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| n ZXLD1371 document numbe r descripti o the zxld13 7 external mos topology cont r through serie s to operate in b the 60v cap a enables it to b in excess of 1 5 the zxld13 7 patent pendin g accuracy in a dimming is a c pwm control. the zxld137 highlights a f a information on features ? 0.5% typi c ? 5 to 60v o ? led driv e ? configura t ? wide dyn a o 10:1 d o 1000: ? up to 1m h ? high tem p ? available ts16949 ? available lead free n ote 1: eu direc t rohs ex e typical a p buck-boo s t h 1 0 r 1. 8 rgi2 75k rgi1 24k c1 10f vin 8v to 22v r : ds35436 rev. 1 o n 7 1 is an le d fets to drive r oller enabling s connected l e b uck, boost an d a bility couple d b e used in a w i 5 leds in seri e 7 1 is a modi f g control sch e a ll three mo d c hieved throu g 1 uses two pi n a ult, while the the exact faul t c al output curr e o perating volta g e r supports bu c t ions a mic range di m d c dimming 1 dimming ran g h z switching p erature contr o in automoti v certification in ?green? m o finish/ rohs t ive 2002/95/ec ( r e mptions applied. p plication s t diagram u c2 330pf zxl d h 1 0 k r 4 8 k pwm vaux vi n gi adj ref tadj shp s nc - 2 60v high a d driver cont r high current it to efficientl y e ds. the mult i d buck-boost c o d with its mult de range of a p e s. f ied hystereti c e me providing d es of operat i g h dc control n s for fault dia g multi-level sta t t . e nt accuracy g e range c k, boost and b m ming g e at 500hz o l of led curre n v e grade w i o lding compo u compliant (n o r ohs) & 2011/65/ e circuit tilizing ther d 1371 l1 33 h r1 0r05 n ism gnd pgnd gate flag status th e w w a ccuracy r oller ic for d leds. it is a y control the c i -topology ena b o nfigurations. i-topology ca p p plications an d c controller u s high output c i on. high ac c and high freq g nosis. a flag o t us pin gives f b uck-boost n t using t adj i th aec-q10 0 u nd (no br, s b o te 1) e u (rohs 2). all a p mistor and t h c 1 0 d1 pds3100 q1 dmn6068lk3 e rmally connected iled = 1a 1 of 42 w w.diodes.com buck/boo s d riving multi- c urrent b les it p ability d drive s ing a c urrent c uracy uency o utput f urther pi n 0 and b ) with p plicable t a dj c out 0 f 1 to 6 leds a di o s t/buck-b o n assignm e curve s h adj ref tadj shp status sgnd pgnd n/c 1 2 3 4 5 6 7 8 a product li n o des incorpo r o ost led d e nts h owing led 1 2 3 4 5 6 7 8 tsso p zx febr u ? diodes n e o f r ated d river-con t current vs. t p -16ep ld1371 u ary 2012 incorporated t roller t led gi pwm flag ism vin vaux gate n/c 16 15 14 13 12 11 10 9
ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 2 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated pin descriptions pin name pin type (note 2) description adj 1 i adjust input (for dc output current control) connect to ref to set 100% output current. drive with dc voltage (125mv ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 3 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated functional block diagram absolute maximum ratings (voltages to gnd unless otherwise stated) (note 3) symbol parameter rating unit v in input supply voltage -0.3 to 65 v v aux auxiliary supply voltage -0.3 to 65 v v ism current monitor input relati ve to gnd -0.3 to 65 v v sense current monitor sense voltage (v in -v ism ) -0.3 to 5 v v gate gate driver output voltage -0.3 to 20 v i gate gate driver continuous output current 18 ma v flag flag output voltage -0.3 to 40 v v pwm , v adj , v tadj , v gi , v pwm other input pins -0.3 to 5.5 v t j maximum junction temperature 150 c t st storage temperature -55 to 150 c stresses greater than the 'absolute maximum ratings' specified a bove, may cause permanent damage to the device. these are stres s ratings only; functional operation of the device at these or any ot her conditions exceeding those indicated in this specification is not implied. device reliability may be affected by exposure to absolute maximum rating conditions for extended periods of time. semiconductor devices are esd sensitive and may be damaged by expos ure to esd events. suitable esd precautions should be taken when handling and transporting these devices. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 4 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated package thermal data thermal resistance package typical unit junction-to-ambient, ja (note 4) tssop-16 ep 50 c/w junction-to-case, jc tssop-16 ep 23 c/w recommended operating conditions (-40c t j 125c) symbol paramete r performance/comment min max unit v in input supply voltage range normal operation 8.0 60 v (note 5) reduced performance operation 5.0 8.0 v aux auxiliary supply voltage range (note 6) normal operation 8.0 60 v (note 5) reduced performance operation 5.0 8.0 v sense differential input voltage v in -v ism , with 0 v adj 2.5 0 450 mv v adj external dc control voltage applied to adj pin to adjust output current dc brightness control mode from 10% to 100% 0.125 1.25 v i ref reference external load current ref sourcing current 1 ma f max recommended switching frequency range (note 7) 300 1000 khz v tadj temperature adjustment (t adj ) input voltage range 0 v ref v f pwm recommended pwm dimming frequency range to achieve 1000:1 resolution 100 500 hz to achieve 500:1 resolution 100 1000 hz t pwmh/l pwm pulse width in dimming mode pwm input high or low 0.002 10 ms v pwmh pwm pin high level input voltage 2 5.5 v v pwml pwm pin low level input voltage 0 0.4 v t j operating junction temperature range -40 125 c gi gain setting ratio for boost and buck-boost modes ratio= v gi /v adj 0.20 0.50 notes: 3. for correct operation sgnd and pgnd shoul d always be connected together. 4. measured on ?high effective thermal condu ctivity test board" according to jesd51. 5. device starts up above 5.4v and as such the minimum appli ed supply voltage has to be above 5.4v (plus any noise margin). the ZXLD1371 will, however, continue to function when the input voltage is reduced from t 8v down to 5.0v. when operating with input voltages below 8v the output current and device parameters may deviate from their normal values; and is dependent on power mosfet switch, load and ambient temperature condi tions. to ensure best operation in boost and buck-boost modes wi th input voltages, v in , between 5.0 and 8v a suitable boot-strap network on v aux pin is recommended. performance in buck mode will be reduced at input voltages (v in , v aux ) below 8v. ? a boot-strap network ca nnot be implemented in buck mode. 6. v aux can be driven from a voltage higher than v in to provide higher efficiency at low v in voltages, but to avoid false operation; a voltage should not be applied to v aux in the absence of a voltage at v in . v aux can also be operated at a lower voltage than v in to increase efficiencies at high v in . 7. the device contains circuitry to control the switching frequency to approximately 400khz. the maximum and minimum operating frequency is not tested in production. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 5 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated electrical characteristics (test conditions: v in = v aux = 12v, t a = 25c, unless otherwise specified.) symbol parameter conditions min typ max units supply and reference parameters v uv- under-voltage detection threshold normal operation to switch disabled v in or v aux falling (note 8) 4.5 v v uv+ under-voltage detection threshold switch disabled to normal operation v in or v aux rising (note 8) 4.9 v i q-in quiescent current into v in pwm pin floating. output not switching 1.5 3 ma i q-aux quiescent current into v aux 150 300 a i sb-in standby current into v in . pwm pin grounded for more than 15ms 90 150 a i sb-aux standby current into v aux . 