open, the max17105 shuts the step-up regulator down. www.maximintegrated.com maxim integrated 17 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
dimming control the max17105 has two dimming modes that are selected by the dfset pin as shown in table 3. the led on/off dpwm frequency is generated by the internal oscillator for smbus mode, while led is directly on/off by the pwmi pin in direct-pwm mode. the dpwm frequency is adjust - able through an external setting resistor and has 3% accuracy over the 200hz to 2khz range. in smbus mode, the duty cycle of this dpwm signal can be controlled externally through two interfaces: pwm and smbus. the iset pin sets the amplitude of the current sources for each led string (figure 2). the internal dpwm signal directly controls the duty cycle of these current sources. the resulting current is chopped and synchronized to the dpwm signal. when filtered by the slow response time of the human eye, the overall brightness is modulated in a consistent flicker-free manner. full-scale led and low-level led current the full-scale led current iiset is determined by the resistors connected from iset to sgnd: led_max iset 20ma 50k i r = oi the acceptable resistance range is 33.3k w < r iset < open, which corresponds to full-scale led current of 30ma > i led_max > 0ma. connect iset to sgnd sets the test mode for 0.3ma (typ) full-scale led current. dpwm frequency setting the max17105 uses an internal dpwm signal to perform dimming control while operating at smbus mode. the dpwm frequency is specified by an external resistor con - nected from the dfset pin to sgnd: dpwm dfset 200hz 250k f r = oi the adjustable range for the dfset resistor, r dfset , is from 10k w to 500k w , corresponding to the dpwm fre - quency of 5khz > f dpwm > 100hz. dimming control interfaces for smbus mode the max17105s dimming control circuit consists of two interfaces: pwm and smbus. the block diagram of these two input interfaces is shown in figure 4. the dim - ming can be performed in three modes: pwm, smbus, or dpst. in pwm mode, the brightness is adjusted by the pwm signal applied to the pwmi pin. in smbus mode, the brightness is adjusted by an i 2 c command from an uplink processor through a 2-wire smbus. in dpst mode, the brightness is adjusted by the product of the pwm duty cycle and smbus command value. this dpwm control provides a dimming range with 8-bit reso - lution down to 0% and supports intel dpst to maximize battery life. the smbus interface can be used to adjust the dim - ming, as well as shut down the max17105. before the max17105 receives a turn-on command from the smbus, it automatically remains off. in this low-power state, most of the control circuits are turned off, and only part of the ldo is active to provide a loosely regulated output of approxi - mately 4.5v on the v cc pin to power the smbus interface. figure 4. pwm and smbus interface circuit pwmi pwmo buffer a mux 0x03 identification register d 1 1 scl smbus interface dpwmsetting digital multiplier backlighton/off sda 0x02 0x01 0x00 mux pwm_sel pwm_md smbus and pwm input block fault/status register device control register brightness control register www.maximintegrated.com maxim integrated 18 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
the max17105 uses two multiplexers internally to direct the dimming signal processing (figure 4). these two multiplex - ers are controlled by 2 bits of the device control register, pwm_sel and pwm_md, respectively. the pwm_sel bit selects either the smbus or the pwmi input to control the brightness. the pwm_md bit selects the mode in which the pwmi input is to be interpreted. table 4 provides a complete setting of the three dimming modes. device control register: address is 0x01. this register is both readable and writeable for bit 0 to bit 2. bit 0, also named bl_ctl, is used as on/off control for the output leds. bit 1 and bit 2, named pwm_sel and pwm_md respec tively, control the operating mode of the backlight controller. bit 3 to bit 7 are reserved bits. all reserved bits, return 0 when read, and are ignored by the controller when written. a value of 1 written to bl_ctl turns on the backlight in 10ms or less after the write cycle completes. a value of 0 written to bl_ctl immediately turns off the backlight. dimming control register descriptions the max17105 includes four registers to monitor and control brightness, fault status, driver id, and operating mode. brightness control register: address is 0x00. this register is both readable and writeable for all 8 bits, and for brt0, and brt7, which are used to control the led brightness level. in smbus dimming