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lt3475/lt3475-1 1 3475fb dual step-down 1.5a led driver the lt ? 3475/lt3475-1 are dual step-down dc/dc converters designed to operate as a constant-current source. an internal sense resistor monitors the output current allowing accurate current regulation ideal for driving high current leds. the high side current sense al- lows grounded cathode led operation. high output current accuracy is maintained over a wide current range, from 50ma to 1.5a, allowing a wide dimming range. unique pwm circuitry allows a dimming range of 3000:1, avoid- ing the color shift normally associated with led current dimming. the high switching frequency offers several advantages, permitting the use of small inductors and ceramic capaci- tors. small inductors combined with the 20 lead tssop surface mount package save space and cost versus alternative solutions. the constant switching frequency combined with low-impedance ceramic capacitors result in low, predictable output ripple. with its wide input range of 4v to 36v, the lt3475/lt3475-1 regulate a broad array of power sources. a current mode pwm architecture provides fast transient response and cycle-by-cycle current limiting. frequency foldback and thermal shutdown provide additional protection. automotive and avionic lighting architectural detail lighting display backlighting constant-current sources true color pwm tm delivers constant color with 3000:1 dimming range wide input range: 4v to 36v operating, 40v maximum accurate and adjustable control of led current from 50ma to 1.5a high side current sense allows grounded cathode led operation open led (lt3475) and short circuit protection lt3475-1 drives led strings up to 25v accurate and adjustable 200khz to 2mhz switching frequency anti-phase switching reduces ripple uses small inductors and ceramic capacitors available in the compact 20-lead tssop thermally enhanced surface mount package dual step-down 1.5a led driver applicatio s u features descriptio u typical applicatio u ef? ciency , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. patents pending. boost1 boost2 sw1 sw2 out1 out2 led1 led2 pwm1 pwm2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in v in 5v to 36v shdn 0.22f 4.7f 10h 10h 0.22f 24.3k 2.2f 2.2f dimming*control *dimming control 0.1f 0.1f 3475 ta01 1.5a ledcurrent 1.5a led current f sw = 600khz *see applications section for details led current (a) 0 efficiency (%) 70 75 80 3475 ta01b 65 60 55 0.5 1 85 90 95 1.5 two series connected white 1.5a leds single white 1.5a led v in = 12v downloaded from: http:///
lt3475/lt3475-1 2 3475fb v in pin .........................................................(-0.3v), 40v boost pin voltage ...................................................60v boost above sw pin ...............................................30v out, led, pins (lt3475) ...........................................15v out, led pins (lt3475-1) .........................................25v pwm pin ...................................................................15v v adj pin ......................................................................6v v c , r t , ref pins ..........................................................3v shdn pin ...................................................................v in maximum junction temperature (note 2)............. 125c operating temperature range (note 3) lt3475e/lt3475e-1 ............................. C40c to 85c lt3475i/lt3475i-1 ............................. C40c to 125c storage temperature range ................... C65c to 150c lead temperature range (soldering, 10 sec) ....... 300c (note 1) the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v boost = 16v, v out = 4v unless otherwise noted (note 3) parameter conditions min typ max units minimum input voltage 3.7 4 v input quiescent current not switching 6 8 ma shutdown current shdn = 0.3v, v boost = v out = 0v 0.01 2 a electrical characteristics absolute axi u rati gs w ww u fe package 20-lead plastic tssop 12 3 4 5 6 7 8 9 10 top view 2019 18 17 16 15 14 13 12 11 out1 led1 boost1 sw1 v in v in sw2 boost2 led2 out2 pwm1v adj1 v c1 refshdn gnd r t v c2 v adj2 pwm2 21 t jmax = 125c, ja = 30c/w, jc = 8c/w exposed pad (pin 21) is ground and must be electrically connected to the pcb. lead free finish tape and reel part marking* package description temperature range lt3475efe#pbf lt3475efe#trpbf lt3475efe 20-lead plastic tssop C40c to 85c lt3475ife#pbf lt3475ife#trpbf lt3475ife 20-lead plastic tssop C40c to 125c lt3475efe-1#pbf lt3475efe-1#trpbf lt3475fe-1 20-lead plastic tssop C40c to 85c lt3475ife-1#pbf lt3475ife-1#trpbf lt3475fe-1 20-lead plastic tssop C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ pin configuration order information downloaded from: http:///