0.7 10 a v ref internal refe rence voltage no load 1.237 1.25 1.263 v v ref change in reference voltage with output current sourcing 1ma -5 mv sinking 25a 5 v ref_line reference voltage line regulation v in = v aux , 8.0v ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 6 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated electrical characteristics (cont.) (test conditions: v in = v aux = 12v, t a = 25c, unless otherwise specified.) symbol parameter conditions min typ max units output parameters v flagl flag pin low level output voltage output sinking 1ma 0.5 v i flagoff flag pin open-drain leakage current v flag = 40v 1 a v status status flag no-load output voltage (note 10 ) normal operation 4.2 4.5 4.8 v out of regulation (v shp out of range) (note 11 ) 3.3 3.6 3.9 v in under-voltage (v in < uvlo) 3.3 3.6 3.9 switch stalled (t on or t off > 100s) 3.3 3.6 3.9 over-temperature (t j > 125c) 1.5 1.8 2.1 excess sense resistor current (v sense > 0.32v) 0.6 0.9 1.2 r status output impedance of status output normal operation 10 k ? driver output (pin gate) v gateh high level output voltage no load sourcing 1ma (note 12 ) 10 11 v v gatel low level output voltage sinking 1ma, ( note 13 ) 0.5 v v gatecl high level gate clamp voltage v in = v au x = v ism = 18v i gate = 1ma 12.8 15 v i gate dynamic peak current available during rise or fall of output voltage charging or discharging gate of external switch with q g = 10nc and 400khz 300 ma t stall time to assert ?stall? flag and warning on status output (note 14 ) gate low or high 100 170 s led thermal control circuit (t adj ) parameters v tadjh upper threshold voltage onset of output current reduction (v tadj falling) 560 625 690 mv v tadjl lower threshold voltage output current reduced to <10% of set value (v tadj falling) 380 440 500 mv i tadj t adj pin input current v tadj = 1.25v 1 a notes: 10. in the event of more than one fault/warning condition occurring, the higher priority condition will take precedence. for example ?excessive coil current? and ?out of regulat ion? occurring together will produce an output of 0.9v on the sta tus pin. these status pin voltages apply for an input voltage to v in of 7.5v < v in < 60v. below 7.5v the status pin voltage levels reduce and therefore may not report the correct status. for 5.4v < v in < 7.5v the flag pin still reports any error by going low. at low v in in boost and buck-boost modes an over-current status may be indicated when operating at high boost ratios ? this due to the feedback l oop increasing the sense voltage. for more information see the applic ation information section about flag/status levels. 11. flag is asserted if v shp < 1.5v or v shp > 2.5v 12. gate is switched to the supply voltage v aux for low values of v aux (5v v aux ~12v). for v aux > 12v, gate is clamped internally to prevent it exceeding 15v. 13. gate is switched to pgnd by an nmos transistor 14. if t on exceeds t stall , the device will force gate low to turn off the external switch and then initiate a restart cycle. during this phase, adj is grounded internally and the shp pin is switched to its nominal operating voltage, before operation is allowed to resume. restart cycles will be repeated automatically until the operating condi tions are such that normal operation can be sustained. if t off exceeds t stall , the switch will remain off until normal operation is possible. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 7 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics 0 0.5 1 1.5 2 2.5 3 5 101520 253035 4045 505560 supply voltage (v) supply voltage vs. supply current supply current (ma) 1.248 1.2485 1.249 1.2495 1.25 1.2505 1.251 1.2515 1.252 -40 -25 -10 5 20 35 50 65 80 95 110 125 junction temperature ( c) reference voltage vs. junction temperature reference v o lta g e (v) 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 6121824303642485460 input voltage (v) duty cycle vs. input voltage duty (%) t=25c l=33h r=146m buck mode 2 leds a s ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 8 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? linear/dc dimming 0 150 300 450 600 750 0 150 300 450 600 750 0 0.25 0.5 0.75 1 1.25 adj voltage (v) led current and switching frequency vs. adj voltage in buck mode led current (ma) switching frequency (khz) switching frequency t=25c v=v=12v 2 leds, l=33h r =300m a aux in s 0 200 400 600 800 1000 1200 1400 0 100 200 300 400 500 600 700 0 0.25 0.5 0.75 1 1.25 adj voltage (v) led current and switching frequency vs. adj voltage in buck-boost mode led current (ma) switching frequency (khz) switching frequency t= 25c v= v= 24v 8 leds, l = 33h gi = 0.23, r = 300m a aux in s led current 0 100 200 300 400 500 600 700 0 50 100 150 200 250 300 350 0 0.25 0.5 0.75 1 1.25 adj voltage (v) led current and switching frequency vs. adj voltage in boost mode led current (ma) switching frequency (khz) switching frequency i led t=25c v=v=12v 12 leds, l=33h gi=0.23, r =300m a aux in s ZXLD1371 document numbe r typical c r : ds35436 rev. 1 c haracteri s 0 250 500 750 1000 1250 1500 0 led current (ma) 0% 20% 40% 60% 80% 100% 0 led current dimming factor - 2 s tics ? pw m i led v s 10 2 0 t = 25c a v= v = l = 33h, r in aux s f= 100h z pwm 25 0 w w m /thermal s . time - p w p led c 0 30 4 24v =150m z 0 5 0 t led current a 9 of 42 w w.diodes.com dimming w m pin tran s p wm duty cycl e c urrent vs. pwm d 4 050 0 0 pin voltage ( dimming factor v s a dj a di o s ient respo n e (%) d uty cycle 60 70 750 ( mv) s . t voltage a dj a product li n o des incorpo r n se i led 80 90 1000 zx febr u ? diodes n e o f r ated 100 1250 ld1371 u ary 2012 incorporated ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 10 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? buck mode ? r s = 75m ? ? l = 33h ? i led = 2.9a 2.6 2.7 2.8 2.9 3.0 3.1 3.2 5 10152025303540 input voltage (v) led current (a) 0 100 200 300 500 600 700 510152025303540 input voltage (v) 1 led 3 leds 2 leds 4 leds switching frequency (khz) t = 25c, v = v l = 15h, r = 75m c = 100f, v = 3.8v aa u xi n s in led 4 leds l = 22h 400 ? 5 10152025 30 35 40 input voltage (v) 0 100 300 500 600 700 efficiency (%) 400 ZXLD1371 document numbe r typical c r : ds35436 rev. 1 c haracteri s 0 1 0 2 0 3 0 6 0 7 0 led c u r r en t (a) 4 0 10 0 e f f i c ien c y (%) 9 5 9 0 8 5 8 0 7 5 7 0 6 5 6 0 - 2 s tics ? buc k 0 0 0 0 0 0 0 0 0 0 0 0 0 510 1 led 1 t = 25c, l = 33h, r c = 100f a s in v i n 5101 0 5 0 5 0 5 0 5 0 3 leds 1 led 2 leds 4 l e t = 25c, l = 33h, r = c = 100f a s in v = in w w k mode ? r s 15 1 02 0 v = 150m aux, = n 5202 5 5 leds e ds 6 leds v 150m aux, = 11 of 42 w w.diodes.com =150m ? - l 20 25 input volta g 0 1 ~ 1 6 5 30 35 v (v) in 8 leds 10 l e a di o = 33h ? i le g e (v ) 25 30 6 leds 40 45 e ds 12 leds a product li n o des incorpo r d = 1.45a 30 35 50 55 14 leds 16 le d zx febr u ? diodes n e o f r ated 40 60 d s ld1371 u ary 2012 incorporated ZXLD1371 document numbe r typical c r : ds35436 rev. 1 c haracteri s 10 0 effi c ie n c y (%) 9 0 8 0 7 0 6 0 5 0 4 0 - 2 s tics ? bo o 4 leds 0 0 0 0 0 0 0 w w o st mode ? i l 6 leds 8 l 12 of 42 w w.diodes.com l ed = 350m a l eds 10 leds a di o a ? r s = 150 m t = l = 3 r9 = c = a in 12 leds 14 a product li n o des incorpo r m ? ? gi rati o 25c, 3h, r = 150m , 120k , r10 = 36k = 100 f s v = v aux in ? leds 16 led s zx febr u ? diodes n e o f r ated o = 0.23 s ld1371 u ary 2012 incorporated ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 13 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? boost mode ? i led = 350ma ? gi ratio = 0.23 ? bootstrap comparison 6 57810 9 1112131415161718 19 20 v (v) in t = 25 c, l = 33h r = 150m , r9 = 120k r10 = 36k , c = 100f a s in ? 0.25 0.27 0.29 0.33 0.31 0.35 0.37 0.39 0.41 0.43 0.45 led c u r r en t (a) 8 leds 8 leds bootstrap 6 57810 911121314 15 16 17 18 19 20 v (v) in t = 25 c, l = 33h r = 150m , r9 = 120k r10 = 36k , c = 100f a s in ? 8 leds bootstrap 8 leds 0 50 100 150 200 250 300 350 400 450 500 swi t c h in g f r e q u en c y (k h z) 6 57810 911121314 15 16 17 18 19 20 v (v) in t = 25 c, l = 33h r = 150m , r9 = 120k r10 = 36k , c = 100f a s in ? 8 leds bootstrap 8 leds 40 50 60 70 80 90 100 e f f i c ien c y % ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 14 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? boost mode ? i led = 350ma ? r s = 150m ? ? gi ratio = 0.23 6 57810 9 11121314151617181920 v (v) in l = 100h 0.25 0.29 0.35 0.37 0.41 0.45 led c u r r en t (a) t = 25 c, v = v 8 leds, rs = 150m r9 = 120k , r10 = 36k , c = 100f aa u xi n in ? l = 33h l = 68h 0.43 0.39 0.33 0.31 0.27 6 57810 911121314 15 16 17 18 19 20 v (v) in t = 25 c, v = v 8 leds, r = 150m , r9 = 120k , r10 = 36k , c = 100f aa u xi n s in ? l = 33h 0 50 100 150 200 250 300 350 400 450 500 switching frequency (khz) l = 100h l = 68h 6 57810 9 111213141516171819 v (v) in 40 50 60 70 80 90 100 efficiency % l = 33h l = 100h l = 68h t = 25c, v = v 8 leds, r = 150m , r9 = 120k , r10 = 36k , c = 100f aa u xi n s in ? ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 15 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? boost mode ? 8 leds ? gi ratio = 0.23 6 57810 911121314 15 16 17 18 19 20 v (v) in i = 500ma led i = 150ma led 0.10 0.20 0.30 0.40 0.50 0.60 led c u r r e n t (a) i = 350ma led t = 25 c, v = v 8 leds, l = 33h r9 = 120k , r10 = 36k , c = 100f aa u xi n in ? 6 57810 911121314 15 16 17 18 19 20 v (v) in i = 150ma led i = 500ma led 0 100 200 300 400 500 600 700 800 swi t c h i n g f r e q u e n c y (k h z) i = 350ma led t = 25 c, v = v 8 leds, l = 33h r9 = 120k , r10 = 36k , c = 100f aa u xi n in ? 6 57810 911121314 15 16 17 18 19 20 v (v) in i = 150ma led i = 500ma led 40 60 70 80 90 100 e f f i c ien c y (%) i = 350ma led t = 25 c, v = v 8 leds, l = 33h r9 = 120k , r10 = 36k , c = 100f aa u xi n in ? 