mode, an smbus write byte cycle to register 0x00 sets the output brightness level. the smbus setting of 0xff for this register sets the backlight controller to the maximum brightness output, and 0x00 sets 0.4% backlight brightness. the default value for register 0x00 is 0xff. a write byte cycle to register 0x00 has no effect when the backlight controller is in pwm mode. the smbus read byte cycle to register 0x00 returns the current brightness level regardless of the dimming mode. x = dont care table 4. operating modes selected by device control register bits 1 and 2 pwm_md pwm_sel mode dpwm duty-cycle setting x 1 pwm mode pwmi input duty cycle 1 0 smbus mode smbus command 0 0 dpst mode product of pwmi input duty cycle and smbus command register 0x01 device control register default value 0x00 reserved reserved reserved reserved reserved pwm_md pwm_sel bl_ctl bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 (r/w) bit 1 (r/w) bit 0 (r/w) bit field definitions: bit field definition description bit 2 pwm_md pwm mode select (1 = absolute brightness, 0 = % change) default = 0 bit 1 pwm_sel brightness mux select (1 = pwmi pin, 0 = smbus value) default = 0 bit 0 bl_ctl bl on/off (1 = on, 0 = off) default = 0 register 0x00 brightness control register default value 0xff brt7 brt6 brt5 brt4 brt3 brt2 brt1 brt0 bit 7 (r/w) bit 6 (r/w) bit 5 (r/w) bit 4 (r/w) bit 3 (r/w) bit 2 (r/w) bit 1 (r/w) bit 0 (r/w) bit field definitions: bit field definition description bit [7:0] brt [7:0] 8-bit brightness setting, adjusting brightness levels in 256 steps, default value is 0xff. www.maximintegrated.com maxim integrated 19 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
the list of id values for vendors is shown below, based on the current backlight controller vendor list and is not sorted in any particular order. identification register: address is 0x03. the id register contains two bit fields to denote the manufacturer and the silicon revision of the controller ic. the bit field widths were chosen to allow up to 32 vendors with up to eight silicon revisions each. this register is read only. in pwm mode, the output led brightness is solely controlled by the percentage duty cycle of the input signal to pwmi. in smbus mode, the input of pwmi has no effect on the dimming control, and only the smbus command to the brightness control register adjusts the output brightness. in dpst mode, the overall brightness level is the normalized product of the smbus command setting and pwm input duty cycle. the pwm signal starts from 100% when operating in dpst mode. the default value for register 0x01 is 0x00. fault/status register: address is 0x02. this register has 6 status bits that allow monitoring the backlight controllers operating state. bit 6 and bit 7 are reserved bits, and bit 3 is the status indicator or backlight. the other 5 bits are fault indicators. bit 0 is a logical or of all fault codes to simplify error detection (global fail, ovcurr fail, 1ch short/open, 2 ch short/open, thrm_shdn). all the bits in this register are read only. the reserved bits return a 0 when read. table 5. vendor ids id vendor 5 st 6 analog devices 7-14 reserved 15 vendor id register not implemented id vendor 0 maxim 1 micro semi 2 mps 3 o2 micro 4 ti register 0x03 id register default value 0x80 led panel mfg3 mfg2 mfg1 mfg0 rev2 rev1 rev0 bit 7 = 1 bit 6 (r) bit 5 (r) bit 4 (r) bit 3 (r) bit 2 (r) bit 1 (r) bit 0 (r) bit field definitions: bit field definition description bit 7 led panel display panel using led backlight, bit 7 = 1 bit [6:3] mfg[3:0] manufacturer id, see table 5, default = 0 bit [2:0] rev[2:0] silicon rev (revs 0 to 7 allowed for silicon spins), default = 0 register 0x02 fault/status register default value 0x00 reserved reserved 2_ch_sd 1_ch_sd bl_stat ovcurr thrm_shdn fault bit 7 (r) bit 6 (r) bit 5 (r) bit 4 (r) bit 3 (r) bit 2 (r) bit 1 (r) bit 0 (r) bit field definitions: bit field definition description bit 5 2_ch_sd two or more led output channels are shut down (1 = shutdown, 0 = okay) bit 4 1_ch_sd one led output channel is shut down (1 = shutdown, 0 = okay) bit 3 bl_stat backlight status (1 = bl on, 0 = bl off) bit 2 ov_curr input overcurrent (1 = overcurrent condition, 0 = current okay) bit 1 thrm_shdn thermal shutdown (1 = thermal fault, 0 = thermal okay) bit 0 fault fault occurred (logic or of all the fault conditions) www.maximintegrated.com maxim integrated 20 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