lt3475/lt3475-1 3 3475fb note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: this ic includes overtemperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed 125c when overtemperature protection is active. continuous operation above the speci? ed maximum operating junction temperature may impair device reliability. note 3: the lt3475e and lt3475e-1 are guaranteed to meet performance speci? cations from 0c to 85c. speci? cations over the C40c to 85c operating temperature range are assured by design, characterization and correlation with statistical process controls. the lt3475i and lt3475i-1 are guaranteed to meet performance speci? cations over the C40c to 125c operating temperature range. note 4: current ? ows out of pin. note 5: current ? ows into pin. note 6: current limit is guaranteed by design and/or correlation to static test. slope compensation reduces current limit at higher duty cycles. the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v boost = 16v, v out = 4v unless otherwise noted (note 3) parameter conditions min typ max units led pin current v adj tied to v ref ? 2/3 v adj tied to v ref ? 7/30 lt3475e/lt3475e-1 0c to 85c 0.970.94 0.3360.325 0.31 1.00 0.350 1.031.04 0.3640.375 0.385 aa a a a ref voltage 1.22 1.25 1.27 v reference voltage line regulation 4v < v in < 40v 0.05 %/v reference voltage load regulation 0 < i ref < 500a 0.0002 %/a v adj pin bias current (note 4) 40 400 na switching frequency r t = 24.3k 530 600 640 khz maximum duty cycle r t = 24.3k r t = 4.32k r t = 100k 90 95 8098 %% % switching phase r t = 24.3k 150 180 210 deg foldback frequency r t = 24.3k, v out = 0v 80 khz shdn threshold (to switch) 2.5 2.6 2.74 v shdn pin current (note 5) v shdn = 2.6v 791 1 a pwm threshold 0.3 0.8 1.2 v v c switching threshold 0.8 v v c source current v c = 1v 50 a v c sink current v c = 1v 50 a led to v c transresistance 500 v/a led to v c current gain 1 ma/a v c to switch current gain 2.6 a/v v c clamp voltage 1.8 v v c pin current in pwm mode v c = 1v, v pwm = 0.3v 10 400 na out pin clamp voltage (lt3475) 13.5 14 14.5 v out pin current in pwm mode v out = 4v, v pwm = 0.3v 25 50 a switch current limit (note 6) 2.3 2.7 3.2 a switch v cesat i sw =1.5a 350 500 mv boost pin current i sw =1.5a 25 40 ma switch leakage current 0.1 10 a minimum boost voltage above sw 1.8 2.5 v electrical characteristics downloaded from: http:///
lt3475/lt3475-1 4 3475fb switch current limitvs duty cycle switch current limit vs temperature oscillator frequency vs temperature oscillator frequency foldback oscillator frequency vs r t current limit vs output voltage typical perfor a ce characteristics uw led current vs v adj led current vs temperature switch on voltage v adj (v) 0 0 led current (a) 0.25 0.50 0.75 1.00 1.25 1.50 0.25 0.5 0.75 1 3475 g01 1.25 t a = 25c temperature (?c) C50 led current (a) 0.8 1.0 1.2 25 75 3475 g02 0.6 0.4 C25 0 50 100 125 0.2 0 v adj = v ref ? 7/30 v adj = v ref ? 2/3 switch current (a) 0 0 switch on voltage (mv) 100 200 300 400 0.5 1.0 1.5 2.0 3475 g03 500 600 t a = 25c duty cycle (%) 0 0 current limit (a) 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 80 3475 g04 100 minimum typical t a = 25c temperature (c) C50 2.0 2.5 3.5 25 75 3475 g05 1.51.0 C25 0 50 100 125 0.5 0 3.0 current limit (a) v out (v) 0 0 current limit (a) 0.5 1.0 1.5 2.0 0.5 2.5 3.0 3.5 2.0 1.0 1.5 4.0 3475 g06 2.5 3.0 t a = 25c temperature (?c) C50 oscillator frequency (khz) 600 650 700 25 75 3475 g07 550 500 C25 0 50 100 125 450 400 r t = 24.3k v out (v) 0 500 600 700 2.0 3475 g08 400300 0.5 1.0 1.5 2.5 200 100 0 oscillator frequency (khz) t a = 25c r t = 24.3k r t (k) 1 10 oscillator frequency (khz) 1000 10 100 3475 g09 t a = 25c downloaded from: http:///