50 ZXLD1371 document numbe r typical c r : ds35436 rev. 1 c haracteri s 0.4 5 led c u r r en t (a) 0.2 5 0.4 2 0.2 7 0.3 0 0.4 0 0.3 7 0.3 2 0.3 5 9 0 e f f i c ien c y % 4 0 8 5 4 5 8 0 5 0 7 5 5 5 7 0 6 0 6 5 - 2 s tics ? bu c 5 5 0 5 0 2 5 7 5 0 0 0 0 7 5 2 5 5 0 1 le d 5 leds 6 leds 8 leds 9 leds 7 leds 5 0 0 5 5 0 0 5 5 0 0 5 7 led 8 l e w w c k-boost m o 8 d 2 leds 3 leds 8 s e ds 9 leds 16 of 42 w w.diodes.com o de ? r s = 1 11 4 leds 11 a di o 50m ? - i led 14 t a rs r1 t = 25c, a l = rs = 150m r r10 = 36k v , , a 14 4 le d 5 leds a product li n o des incorpo r = 350ma ? g 17 = 25c, l = 33h, = 150m r9 = 120k 0 = 36k v = v , , aux i n 33h, r 9 = 120k = v , a ux i n 17 1 led 2 leds 3 leds d s 6 led zx febr u ? diodes n e o f r ated g i ratio = 0.2 20 , n 20 s ld1371 u ary 2012 incorporated 3 ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 17 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? buck-boost mode ? r s = 150m ? - i led = 350ma ? gi ratio = 0.23 6 57810 9 1112131415161718 v (v) in t = 25 c, l = 33h r = 150m , r9 = 120k r10 = 36k a s ? 0.25 0.27 0.29 0.33 0.31 0.35 0.37 0.39 0.41 0.43 0.4 5 led c u r r en t (a) 5 leds 5 leds bootstrap 56 9 14 15 18 81 21 6 1 7 13 10 11 7 swi t c h in g f r e q u en c y ( k h z) 600 500 400 200 100 0 300 6 5781 0 9 11 12131415161718 v (v) in 90 efficiency % 5 leds bootstrap 5 leds t = 25 c, l = 33h r = 150m , r9 = 120k , r10 = 36k , c = 100f a s in ? 85 80 50 45 40 75 70 65 60 55 ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 18 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? buck-boost mode ? r s = 150m ? - i led = 350ma ? gi ratio = 0.23 56 9 14 15 18 81 21 6 1 7 13 10 11 7 led c u r r e n t (a) 0.450 0.425 0.400 0.375 0.325 0.300 0.275 0.250 0.350 l = 33h l = 68h t = 25c, v = v , 5 leds, r = 150m c = 100f ai na u x s in , r9 = 120k r10 = 36k 56 9 1415 18 81 21 6 1 7 13 10 11 7 0 100 200 300 400 500 l = 100h swi t c h in g f r e q u en c y ( k h z) 600 l = 33h t = 25c, v = v , 5 leds, r = 150m c = 100f ai na u x s in , r9 = 120k r10 = 36k 56 9 1415 18 81 21 6 1 7 13 10 11 7 90 e f f i c ien c y % 40 85 45 80 50 75 55 70 60 65 l = 100h l = 68h ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 19 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated typical characteristics ? buck-boost mode ?5 leds gi ratio = 0.23 i = 150ma led i = 500ma led i = 350ma led t = 25c, v = v , 5 leds c = 100f ai na u x in l = 33h, r9 = 120k r10 = 36k , 56 9 1415 18 81 21 6 1 7 13 10 11 7 0.60 led c u r r en t (a) 0.10 0.55 0.15 0.50 0.20 0.45 0.25 0.40 0.30 0.35 i = 150ma led i = 350ma led t = 25c, v = v , 5 leds, l = 33 h, c = 100f ai na u x in r9 = 120k r10 = 36k , 56 9 1415 18 81 21 6 1 7 13 10 11 7 0 100 200 300 400 500 600 700 800 900 1000 i = 500ma led swi t c h in g f r e q u en c y ( k h z) i = 150ma led i = 500ma led i = 350ma led t = 25c, v = v , 5 leds c = 100f ai na u x in l = 33h, r9 = 120k r10 = 36k , 56 9 1415 18 81 21 6 1 7 13 10 11 7 90 effi c ie n c y % 40 85 45 80 50 75 55 70 60 65 ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 20 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information the ZXLD1371 is a high accuracy hysteretic inductive buck/ boost/buck-boost controller designed to be used with an external nmos switch for current-driving single or multiple series-connected leds. t he device can be configured to operate in buck, boost, or buck-boost modes by suitable conf iguration of the external components as shown in the schematics shown in the device operation description. device description a) buck mode ? the most simple buck circuit is shown in figure 1 control of the led current buck mode is achieved by sensing the coil current in the sense resistor rs, connected between the two inputs of a current monitor within the control loop block. an output from the control loop drives the input of a comparator wh ich drives the gate of the external nmos switch transisto r q1 via the internal gate driver. when the switch is on, the drain voltage of q1 is near zero. current flows from vin, via rs, led, coil and switch to ground. this current ramps up until an upper threshold value is reached (see figure 2 ). at this point gate goes low, the switch is turned off and the drain voltage increases to vin plus the forward voltage, v f , of the schottky diode d1. current flows via rs, led, coil and d1 back to vin. when the coil current has ramped down to a lower threshold value, gate goes high, the switch is turned on again and the cycle of events repeats, resulting in continuous oscillation. the feedback loop adjusts the nmos switch duty cycle to stabilize the led current in response to changes in external conditions, including input voltage and load voltage. figure 1. buck configuration the average current in the s ense resistor, led and coil is equal to the average of the maximum and minimum threshold currents. the ripple current (hysteresis) is equal to the difference between the thresholds. the control loop maintains the average led current at the set level by adjusting the switch duty cycle continuously to force the average sense resistor current to the value demanded by the voltage on the adj pin. this minimizes variation in output current with changes in operating conditions. the control loop also regulates the switching frequency by varying the level of hysteresis. the hysteresis has a defined minimum (typ 5%) and a maximum (typ 30%). the frequency may deviate from nominal in some conditions. this depends upon the desired led current, the coil inductance and the voltages at the input and the load. loop compensation is achieved by a single external capacitor c2, connected between shp and sgnd. the control loop sets the duty cycle so that the sense voltage is v sense = 0.218 v adj v ref therefore, i led = 0.218 r s v adj v ref (buck mode) equation 1 if the adj pin is connected to the ref pin, this simplifies to i led = 0.218 r s (buck mode). figure 2. operating waveforms (buck mode) gate voltage +11v to 15v typ. 0v q1 drain voltage vin + vf 0v coil & led current 0a i pk sense voltage v in -v ism mean = 218mv t off t on ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 21 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) b) boost and buck-boost modes ? the most simple boost/buck-boost circuit is shown in figure 3 control in boost and buck-boost mode is achieved by sensing the coil current in the series resistor rs, connected between the two inputs of a current monitor within the control loop block. an output from the control loop drives the input of a comparator wh ich drives the gate of the external nmos switch transisto r q1 via the internal gate driver. when the switch is on , the drain voltage of q1 is near zero. current flows from vin, via rs, coil and switch to ground. this current ramps up until an upper threshold value is reached (see figure 4 ). at this point gate goes low, the switch is turned off and the drain voltage increases to either: 1) the load voltage vleds plus the forward voltage of d1 in boost configuration, or 2) the load voltage vleds plus the forward voltage of d1 plus vin in buck-boost configuration. current flows via rs, coil, d1 and led back to vin (buck- boost mode), or gnd (boost mode). when the coil current has ramped down to a lower threshold value, gate goes high, the switch is turned on again and the cycle of events repeats, resulting in continuous oscillation. figure 3. boost and buck-boost configuration the feeback loop adjusts the nmos switch duty cycle to stabilize the led current in response to changes in external conditions, including input vo ltage and load voltage. loop compensation is achieved by a single external capacitor c2, connected between shp and sgnd. note that in reality, a load capacitor c out is used, so that the led current waveform shown is smoothed. the average current in the sense resistor and coil, i rs , is equal to the average of the maximum and minimum threshold currents and the ripple current (hysteresis) is equal to the difference between the thresholds. the average current in the led, i led , is always less than i rs . the feedback control loop adjusts the switch duty cycle, d, to achieve a set point at the sense resistor. this controls i rs . during the interval t off , the coil current flows through d1 and the led load. during t on , the coil current flows through q1, not the leds. therefore the set point is modified by d using a gating function to control i led indirectly. in order to compensa te internally fo r the effect of the gating function, a control factor, gi_adj is used. gi_adj is set by a pair of external resistors, r gi1 and r gi2 . ( figure 3. ) this allows the sense voltage to be adjusted to an optimum level for power efficiency without significant error in the led controlled current. gi_adj = rgi1 rgi1 +rgi2 equation 2 (boost and buck-boost modes) the control loop sets the duty cycle so that the sense resistor current is i rs = 0.225 r s gi_adj 1-d v adj v ref equation 3 (boost and buck-boost modes) figure 4. operating waveforms (boost and buck-boost modes) i rs equals the coil current. the coil is connected only to the sw itch and the schottky diode. the schottky diode passes the led current. therefore the average led current is the coil current multiplied by the schottky diode duty cycle, 1-d. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 22 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) i led = i rs 1-d = 0.225 r s gi_adj v adj v ref (boost and buck-boost) equation 4 this shows that the led current depends on the adj pin volt age, the reference voltage and 3 resistor values (rs, rgi1 and rgi2). it is independent of the input and output voltages. if the adj pin is connected to the ref pin, this simplifies to i led = 0.225 r s gi_adj (boost and buck-boost) now i led is dependent only on the 3 resistor values. considering power dissipation and accuracy, it is useful to know how the mean sense voltage varies with input voltage and other parameters. v rs = i rs r s = 0.225 gi_adj 1-d v adj v ref (boost and buck-boost) equation 5 this shows that the sense voltage varies with duty cycle in boost and buck-boost configurations. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 23 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) application circuit design external component selection is driven by the characteristics of the load and the input supply, since this will determine the kind of topology being used for the system. component selection begins with the current setting procedure, the inductor/frequency setting and the mosfet selection. fi nally after selecting the freewheeling diode and the output capacitor (if needed), the application section will cover th e pwm dimming and thermal feedback. the full procedure is greatly accelerated by the web calculator spreadsheet, which includes fully automated component selection, and is available on the diodes web site. however the full calculation is also given here. please note the following particular feature of the web calculator . the gi ratio can be set for automatic calculation, or it c an be fixed at a chosen value. when optimiz ing a design, it is best first to optimize for the chosen voltage range of most interest, using the automatic setti ng. in order to subsequently evaluate perform ance of the circuit over a wider input voltage range, fix the gi ratio in the calculator input fi eld, and then set the desired input voltage range. some components depend upon the switching frequency and the duty cycle. the switching frequency is regulated by the ZXLD1371 to a large extent, depending upon conditions. this is discussed in a later paragraph dealing with coil selection. duty cycle calculation and topology selection the duty cycle is a function of the inpu t and output voltages. approximately, the mosfet switching duty cycle is d buck v out v in for buck d boost v out - v in v out for boost d bb v out v out + v in for buck-boost equation 6 because d must always be a positive number less than 1, these equations show that v out < v in for buck (voltage step-down) v out > v in for boost (voltage step-up) v out > or = or < v in for buck-boost (voltage step-down or step-up) this allows us to select the topol ogy for the required voltage range. more exact equations are used in the web calculator. these are: d buck = v out + v f + i out r s +r coil v in + v f - v dson for buck d boost = v out - v in + i in r s +r coil + v f v out + v f - v dson for boost d bb = v out + v f + i in +i out r s +r coil v ou t + v in + v f - v d so n for buck-boost equation 7 where v f = schottky diode forward voltage, estimated for the expected coil current, i coil v dson = mosfet drain source volta ge in the on condition (dependent on r dson and drain current = i coil ) r coil = dc winding resistance of l1 ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 24 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) the additional terms are relatively small, so the exact equations will only make a significant difference at lower operating voltages at the input and output, i.e. low input voltage or a small number of leds connected in series. the estimates of v f and v dson depend on the coil current. the mean coil current, i coil depends upon the topology and upon the mean terminal currents as follows: i coil = i led for buck i coil = i in for boost i coil = i in + i led for buck-boost equation 8 i led is the target led current and is already known. i in will be calculated with some accuracy later, but can be estimated now from the electrical power efficiency. if the expected efficiency is roughly 90%, the output power p out is 90% of the input power, p in , and the coil current is estimated as follows. p out 0.9 p in or i led n v led 0.9 i in v in where n is the number of leds connected in series, and v led is the forward voltage drop of a single led at i led . so i in i led n v led 0.9 v in equation 9 equation 9 can now be used to find i coil in equation 8 , which can then be used to estimate the small terms in equation 7 . this completes the calculation of duty cycle and t he selection of buck, boost or buck-boost topology. an initial estimate of duty cycle is required before we can choose a coil. in equation 7, the following approximations are recommended: v f = 0.5v i in (r s +r coil ) = 0.5v i out (r s +r coil ) = 0.5v v dson = 0.1v (i in +i out )(r s +r coil ) = 1.1v then equation 7 becomes d buck v out + 1 v in + 0.4 for buck d boost v out - v in + 1 v out + 0.4 for boost d bb v out + 1.6 v ou t + v in + 0.4 for buck-boost equation 7a setting the led current the led current requirement determines the choice of the sense resistor rs. this also depends on the voltage on the adj pin and the voltage on the gi pin, acco rding to the topology required. the adj pin may be connected directly to the internal 1.25v reference (v ref ) to define the nominal 100% led current. the adj pin can also be driven with an external dc voltage be tween 125mv and 1.25v to adjust the led current proportionally between 10% and 100% of the nominal value. for a divider ratio gi_adj greater than 0.65v, the ZXLD1371 operates in buck mode when v adj = 1.25v. if gi_adj is less than 0.65v (typical), the device operates in boost or buck- boost mode, according to the load connection. this 0.65v threshold varies in proportion to v adj , i.e., the buck mode threshold voltage is 0.65 v adj /1.25 v. adj and gi are high impedance inputs within their normal oper ating voltage ranges. an internal 1.3v clamp protects the device against excessive input voltage and limits the maximum output current to approximately 4% above the maximum current set by v ref if the maximum input voltage is exceeded. ZXLD1371 document numbe r applicati o buck topol o in buck mo d current dep e above, if adj is dir e boost and b for boost a upon the r e above. th e there is not a if adj is co n gi_adj is g note that fro m 22k ? the addition a as mentione d buck-boost c 0.2 < the mean vo v rs = note that if g same coil c u consider eq u this shows t h example if d increase the offset uncert a 100mv. r : ds35436 rev. 1 o ns infor m o gy d e, gi is con n e nds only upo e ctly connecte d b uck-boost t o a nd buck-boo s e sistors, r s , r e re is more tha n a unique soluti n nected to re f iven by equat i m consideratio n ? < r gi1 < 10 a l degree of fr e d above, the w onfiguration is < gi_adj < 0 ltage across t h = i coil r s g i_adj is mad e u rrent, the dis s u ation 5 . if a d h at v rs beco m = 0.2, and gi led current e a inty in the z x - 2 m ation (con t n ected to adj n r s , v adj an d d to vref, thi s o pology s t topologies, r gi1 , and r gi2 n one degree o on. from eq u f , this become s i on 2, repeate d n s of zxld13 7 0k ? e edom allows w orking voltag e 0 .5 h e sense resist e larger, these s ipation in r s d j is connecte d m es smaller th a _adj is the m rror due to sm x ld1371. if n o w w t .) as in figure 5 d v ref . from e q s becomes: the led curr e as in equati o o f freedom. t h u ation 4 , e q s d here for con v 7 1 input bias c u us to select g e range at the or is equations sh o is increased. d to ref, this b a n 225mv if g i m inimum value all offsets in t h o w, gi_adj is 25 of 42 w w.diodes.com 5 . the led equation 1 q uation 10 e nt depends o ns 4 and 2 h at is to say, q uation 11 v enience: u rrent, the rec o i_adj within l i gi pin is rest o w that r s is i n so, in some b ecomes i _adj < 1 - d . of 0.2, then v h e system, su c increased to a di o figur e figure 6. b o mmended limi i mits but this m ricted. the p e n creased and v cases, it is b e . if also d is s v rs becomes 0 c h as mv drop 0.4 or 0.5, v r s a product li n o des incorpo r e 5. setting l e configu setting led b uck-boost c o ts for r gi1 are: e m ay affect ove r e rmitted range e e v rs is increa s e tter to minim i s mall, v rs can .225* 0.2 / 0. 