thermal shutdown the max17105 includes a thermal-protection circuit. when the local ic temperature exceeds +150c (typ), the controller and current sources shut down and do not restart until the next enable signal is sent. design procedure all max17105 designs should be prototyped and tested prior to production. external component value choice is primarily dictated by the output voltage and the maximum load current, as well as maximum and minimum input voltages. begin by selecting an inductor value. once the inductor is known, choose the diode and capacitors. step-up converter current calculation to ensure stable operation, the max17105 includes slope compensation, which sets the minimum induc - tor value. in continuous-conduction mode (ccm), the minimum inductor value is calculated with the following equation: ( ) out(max) diode s(min) s ccm(min) sw(min) v v -2 v r l 2 25.5mv f + = oo oo where 25.5mv is a scale factor from the slope com - pensation, l ccm(min) is the minimum inductor value for stable operation in ccm, and r s =13.7m w (typ) is the equivalent sensing-scale factor from the controllers internal current-sense circuit. the controller can also operate in discontinuous-con - duction mode (dcm). in this mode, the inductor value can be lower, but the peak inductor current is higher than in ccm. in dcm, the maximum inductor value is calculated with the following equation: s(min) dcm(max) out(max) diode 2 s(min) sw(max) out(max) out(max) v l 1- vv v 2f v i ???? = ?? + ?? oe o oo o where l dcm(max) is the maximum inductor value for dcm, is the nominal regulator efficiency (85%), and i out(max) is the maximum output current. the output current capability of the step-up regulator is a function of current limit, input voltage, operating frequen - cy, and inductor value. because the slope compensation is used to stabilize the feedback loop, the inductor current limit depends on the duty cycle, and is determined with the following equation: ( ) lim s 25.5mv 0.75 - d i 2a r = + o where 25.5mv is the scale factor from the slope compen - sation, 2a is the current limit specified at 75% duty cycle, and d is the duty cycle. the output current capability depends on the current- limit value and operating mode. the maximum output current in ccm is governed by the following equation: ss out_ccm(max) lim sw out 0.5 d v v i i- flv ?? = ???? oo o oe o where i lim is the current limit calculated above, is the nominal regulator efficiency (85%), and d is the duty cycle. the corresponding duty cycle for this current is: out s diode out lim on diode v -v v d v -i r v + = + o where v diode is the forward voltage of the rectifier diode and r on is the internal mosfets on-resistance (0.15 w (typ)).the maximum output current in dcm is governed by the following equation: ( ) ( ) 2 lim sw out diode out_dcm(max) out out diode s li f v v i 2 v v v -v + = + o o oeo oo inductor selection the inductance, peak current rating, series resistance, and physical size should all be considered when select - ing an inductor. these factors affect the converters oper- ating mode, efficiency, maximum output load capability, transient-response time, output voltage ripple, and cost. www.maximintegrated.com maxim integrated 21 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
the maximum output current, input voltage, output volt - age, and switching frequency determine the inductor value. very high inductance minimizes the current ripple, and therefore reduces the peak current, which decreases core losses in the inductor and i 2 r losses in the entire power path. however, large inductor values also require more energy storage and more turns of wire, which increases physical size and i 2 r copper losses. low inductor values decrease the physical size but increase the current ripple and peak current. finding the best inductor involves the compromises among circuit effi - ciency, inductor size, and cost. in choosing an inductor, the first step is to determine the operating mode: continuous-conduction mode (ccm) or discontinuous-conduction mode (dcm). the max17105 has a fixed internal slope compensation, which requires minimum inductor value. when ccm mode is chosen, the ripple current and the peak current of the inductor can be minimized. if a small-size inductor is required, dcm mode can be chosen. in dcm mode, the inductor value and size can be minimized, but the inductor ripple current and peak current are higher than those in ccm. the controller can be stable, independent of the internal slope-compensation mode, but there is a maximum induc - tor value requirement to ensure the dcm operating mode. the equations used here include a constant lir, which is the ratio of the inductor peak-to-peak ripple current to the average dc inductor current at the full load current. the controller operates in dcm mode when