lt3475/lt3475-1 5 3475fb pi fu ctio s uuu boost pin current quiescent current out1, out2 (pins 1, 10): the out pin is the input to the current sense resistor. connect this pin to the inductor and the output capacitor. led1, led2 (pins 2, 9): the led pin is the output of the current sense resistor. connect the anode of the led here. v in (pins 5, 6): the v in pins supply current to the internal circuitry and to the internal power switches and must be locally bypassed. sw1, sw2 (pins 4, 7): the sw pin is the output of the internal power switch. connect this pin to the inductor, switching diode and boost capacitor. boost1, boost2 (pins 3, 8): the boost pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. gnd (pins 15, exposed pad pin 21): ground. tie the gnd pin and the exposed pad directly to the ground plane. the exposed pad metal of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. the exposed pad must be soldered to the circuit board for proper operation. use a large ground plane and thermal vias to optimize thermal performance. open-circuit output voltage and input current reference voltage minimum input voltage, single 1.5a white led minimum input voltage, two series connected 1.5a white leds typical perfor a ce characteristics uw switch current (a) 0 boost pin current (ma) 20 25 30 1.5 3475 g10 1510 0.5 1.0 2.0 5 0 35 t a = 25c v in (v) 0 output voltage (v) input current (ma) 25 4035 40 3475 g12 10 0 10 20 30 5020 15 30 5 8 124 0 146 102 45 t a = 25c lt3475-1 lt3475-1 lt3475 lt3475 output voltage input current v in (v) 0 input current (ma) 4 5 6 30 3475 g11 32 10 20 40 1 0 7 t a = 25c led current (a) 0 0 v in (v) 1 2 3 4 6 0.5 1 3475 g14 1.5 5 t a = 25c to run led voltage to start temperature (?c) C50 v ref (v) 1.26 1.27 1.28 25 75 3475 g13 1.25 1.24 C25 0 50 100 125 1.23 1.22 led current (a) 0 5 v in (v) 6 7 8 9 10 0.5 1 3475 g15 1.5 t a = 25c to run led voltage to start downloaded from: http:///
lt3475/lt3475-1 6 3475fb 3475 bd qrqs int reg and uvlo slave osc slope comp slope comp mosc 1 mosc 2 frequency foldback frequencyfoldback +C qr qs master osc slave osc +C driver gm1 gm2 boost1 boost2 d1 d2 d3 d4 d led1 d led 2 q1 q2 q3 q4 c1 c2 c in c out1 c out2 v c2 c c1 v c1 v adj1 v adj2 v in v in v in r t shdn c c2 r t gnd exposed pad ref pwm 1 pwm2 led1 led2 sw1 sw2 out1 out2 0.067 100 0.067 100 1.25k 1.25v 1.25k 2v 2v l1 l2 driver c1 c2 block diagram r t (pin 14): the r t pin is used to set the internal oscillator frequency. tie a 24.3k resistor from r t to gnd for a 600khz switching frequency. shdn (pin 16): the shdn pin is used to shut down the switching regulator and the internal bias circuits. the 2.6v switching threshold can function as an accurate undervoltage lockout. pull below 0.3v to shut down the lt3475/lt3475-1. pull above 2.6v to enable the lt3475/ lt3475-1. tie to v in if the shdn function is unused. ref (pin 17): the ref pin is the buffered output of the internal reference. either tie the ref pin to the v adj pin for a 1.5a output current, or use a resistor divider to generate a lower voltage at the v adj pin. leave this pin unconnected if unused. v c1 , v c2 (pins 18, 13): the v c pin is the output of the internal error amp. the voltage on this pin controls the peak switch current. use this pin to compensate the control loop. v adj1 , v adj2 (pins 19, 12): the v adj pin is the input to the internal voltage-to-current ampli? er. connect the v adj pin to the ref pin for a 1.5a output current. for lower output currents, program the v adj pin using the following formula: i led = 1.5a ? v adj /1.25v. pwm1, pwm2 (pins 20, 11): the pwm pin controls the connection of the v c pin to the internal circuitry. when the pwm pin is low, the v c pin is disconnected from the internal circuitry and draws minimal current. if the pwm feature is unused, leave this pin unconnected. pi fu ctio s uuu downloaded from: http:///
lt3475/lt3475-1 7 3475fb the lt3475 is a dual constant frequency, current mode regulator with internal power switches capable of gen- erating constant 1.5a outputs. operation can be best understood by referring to the block diagram. if the shdn pin is tied to ground, the lt3475 is shut down and draws minimal current from the input source tied to v in . if the shdn pin exceeds 1v, the internal bias circuits turn on, including the internal regulator, reference and oscillator. the switching regulators will only begin to operate when the shdn pin exceeds 2.6v. the switcher is a current mode regulator. instead of directly modulating the duty cycle of the power switch, the feedback loop controls the peak current in the switch during each cycle. compared to voltage mode control, current mode control improves loop dynamics and provides cycle-by- cycle current limit. a pulse from the oscillator sets the rs ? ip-? op and turns on the internal npn bipolar power switch. current in the switch and the external inductor begins to increase. when this current exceeds a level determined by the voltage at v c , current comparator c1 resets the ? ip-? op, turning off the switch. the current in the inductor ? ows through the external schottky diode and begins to decrease. the cycle begins again at the next pulse from the oscillator. in this way, the voltage on the v c pin controls the current through the inductor to the output. the internal error ampli? er regulates the output current by continually adjusting the v c pin voltage. the threshold for switching on the v c pin is 0.8v, and an active clamp of 1.8v limits the output current. the voltage on the v adj pin sets the current through the led pin. the npn, q3, pulls a current proportional to the voltage on the v adj pin through the 100 resistor. the gm ampli? er servos the v c pin to set the current through the 0.067 resistor and the led pin. when the voltage drop across the 0.067 resistor is equal to the voltage drop across the 100 resistor, the servo loop is balanced. tying the ref pin to the v adj pin sets the led pin current to 1.5a. tying a resistor divider to the ref pin allows the programming of led pin currents of less than 1.5a. led pin current can also be programmed by tying the v adj pin directly to a voltage source. an led can be dimmed with pulse width modulation using the pwm pin and an external nfet. if the pwm pin is unconnected or is pulled high, the part operates nominally. if the pwm pin is pulled low, the v c pin is dis- connected from the internal circuitry and draws minimal current from the compensation capacitor. circuitry draw- ing current from the out pin is also disabled. this way, the v c pin and the output capacitor store the state of the led pin current until the pwm is pulled high again. this leads to a highly linear relationship between pulse width and output light, allowing for a large and accurate dimming range. the r t pin allows programming of the switching frequency. for applications requiring the smallest external components possible, a fast switching frequency can be used. if low dropout or very high input voltages are required, a slower switching frequency can be programmed. during startup v out will be at a low voltage. the npn, q3, can only operate correctly with suf? cient voltage of 1.7v at v out , a comparator senses v out and forces the v c pin high until v out rises above 2v, and q3 is op- erating correctly. the switching regulator performs frequency foldback during overload conditions. an ampli? er senses when v out is less than 2v and begins decreasing the oscillator frequency down from full frequency to 15% of the nominal frequency when v out = 0v. the out pin is less than 2v during startup, short circuit, and overload conditions. frequency foldback helps limit switch current under these conditions. the switch driver operates either from v in or from the boost pin. an external capacitor and schottky diode are used to generate a voltage at the boost pin that is higher than the input supply. this allows the driver to saturate the internal bipolar npn power switch for ef? cient operation. operation downloaded from: http:///
lt3475/lt3475-1 8 3475fb open circuit protection the lt3475 has internal open-circuit protection. if the led is absent or is open circuit, the lt3475 clamps the voltage on the led pin at 14v. the switching regulator then oper- ates at a very low frequency to limit the input current. the lt3475-1 has no internal open circuit protection. with the lt3475-1, be careful not to violate the absmax voltage of th boost pin; if v in > 25v, external open circuit protection circuitry (as shown in figure 1) may be necessary.the output voltage during an open led condition is shown in the typical performance characteristics section. undervoltage lockout undervoltage lockout (uvlo) is typically used in situations where the input supply is current limited, or has high source resistance. a switching regulator draws constant power from the source, so the source current increases as the source voltage drops. this looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. uvlo prevents the regulator from operating at source voltages where these problems might occur. an internal comparator will force the part into shut- down when v in falls below 3.7v. if an adjustable uvlo threshold is required, the shdn pin can be used. the threshold voltage of the shdn pin comparator is 2.6v. an internal resistor pulls 9a to ground from the shdn pin at the uvlo threshold. choose resistors according to the following formula: r2 = 2.6v v th ?2.6v r1 ?9 a v th = uvlo threshold example: switching should not start until the input is above 8v. v th = 8v r1=100k r2 = 2.6v 8v ? 