8 in the copper s is increased zx febr u ? diodes n e o f r ated e d current in b ration current in bo o o nfigurations e quation 12 r all performan c of gi_adj in e quation 13 e quation 14 s ed . therefo r i ze gi_adj. become too s m 8 = 56.25 mv. printed wiring to a value gr e ld1371 u ary 2012 incorporated b uck o st and c e a little. boost or r e, for the however, m all. for this will circuit, or e ater than ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 26 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) this will give small enough i led error for most practical purposes. sa tisfactory operation will be obtained if v rs is more than about 80mv. this means gi_adj should be greater than (1-d min ) * 80/225 = (1- d min ) * 0.355. there is also a maximum limit on v rs which gives a maximum limit for gi_adj. if v rs exceeds approximately 300mv, or 133% of 225mv, the status out put may indicate an over-current condition. this will happen for larger d max . therefore, together with the requirement of equation 13 , the recommended range for gi_adj is 0.355 ( 1-d min ) < gi_adj < 1.33 ( 1-d max ) equation 15 an optimum compromise for gi_adj has been suggested, i.e. gi_adj auto = 1 - d max equation 16 this value has been used for the ?automat ic? setting of the web calculator. if 1-d max is less than 0.2, then gi_adj is set to 0.2. if 1- d max is greater than 0.5 then gi_adj is set to 0.5. once gi_adj has been selected, a value of rgi1 can be selected from equation 12 . then rgi2 is calculated as follows, rearranging equation 2 : r gi2 = r gi1 1-gi_adj gi_adj equation 17 for example to drive 12 leds at a current of 350ma from a 12v supply requires boost configuration. each led has a forward voltage of 3.2v at 350ma, so vout = 3.2*12 = 38.4v. from equation 6 , the duty cycle is approximately v out -v in v out = 38.4-12 38.4 = 0.6875 from equation 16 , we set gi_adj to 1 ? d = 0.3125. if r gi1 = 33k ? , then from equation 17 , r gi2 = 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6k ? . let us choose the preferred value r gi2 = 75k? . now gi_adj is adjusted to the new value, using equation 2 . gi_adj = rgi1 rgi1 +rgi2 = 33k 33k +75k =0.305 now we calculate rs from equation 11 . assume adj is connected to ref. r sboostbb = 0.225 i led gi_adj v adj v ref = 0.225 0.35 * 0.305 = 0.196 ? a preferred value of r sboostbb = 0.2 ? will give the desired led current with an error of 2% due to the preferred value selection. table 1 shows typical resistor values used to det ermine the gi_adj ratio with e24 series resistors. table 1 gi ratio rgi1 rg2 0.2 30k 120k 0.25 33k 100k 0.3 39k 91k 0.35 30k 56k 0.4 100k 150k 0.45 51k 62k 0.5 30k 30k this completes the led current setting. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 27 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) inductor selection and frequency control the selection of the inductor coil, l1, requires knowledge of the switching frequency and current ripple, and also depends on the duty cycle to some extent. in the hysteretic converte r, the frequency depends upon t he input and output voltages and the switching thresholds of t he current monitor. the peak-to-peak coil current is adjusted by the ZXLD1371 to control the frequency to a fixed value. this is done by controlling the switching thresholds within particular limits. this effectively m uch reduces the overall frequency range for a given input voltage range. where the input voltage range is not excessive, the frequency is regulated to approximately 39 0khz. this is helpful in terms of emc and other system requirements. figure 7 shows practical results of switching frequency driving 8 leds at 350ma. figure 7. frequency vs. v in for boost led driver with 350ma led current and various inductor values for larger input voltage variation, or when the choice of co il inductance is not optimum, t he switching frequency may depart from the regulated value, but the regulati on of led current remains successful. if desired, the frequency can to some extent be increased by using a smaller inductor, or decreased using a larger inductor. the web calculator will evaluate the frequency across the input voltage range and the effect of this upon power efficiency and junction temperatures. determination of the input voltage range for which the frequenc y is regulated may be required. this calculation is very involved, and is not given here. however the performance in this respect can be evaluated within the web calculator for the chosen inductance. the inductance is given as follows in terms of peak-to-peak ripple current in the coil, i l and the mosfet on time, t on . l1 = v in - n v led - i out r dson + r coil + r s t on ? i l for buck l1 = v in - i in r dson + r coil + r s t on ? i l for boost l1 = v in - (i in + i out ) r dson + r coil + r s t on ? i l for buck-boost equation 18 therefore in order to calculate l1, we need to find i in , t on , and i l . the effects of the resistances are small and will be estimated. i in is estimated from equation 9 . t on is related to switching frequency, f, and duty cycle, d, as follows: t on = d f equation 19 as the regulated frequency is known, and we have already found d from equation 7 or the approximation equation 7b , this allows calculation of t on . 6 57810 911121314 15 16 17 18 19 20 v (v) in t = 25 c, v = v 8 leds, r = 150m , r9 = 120k , r10 = 36k , c = 100f aa u xi n s in ? l = 33h 0 50 100 150 200 250 300 350 400 450 500 switching frequency (khz) l = 100h l = 68h ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 28 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) the ZXLD1371 sets the ripple current, i l , to between nominally 10% and 30% of the mean coil current, i coil , which is found from equation 8 . the device adjusts the ripple current within this range in order to regulate the switching frequency. we therefore need to use a i l value of 20% of i coil to find an inductance which is optimiz ed for the input voltage range. the range of ripple current control is also mo dulated by other circuit parameters as follows. ? i lmax = 0.06 +0.24 v adj v ref 1-d gi_adj i coil ? i lmin = 0.02 +0.08 v adj v ref 1-d gi _ adj i coil equation 20 ? i lmid = 0.04 +0.16 v adj v ref 1-d gi_adj i coil if adj is connected to ref, this simplifies to ? i lmax = 0.3 1-d gi_adj i coil ? i lmin = 0.1 1-d gi _ adj i coil equation 20a ? i lmid = 0.2 1-d gi_adj i coil where i lmid is the value we must use in equation 18 . we have now established the inductance value. the chosen coil should saturate at a curr ent greater than the peak sensed current. this saturation current is the dc current for which the inductance has decreased by 10% compared to the low current value. assuming 10% ripple current, we c an find this peak current from equation 8 , adjusted for ripple current: i coilpeak = 1.1 i led for buck i coilpeak = 1.1 i inmax for boost equation 21 i coilpeak = 1.1 i inmax + i led for buck-boost where i inmax is the value of i in at minimum v in . the mean current rating is also a factor, but normally the saturation current is the limiting factor. the following websites may be useful in finding suitable components www.coilcraft.com www.niccomp.com www.wuerth-elektronik.de mosfet selection the ZXLD1371 requires an external nmos fet as the main power switch with a voltage rating at least 15% higher than the maximum circuit voltage to ensure safe operation during the overshoot and ringing of t he switch node. the current rating is recommended to be at least 10% higher than the average transistor current. the power rating is then verified by calculating the resistive and switching power losses. resistive power losses the resistive power losses are calculated using the rm s transistor current and the mosfet on-resistance. calculate the current for the di fferent topologies as follows: buck mode boost / buck-boost mode switching p resistive pp + = led i mosfetmax i = max d1 led i mosfetmax i ? = ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 29 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) during the on-time, the mosfet switch current is equa l to the coil current. the rms mosfet current is i coil d where i coil is the mean coil current. therefore the approximate rms current in the mosfet during t on is: buck mode boost / buck-boost mode the resistive power dissipation of the mosfet is: switching power losses calculating the switching mosfet's sw itching loss depends on many factors t hat influence both tu rn-on and turn-off. using a first order rough approximation, the switching power dissipation of the mosfet is: where c rss is the mosfet's reverse-transfer capacitance (a data sheet parameter), f sw is the switching frequency, i gate is the mosfet gate-dr iver's sink/source current at t he mosfet's turn-on threshold. matching the mosfet with the controller is primarily based on the rise and fall time of the gate voltage. the best rise/fall time in the application is based on many requirements, such as emi (conducted and radiated), switching losses, lead/circuit inductance, switching frequency, etc. how fast a mosfet can be turned on and off is related to how fast the gate capacitance of the mosfet can be char ged and