lir is higher than 2.0, and it works in ccm mode when lir is lower than 2.0. the best trade-off between inductor size and converter efficiency for step-up regulators generally has an lir between 0.3 and 0.5. however, depending on the ac characteristics of the inductor core material and ratio of inductor resistance to other power path resistances, the best lir can shift up or down. if the inductor resistance is relatively high, more ripples can be accepted to reduce the number of required turns and increase the wire diam - eter. if the inductor resistance is relatively low, increasing inductance to lower the peak current can reduce losses throughout the power path. if extremely thin high-resis - tance inductors are used, as is common for lcd panel applications, lir higher than 2.0 can be chosen for dcm operating mode. once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. the detail design procedure for ccm can be described as: calculate the approximate inductor value using the typical input voltage (v s ), the maximum output cur - rent (i out(max) ), the expected efficiency ( typ) taken from an appropriate curve in the typical operating characteristics , and an estimate of lir based on the above discussion: 2 s(min) out s(min) typ out out(max) sw v v -v l v i f lir ?? ?? ?? ?? = ?? ?? ?? ?? ?? ?? ?? e o the max17105 has a minimum inductor value limitation for stable operation in ccm mode at low input voltage, because of the internal fixed slope compensation. the minimum inductor value for stability is calculated with the following equation: ( ) out(max) diode s(min) s ccm(min) sw(min) v v -2 v r l 2 25.5mv f + = oo oo where 25.5mv is a scale factor from slope compensa - tion, and r s is the equivalent current-sensing scale fac - tor (13.7m w typ). choose an available inductor value from an appropriate inductor family. calculate the maximum dc input current at the minimum input voltage v s(min) , using conserva - tion of energy and the expected efficiency at that operat - ing point ( min) taken from an appropriate curve in the typical operating characteristics : out(max) out in(dc,max) s(min) min iv i v = o oe calculate the ripple current at that operating point and the peak current required for the inductor: ( ) s(min) out(max) s(min) ripple out(max) sw v v -v i lv f = o oo ripple peak in(dc,max) i ii 2 = + when the dcm operating mode is chosen to minimize the inductor value, the calculations are different from those above in ccm mode. the maximum inductor value for dcm mode is calculated with the following equation: s(min) dcm(max) out(max) diode 2 s(min) sw(max) out(max) out(max) v l 1- vv v 2f v i ?? = ???? + ?? oe o oo o www.maximintegrated.com maxim integrated 22 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
the peak inductor current in dcm is calculated with the following equation: ( ) ( ) out(max) out(max) out(max) diode s(min) peak sw(min) out(max) diode i 2 v v v -v i lf v v + = + oo o o oeo the inductors saturation current rating should exceed ipeak, and the inductors dc current rating should exceed i in(dc,max) . for good efficiency, choose an inductor with less than 0.1 w series resistance. considering the circuit with eight 10-led strings and 20ma led full-scale current, the maximum load current (i out(max) ) is 160ma with a 32v output and a minimal input voltage of 7v. choosing a ccm operating mode with lir = 0.7 at 1mhz and an estimating efficiency of 85% at this operating point: 2 7v 32v - 7v 0.85 l 9.08h 32v 160ma 1mhz 0.7 ?? ?? ? ? = = ?? ?? ? ? ?? ? ? ?? in ccm, the inductor has to be higher than l ccm(min) : ( ) ccm(min) 32v 0.4v - 2 7v 13.7m l 5.5h 2 25.5mv 0.9mhz + = = oo i oo 10h inductor is chosen, which is higher the minimum l that guarantees stability in ccm. the peak inductor current at minimum input voltage is calculated as follows: ( ) peak 7v 32v - 7v 160ma 32v i 1.16a 7v 0.85 2 10h 32v 0.9mhz = += o o o o oo alternatively, choosing a dcm operating mode will lower inductance and estimate efficiency of 85% at this oper - ating point. since dcm will have higher peak inductor current at lower input, it will cause current limit when the parameters are not chosen properly. consider the case with six 10-led strings and 20ma led full-scale current to prevent excessive switch current from causing current limit: dcm(max) 2 7v l 1- 32v 0.4v (7v) 0.85 3.9h 2 1.1mhz 32v 120ma ?? = ?? + ?? = o o o oo 3.3h inductor is chosen. the peak inductor current at minimum input voltage is calculated as follows: ( ) ( ) peak 120ma 2 32v 32v 0.4v - 7v i 1.40a 3.3h 1.1mhz 0.85 32v 0.4v + = = + oo o o oo output capacitor selection the total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to the capacitors equivalent series resistance (esr): ripple ripple(c) ripple(esr) vv v = + out(max) out(max) s(min) ripple(c) out out(max) sw i v -v v cv f ?? ?? ?? ?? o and: ripple(esr) peak esr(cout) v ir where i peak is the peak inductor current (see the inductor selection section). the output voltage ripple should be low enough for the fb_ current source regulation. the ripple voltage should be less than 200mv p-p . for ceramic capacitors, the out - put voltage ripple is typically dominated by v ripple(c) . the voltage rating and temperature characteristics of the output capacitor must also be considered. rectiier diode selection the max17105s high switching frequency demands a high-speed rectifier. schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. the diode should be rated to handle the output voltage and the peak switch current. make sure that the diodes peak current rating is at least ipeak calculated in the inductor selection section and that its breakdown voltage exceeds the output voltage. overvoltage-protection determination the overvoltage-protection circuit should ensure the circuit safe operation; therefore, the controller should limit the output voltage within the ratings of all mosfet, diode, and output capacitor components, while provid - ing sufficient output voltage for led current regulation. the ovp pin is connected to the center tap of a resistive voltage-divider (r1 and r2 in figure 1) from the high- voltage output. when the controller detects the ovp pin voltage reaching the threshold vovp_th, typically 1.25v, overvoltage protection is activated. hence, the step-up converter output overvoltage protection point is: www.maximintegrated.com maxim integrated 23 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
out(ovp) ovp_th r1 v v (1 ) r2 = + o in figure 1, the output ovp voltage is set to: out(ovp) 2.21m v 1.25v (1 ) 39.89v 71.5k = += i o i input capacitor selectionthe input capacitor (c in ) filters the current peaks drawn from the input supply and reduces noise injection into the ic. a 4.7f ceramic capacitor is used in the typical operating circuit (figure 1) because of the high source impedance seen in typical lab setups. actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. in some applications, c in can be reduced below the values used in the typical operating circuit. ensure a low noise supply at in by using adequate c in . alternatively, greater voltage variation can be toler - ated on c in if in is decoupled from c in using an rc lowpass filter. led selection and bias the series/parallel configuration of the led load and the full-scale bias current have a significant effect on regula - tor performance. led characteristics vary significantly from manufacturer to manufacturer. consult the respec - tive led data sheets to determine the range of output voltages for a given brightness and led current. in general, brightness increases as a function of bias cur - rent. this suggests that the number of leds could be decreased if higher bias current is chosen; however, high current increases led temperature and reduces operat - ing life. improvements in led technology are resulting in devices with lower forward voltage while increasing the bias current and light output. led manufacturers specify led color at a given led cur - rent. with lower led current, the color of the emitted light tends to shift toward the blue range of the spectrum. a blue bias is often acceptable for business applications but not for high-image-quality applications such as dvd play - ers. direct-pwm dimming is a viable solution for reducing power dissipation while maintaining led color integrity. careful attention should be paid to switching noise to avoid other display quality problems. using fewer leds in a string improves step-up converter efficiency, and lowers breakdown voltage requirements of the external mosfet and diode. the minimum num - ber of leds in series should always be greater than the maximum input voltage. if the diode voltage drop is lower than maximum input voltage, the voltage drop across the current-sense inputs (fb_) increases and causes excess heating in the ic. between 8 and 12 leds in series are ideal for input voltages up to 20v. applications information led v fb_ variation the forward voltage of each white led may vary up to 25% from part to part, and the accumulated voltage difference in each string equates to additional power loss within the ic. for the best efficiency, the voltage difference between strings should be minimized. the difference between lowest voltage string and highest voltage string should be less than 8v (typ). otherwise, the internal led short-protection circuit disables the high fb string. fb_ pin maximum voltage the current through each fb_ pin is controlled only during the step-up converters on-time. during the con - verters off-time, the current sources are turned off. the output voltage does not discharge and stays high. the max17105 disables the fb current source from which the string is shorted. in this case, the step-up converters output voltage is always applied to the disabled fb_ pin. fb_ pin can withstand 45v. pcb layout guidelines careful pcb layout is important for proper operation. use the following guidelines for good pcb layout: 1) minimize the area of the high-current switching loop of rectifier diode, internal mosfet, and output capacitor to avoid excessive switching noise. 2) connect high-current input and output components with short and wide connections. the high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the internal mosfet, then to the input capacitors negative terminal. the high- current output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode, and to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. avoid using vias in the high-current paths. if vias are unavoidable, use mul - tiple vias in parallel to reduce resistance and induc - tance. www.maximintegrated.com maxim integrated 24 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability downloaded from: http:///
3) create a ground island (pgnd) consisting of the input and output capacitor ground and negative ter - minal of the current-sense resistor. connect all these together with short, wide traces or a small ground plane. maximizing the width of the power ground traces improves efficiency and reduces output volt - age ripple and noise spikes. create an analog ground island (agnd) consisting of the overvoltage-detection divider ground connection, the iset and dfset resistor connections, the comp resistor and capacitor connections, and the devices exposed backside pad. connect the agnd and pgnd islands by connecting the sgnd pins directly to the exposed backside pad. make no other connections between these separate ground planes. 4) place the overvoltage-detection divider resistors as close to the ovp pin as possible. the dividers center trace should be kept short. placing the resistors far away causes the sensing trace to become antennas that can pick up switching noise. avoid running the sensing traces near lx. 5) place the in pin bypass capacitor as close to the device as possible. the ground connection of the in bypass capacitor should be connected directly to sgnd pins with a wide trace. 6) minimize the size of the lx node while keeping it wide and short. keep the lx node away from the feedback node and ground. if possible, avoid running the lx node from one side of the pcb to the other. use dc traces as a shield if necessary. refer to the max17105 evaluation kit for an example of proper board layout. package type package code outline no. land pattern no. 24 tqfn-ep t2444+4 21-0139 90-0022 fb1 pgnd in fault lx ovp fb2fb3 fb4 fb5 fb6 fb7 fb8 iset v cc pwmipwm0 10khz max17105 dfseten scl sda osccomp ep sgnd www.maximintegrated.com maxim integrated 25 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability package information for the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages . note that a +, #, or - in the package code indicates rohs status only. package drawings may show a different suffix character, but the drawing pertains to the package regardless of rohs status. chip information process: bicmos simpliied operating circuit downloaded from: http:///
revision number revision date description pages changed 0 5/09 initial release 1 12/09 updated electrical characteristics , typical operating characteristics graphs 1 and 2, and comp pin function with specifications for final production version 2C7, 11 2 9/14 removed automotive reference from applications 1 maxim integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim integrated product. no circuit patent licenses are implied. maxim integrated reserves the right to change the circuitry and speciications without n otice at any time. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. maxim integrated and the maxim integrated logo are trademarks of maxim integrated products, inc. ? 2014 maxim integrated products, inc. 26 max17105 8-string wled driver with integrated step-up regulator and smbus/pwm dimming capability revision history for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim integrateds website at www.maximintegrated.com. downloaded from: http:///

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