discharged. the relationship between c (and the relative total gate charge qg), turn-on/turn-off time and the mosf et driver current rating can be written as: where dt = turn-on/turn-off time dv = gate voltage c = gate capacitance = qg/v i = drive current ? constant current source (for the given voltage value) here the constant current source? i ? usually is approximated wi th the peak drive current at a given driver input voltage. example 1) using the dmn6068 mosfet (v ds(max) = 60v, i d(max) = 8.5a): ? q g = 10.3nc at v gs = 10v ZXLD1371 i peak = i gate = 300ma assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum frequency allowed in this condition is: t period = 20*dt ? f = 1/ t period = 1.43mhz this frequency is well above the max frequency the device can handle, therefore the dnm6068 can be used with the ZXLD1371 in the whole spectrum of frequencies recommended for the device (from 300khz to 1mhz). di i led mosfetrms = led mosfetrms ix d1 d i ? = )on(ds rx 2 mosfetrms i resistive p = gate i load ix sw fx in 2 vx rss c switching p = i qg i cdv dt = ? = ns35 ma300 nc3.10 peak i g q dt = = = ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 30 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) example 2) using the zxmn6a09k (v ds(max) = 60v, i d(max) = 12.2a): ? q g = 29nc at v gs = 10v ZXLD1371 i peak = 300ma assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum frequency allowed in this condition is: t period = 20*dt ? f = 1/ t period = 515khz this frequency is within the recommended frequency range the device can handle, therefore the zxmn6a09k is recommended to be used with the ZXLD1371 for frequencies from 300khz to 500khz). the recommended total gate charge for the mosfet used in conjunction with the ZXLD1371 is less than 30nc. junction temperature estimation finally, the ZXLD1371 junction temperature can be estimated using the following equations: total supply current of ZXLD1371: i qtot i q + f ? q g where i q = total quiescent current i q-in + i q-aux power consumed by ZXLD1371 p ic = v in ? (i q + f ? qg) or in case of separate voltage supply, with v aux < 15v p ic = v in ? i q-in + v aux ? (i q-aux + f ? qg) t j = t a + p ic ? ja = t a + p ic ? ( jc + ca) where the total quiescent current i qtot consists of the static supply current (i q ) and the current required to charge and discharge the gate of the power mosfet. moreover the part of thermal resistance between case and ambient depends on the pcb characteristics. figure 8. power derating curve for zxld1370 mounted on test board according to jesd51 ns97 ma300 nc29 peak i g q dt = = = 0 0.5 1 1.5 2 2.5 -40 -25 -10 5 20 35 50 65 80 95 110 125 ambient temperature (c) p o we r dissipati o n (mw) ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 31 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) diodes selection for maximum efficiency and performance, the rectifier (d1) should be a fast low capacitance schottky diode* with low reverse leakage at the maximum operating voltage and temper ature. the schottky diode also provides better efficiency than silicon pn diodes, due to a combination of lower forward voltage and reduced recovery time. it is important to select parts with a peak current rating abov e the peak coil current and a continuous current rating higher than the maximum output load current. in particular, it is recommended to have a voltage rating at least 15% higher than the maximum transistor voltage to ensure safe operation during the ringing of the switch node and a current rating at least 10% higher than the average diode current. the power rating is verified by calculating the power loss through the diode. the higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the drain of the external mosfet. if a silicon diode is used, care should be taken to ensure that the total voltage appearing on the drain of the external mosfet, including supply ripple, does not exceed the specified maximum value. *a suitable schottky diode for a switching current of up to about 1.5a would be pds3100 (diodes inc). output capacitor an output capacitor may be required to limit interferen ce or for specific emc purposes. for boost and buck-boost regulators, the output capacitor provides energy to the load wh en the freewheeling diode is reverse biased during the first switching subinterval. an output capacitor in a buck topology will simply reduce the led current ripple below the inductor current ripple. in other words, this capacitor changes the current waveform through the led(s) from a triangular ramp to a more sinusoidal version without altering the mean current value. in all cases, the output capacitor is chosen to provide a desired current ripple of the led current (usually recommended to be less than 40% of the average led current). buck: boost and buck-boost ppled led sw ppl output ixrxf ixd c ? ? = where: ? i l-pp is the ripple of the inductor current, usually 20% of the average sensed current ? i led-pp is the ripple of the led current, it s hould be <40% of the leds average current ? f sw is the switching frequency (from graphs and calculator) ? r led is the dynamic resistance of the leds string (n times the dynamic resistance of the single led from the datasheet of the led manufacturer). the output capacitor should be chosen to account for derating du e to temperature and operating voltage. it must also have the necessary rms current rating. the minimum rms current for the output capacitor is calculated as follows: buck boost and buck-boost ceramic capacitors with x7r dielectric are the best choice due to their high ripple current rating, long lifetime, and performance over the voltage and temperature ranges. ppled led sw ppl output ixrxfx8 i c ? ? = 12 i i ppled rms coutput ? = max max led coutputrms d1 d i i ? = ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 32 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) input capacitor the input capacitor can be calculated knowing the input voltage ripple v in-pp as follows: buck use d = 0.5 as worst case boost buck-boost use d = d max as worst case the minimum rms current for the output capacitor is calculated as follows: buck use d=0.5 as worst case boost buck-boost use d=d max as worst case led current dimming the ZXLD1371 has 3 dimming methods for reducing the average led current 1. dc dimming using the adj pin 2. pwm dimming using the pwm pin 3. dc dimming for thermal protection using the tadj pin. dc dimming the ZXLD1371 has a clamp on the adj pin to prevent over-driving of the led current which results in the maximum voltage being applied to internal circuitry is the reference volt age. this provides a 10:1 dynamic range of dc led current adjustment. the equation for dc dimming of the led current is approximately: i led_dim =i led_nom v adj v ref where i led_dim is the dimmed led current i led_nom is the led current with v adj = 1.25v one consequence of dc dimming is that as the adj pin voltage is reduced the sense voltage will also be reduced which has an impact on accuracy and switching frequency especially at lower adj pin voltages. figure 9. led current and switching frequency vs. adj voltage ppin sw led in vxf ix)d1(xd c ? ? = ppin sw ppl in vxfx8 i c ? ? = ppin vx sw f led ixd in c ? = )d1(dxxii led rmscin ? = ? 12 i i ppl rms cin ? ? = )d1( d xii led rmscin ? = ? 0 150 300 450 600 750 0 150 300 450 600 750 0 0.25 0.5 0.75 1 1.25 adj voltage (v) led current (ma) switching frequency (khz) switching frequency t=25c v=v=12v 2 leds l=33h r=300m a aux in s ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 33 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) pwm output current control & dimming the ZXLD1371 has a dedicated pwm dimming input that allows a wide dimming frequency range from 100hz to 1khz with up to 1000:1 resolution; however higher dimming frequencies can be used ? at the expense of dimming dynamic range and accuracy. typically, for a pwm frequency of 1khz, the error on the current linearity is lower than 5%; in particular the accuracy is better than 1% for pwm from 5% to 100%. this is shown in the graph below: figure 10. led current linearity and accuracy with pwm dimming at 1khz for a pwm frequency of 100hz, the error on the current linearity is lower than 2.5%; it becomes negligible for pwm greater than 5%. this is shown in the graph below: figure 11. led current linearity and accuracy with pwm dimming at 100hz the pwm pin is designed to be driven by both 3.3v and 5v logic levels and as such doesn?t require open collector/drain drive. it can also be driven by an open drain/collector transistor. in this case the designer can either use the internal pull -up network or an external pull-up network in order to speed-up pwm transitions, as shown in the boost/ buck-boost section. buck mode - l=33uh - rs = 150m ? - pwm @ 1khz 0.00 250.00 500.00 750.00 1000.00 1250.00 1500.00 0 102030405060708090100 pwm led current [ma] 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% error pwm @ 1khz error buck mode - l=33uh - rs = 150m ? - pwm @ 100hz 0.00 250.00 500.00 750.00 1000.00 1250.00 1500.00 0 102030405060708090100 pwm led current [ma] 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% error pwm @ 100hz error ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 34 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) led current can be adjusted digitally, by applying a low frequency pwm logic signal to the pwm pin to turn the controller on and off. this will produce an average output current proportional to the duty cycle of the control signal. during pwm operation, the device remains powered up and only the output switch is gated by the control signal. the pwm signal can achieve very high led current resolution. in fact, dimming down from 100% to 0, a minimum pulse width of 2s can be achieved resulting in very high accuracy. while the maximum recommended pulse is for the pwm signal is10ms. figure 12. pwm dimming minimum and maximum pulse the device can be put in standby by taking the pwm pin to ground, or pulling it to a voltage below 0.4v with a suitable open collector npn or open drain nmos transistor, for a time exceeding 15ms (nominal). in the shutdown state, most of the circuitry inside the device is switched off and residual quiescent current will be typically 90a. in particular, the status p in will go down to gnd while the flag and ref pins will stay at their nominal values. figure 13. stand-by state from pwm signal thermal control of led current for thermal control of the leds, the ZXLD1371 monitors the voltage on the tadj pin and reduces output current if the voltage on this pin falls below 625mv. an external ntc ther mistor and resistor can therefore be connected as shown below to set the voltage on the tadj pin to 625mv at the required te mperature threshold. this will give 100% led current below the threshold temperature and a falling current above it as shown in the graph. the temperature threshold can be altered by adjusting the value of rth and/or the thermistor to suit the requirements of the chosen led. the thermal control feature can be disabled by connecting tadj directly to ref. here is a simple procedure to design the thermal feedback circuit: 1) select the temperature threshold t threshold at which the current must start to decrease 2) select the thermistor th1 (both resistive value at 25 ? c and beta) 3) select the value of the resistor r th as r th = th at t threshold pwm 0v < 10 ms 0v 2s gate < 10 ms 2 s ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 35 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) figure 14. thermal feedback network the thermistor resistance, r t , at a temperature of t degrees kelvin is given by r t = r r e b 1 t - 1 t r where r r is the thermistor resistance at the reference temperature, t r t r is the reference temperature, in kelvin, normally 273 + 25 = 298k (25c) b is the ?beta? value of the thermistor. for example, 1) temperature threshold t threshold = 273 + 70 = 343k (70 ? c) 2) th1 = 10k ? at 25 ? c and b = 3900 ? r t = 1.8k ? @ 70 ? c 3) r th = r t at t threshold = 1.8k ? over-temperature shutdown the ZXLD1371 incorporates an over-temperature shutdown circuit to protect against damage caused by excessive die temperature. a warning signal is generated on the status output when die temperature exceeds 125c nominal and the output is disabled when die temperature exceeds 150c nomi nal. normal operation resumes when the device cools back down to 125c. ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 36 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) flag/status outputs the flag/status outputs provide a warning of extreme operat ing or fault conditions. flag is an open-drain logic output, which is normally off, but switches low to indicate that a wa rning, or fault condition exists. status is a dac output, which is normally high (4.5v), but switches to a lower voltage to indicate the nature of the warning/fault. conditions monitored, the method of detection and the nominal status output voltage are given in the following table (note 15): table 2 warning/fault condition severity (note 16) monitored parameters flag nominal status voltage normal operation h 4.5v supply under-voltage 1 v aux < 5.0v l 4.5v 2 v in < 5.6v l < 3.6v output current out of regulation (note 17) 2 v shp outside normal voltage range l 3.6v driver stalled with switch ?on?, or ?off? (note 18) 2 t on , or t off > 100s l 3.6v device temperature above maximum recommended operating value 3 t j > 125c l 1.8v sense resistor current i rs above specified maximum 4 v sense > 0.3v l 0.9v notes: 15. these status pin voltages apply for an input voltage,v in , of 7.5v < v in < 60v. below 7.5v the status pin voltage levels reduce and therefore may not report the correct status. for 5.4v < v in < 7.5v the flag pin still reports an error by going low. at low v in in boost and buck-boost modes an over-current status may be indicated when operating at high boost ratios -? this due to the feedback loop increasing the sen se voltage. 16. severity 1 denotes lowest severity. 17. this warning will be indicated if the output power demand is higher than the available input power; the loop may not be ab le to maintain regulation. 18. this warning will be indicated if the gate pin stays at the same level for greater than 100s (e.g. the output transistor cannot pass enough current to reach the upper switching threshold). figure 15. status levels ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 37 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) in the event of more than one fault/warning condition occurring, the higher severity condition will take precedence. e.g. ?excessive coil current? and ?out of regulation? occurring to gether will produce an output of 0.9v on the status pin. if v adj >1.7v, v sense may be greater than the excess coil current threshold in normal operation and an error will be reported. hence, status and fl ag are only guaranteed for v adj <=v ref . diagnostic signals should be ignored during the device start ? up for 100 s. the device start up sequence will be initiated both during the first power on of the device or after the pwm signal is kept low for more than 15ms, initiating the standby state of the device. in particular, during the first 100 s the diagnostic is signaling an over-current then an out-of-regulation status. these two events are due to the charging of the inductor and are not true fault conditions. figure 16. diagnostic during start-up reduced input voltage operation to facilitate operation in automo tive and other applications, that have large transient reducti ons in system supply voltage, the ZXLD1371 is now capable of operating down to input voltages as low as 5.0v. care must be taken when operating at these lower supply voltages to ensure that the external mosfet is correctly enhanced and that the boosting ratio is not increased to excessive amounts where both the duty cycle and peak-switch current limits are not exceeded. the device will operate down to 5.0v, but for reliable start up v in must be higher than 5.4v. the designer should also take into account any noise that may occur on the supply lines. in buck-boost and boost modes (most common topologies for applications likely to require transient operation down to supply voltages approaching 5.0v) as the input voltage reduces then the peak switch current will increase the ZXLD1371 compensates for this by allowing the sense voltage to increase while maintaining regulation of the led current. however if the boost ratio (switch output voltage/input voltage) is in creased too much then the sense voltage could be increased too much causing an over-current flag to be triggered and/or loss of regulation. in addition to this, increased power dissipation will occur in the external mosfet switch ? especially if the external mosfet has a large threshold. one way of overco ming this is to apply a boot-strap network to the v aux pin ? see next section. if the ZXLD1371 is used in buck mode at low voltages then the boot-strap network cannot be implemented and so a low threshold mosfet with low gate capacitance should be used. some loss of regulation is expected to occur at voltages below 6v ? see buck mode typical characteristics section. when using the ZXLD1371 in applications with transient input voltage excursions we recommend using the web calculator to optimize operation over the normal operating band. then c hange the input range to include the transient excursion while keeping the optimized component selection to check expect ed function during the transient input voltage conditions. vref 0v over current 225mv/r1 0a flag status coil current out of regulation 100us ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 38 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) boosting v aux supply voltage in boost and buck-boost mode this means that depending on the characteristics of the external mosfet, the gate voltage may not be enough to fully enhance the power mosfet. a boot-strap boosting technique can be used to increase the gate drive voltage at low input voltage. see figure 17 for circuit diagram. this can be particula rly important for extended use at low input voltages as this i s when the switch current will be at its greatest ? resu lting in greatest heat gener ation within the mosfet. figure 17. bootstrap circuit for boost and buck-boost low voltage operations the bootstrap circuit guarantees that the mosfet is full y enhanced reducing both the power dissipation and the risk of thermal runaway of the mosfet itself. the bootstrap circuit consists of an extra diode d2 and decoupling capacitor c3 which are used to generate a boosted voltage at v aux . this enables the device to operate with full output current when v in is at the minimum value of 5v. the resistor r2 can be used to limit the current in the bootstrap circuit in order to reduce the impact of the circuit itself on the led accuracy. a typical value would be 100 ohms. the impact on the led current is usually a decrease of maximum 5% compared to the nominal current value set by the sense resistor. the zener diode d3 is used to limit the voltage on the v aux pin to less than 60v. due to the increased number of components and the loss of current accuracy, the bootstrap circuit is recommended only when the system has to operate continuously in conditions of low input voltage (between 5 and 8v) and high load current. other circumstances such as low input voltage at low load current, or transient low input voltage at high current should be evaluated keeping account of the external mosfet?s power dissipation. figure 18. effect of bootstrap on led current in buck-boost mode ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 39 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) over-voltage protection the ZXLD1371 is inherently protected against open-circuit load wh en used in buck configuration. however care has to be taken with open-circuit load conditions in buck-boost or boost configurations. this is because in these configurations there is no internal open-circuit protection mechanism for the exter nal mosfet. in this case an over-voltage-protection (ovp) network should be provided externally to the mosfet to avoid damage due to open circuit conditions. this is shown in figure 19 below, highlighted in the dotted blue box. figure 19. ovp circuit the zener voltage is determined according to: vz = v ledmax +10% where v ledmax is maximum led chain voltage. if the leda voltage exceeds v z the gate of mosfet q2 will rise turning q2 on. this will pull the pwm pin low and switch off q1 until the voltage on the drain of q1 falls below vz. if the voltage at leda remains above v z for longer than 20ms then the ZXLD1371 will enter into a shutdown state. care should be taken such that the maximum gate voltage of the q2 mosfet is not exceeded. take care of the max voltage drop on the q2 mosfet gat e. typical devices for z1 and q2 are bzx84c and 2n7002 ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 40 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated applications information (cont.) pcb layout considerations pcb layout is a fundamental to device performance in all configurations. figure 20 shows a section of a proven ZXLD1371 pcb layout. figure 20. circuit layout here are some considerations useful for the pcb layout using ZXLD1371 in buck, boost and buck-boost configurations: ? in order to avoid ringing due to stray inductances, the inductor l1, the anode of d1 and the drain of q1 should be placed as close together as possible. ? the shaping capacitor c1 is fundamental for the stability of the control loop. to this end it should be placed no more than 5mm from the shp pin. ? input voltage pins, vin and vaux, need to be decoupled. it is recommended to use two ceramic capacitors of 2.2uf, x7r, 100v (c3 and c4). in addition to these capacitors, it is suggested to add two ceramic capacitors of 1uf, x7r, 100v each (c2, c8), as well as a further decoupling capacitor of 100nf close to the vin/vaux pins (c9). vin and vaux pins can be short-circuited when the device is used in buck mode, or can be driven from a separate supply. ? the underside of the pcb should be a solid copper ground plane, electrically bonded to top ground copper at regular intervals using plated-thro via holes. the ground plane should be unbroken as far as possible, particularly in the area of the switching circuit including the ZXLD1371, l1, q1 d, c3 and c4. plated via holes are necessary to provide a short electrical path to minimize stray inductanc e. critical positions of via holes include the decoupling capacitors, the source connection of the mosfet and the ground connections of the ZXLD1371, including the centre paddle. these via holes also serve to conduct heat away from the semiconductors and minimize the device junction temperatures. evaluation boards to support easier evaluation of the ZXLD1371 three evalua tion boards have been developed which available via your diodes sales representative for qualified opportunities: ZXLD1371ev1 buck configuration ZXLD1371ev2 buck-boost configuration ZXLD1371ev3 boost configuration shp pin v in / v aux decou p lin g inductor, switch and freewheeling diode ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 41 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated ordering information device (note 19) packaging status part marking reel quantity tape width reel size ZXLD1371est16tc tssop-16ep preview zxld 1371 yyww 2500 16mm 13? ZXLD1371qesttc tssop-16ep preview zxld 1371 yyww 2500 16mm 13? note: 19. for automotive grade with aec-q100 qualification the ZXLD1371qesttc should be ordered. where yy is last two digits of year and ww is two digit week number package outline dimensions (all dimensions in mm) suggested pad layout tssop-16ep dim min max a 4.9 5.10 b 4.30 4.50 c ? 1.2 d 0.8 1.05 f 1.00 ref. g 0.65 ref. k 0.19 0.30 l 6.40 ref. a1 7 a2 0 8 all dimensions in mm dimensions value (in mm) c 0.650 x 0.450 x1 3.290 x2 5.000 y 1.450 y1 3.290 y2 4.450 y3 7.350 a b l k g c d f a1 a2 pin 1 indent detail ?a? gauge plane seating plane detail ?a ? y2 y 16x y3 c y1 x2 x1 x 16x ZXLD1371 ZXLD1371 document number: ds35436 rev. 1 - 2 42 of 42 www.diodes.com february 2012 ? diodes incorporated a product line o f diodes incorporated important notice diodes incorporated makes no warranty of any kind, express or implied, with regards to this document, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose (and their equivalents under the laws of any jurisdiction). diodes incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or ot her changes without further notice to this document and any product described herein. diodes incorporated does not assume any liabi lity arising out of the application or use of this document or any product described herein; neither does diodes incorporated convey any license under its patent or trademark rights, nor the rights of others. any customer or user of this document or products desc ribed herein in such applications shall assume all risks of such us e and will agree to hold diodes in corporated and all the companies whose products are represented on diodes incorporated website, harmless against all damages. diodes incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthoriz ed sales channel. should customers purchase or use diodes incorporated products for any unintended or unauthorized application, customers shall indemnify and hold diodes incorporated and its representatives harmless against all claims, damages, expenses, and attorney fee s arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized a pplication. products described herein may be covered by one or more united states, international or foreign patents pending. product names and markings noted herein may also be covered by one or more united states, international or foreign trademarks. life support diodes incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the chief executive officer of diodes incorporated. as used herein: a. life support devices or systems are devices or systems which: 1. are intended to implant into the body, or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user. b. a critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support dev ices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of diodes incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by diodes incorporated. further, customers must fully indemnify diodes incorporated and its representatives against any damages arising out of the use of diodes incorporated products in such safety-critical, life support devices or systems. copyright ? 2012, diodes incorporated www.diodes.com |
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