2.6v 100k ?9 a = 57.6k figure 2. undervoltage lockout gnd 9a 2.6v v in v c lt3475 r1 c1 r2 v in shdn 3475 f02 table 1. switching frequencies switching frequency (mhz) r t (k) 2 4.32 1.5 6.81 1.2 9.09 1 11.8 0.8 16.9 0.6 24.3 0.4 40.2 0.3 57.6 0.2 100 keep the connections from the resistors to the shdn pin short and make sure the coupling to the sw and boost pins is minimized. if high resistance values are used, the shdn pin should be bypassed with a 1nf capacitor to prevent coupling problems from switching nodes. setting the switching frequency the lt3475 uses a constant frequency architecture that can be programmed over a 200khz to 2mhz range with a single external timing resistor from the r t pin to ground. a graph for selecting the value of r t for a given operating frequency is shown in the typical applications section. v c 100k 10k 22v out 3475 f01 figure 1. external overvoltage protection circuitry for the lt3475-1 applications information downloaded from: http:///
lt3475/lt3475-1 9 3475fb table 1 shows suggested r t selections for a variety of switching frequencies.operating frequency selection the choice of operating frequency is determined by several factors. there is a tradeoff between ef? ciency and component size. a higher switching frequency allows the use of smaller inductors at the cost of increased switching losses and decreased ef? ciency. another consideration is the maximum duty cycle. in certain applications, the converter needs to operate at a high duty cycle in order to work at the lowest input voltage possible. the lt3475 has a ? xed oscillator off time and a variable on time. as a result, the maximum duty cycle increases as the switching frequency is decreased. input voltage range the minimum operating voltage is determined either by the lt3475s undervoltage lockout of 4v, or by its maximum duty cycle. the duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: dc = v out + v f () v in ?v sw + v f () where v f is the forward voltage drop of the catch diode (~0.4v) and v sw is the voltage drop of the internal switch (~0.4v at maximum load). this leads to a minimum input voltage of: v in min () = v out + v f dc max ?v f + v sw with dc max = 1Ct off(min) ? f where t 0ff(min) is equal to 167ns and f is the switching frequency. example: f = 600khz, v out = 4v dc max = 1 ? 167ns ? 600khz = 0.90 v in min () = 4v + 0.4v 0.9 ? 0.4v + 0.4v = 4.9v the maximum operating voltage is determined by the absolute maximum ratings of the v in and boost pins, and by the minimum duty cycle. v in max () = v out + v f dc min ?v f + v sw with dc min = t on(min) ? f where t on(min) is equal to 140ns and f is the switching frequency. example: f = 750khz, v out = 3.4v dc min = 140ns ? 750khz = 0.105 v in max () = 3.4v + 0.4v 0.105 ? 0.4v + 0.4v = 36v the minimum duty cycle depends on the switching fre- quency. running at a lower switching frequency might allow a higher maximum operating voltage. note that this is a restriction on the operating input voltage; the circuit will tolerate transient inputs up to the absolute maximum ratings of the v in and boost pins. the input voltage should be limited to the v in operating range (36v) during overload conditions (short circuit or start up). minimum on time the lt3475 will regulate the output current at input volt- ages greater than v in(max) . for example, an application with an output voltage of 3v and switching frequency of 1.2mhz has a v in(max) of 20v, as shown in figure 3. figure 4 shows operation at 35v. output ripple and peak inductor v sw 20v/div i l 1a/div v out 500mv/div (ac coupled) 3475 f03 figure 3. operation at v in(max) = 20v. v out = 3v and f sw = 1.2mhhz applications information downloaded from: http:///
lt3475/lt3475-1 10 3475fb current have signi? cantly increased. exceeding v in(max) is safe if the external components have adequate ratings to handle the peak conditions and if the peak inductor current does not exceed 3.2a. a saturating inductor may further reduce performance. table 2. inductors part number value ( h) i rms (a) dcr ( ) height (mm) sumida cr43-3r3 3.3 1.44 0.086 3.5 cr43-4r7 4.7 1.15 0.109 3.5 cdrh4d16-3r3 3.3 1.10 0.063 1.8 cdrh4d28-3r3 3.3 1.57 0.049 3.0 cdrh4d28-4r7 4.7 1.32 0.072 3.0 cdrh6d26-5r0 5.0 2.20 0.032 2.8 cdrh6d26-5r6 5.6 2.0 0.036 2.8 cdrh5d28-100 10 1.30 0.048 3.0 cdrh5d28-150 15 1.10 0.076 3.0 cdrh73-100 10 1.68 0.072 3.4 cdrh73-150 15 1.33 0.130 3.4 cdrh104r-150 15 3.1 0.050 4.0 coilcraft do1606t-332 3.3 1.30 0.100 2.0 do1606t-472 4.7 1.10 0.120 2.0 do1608c-332 3.3 2.00 0.080 2.9 do1608c-472 4.7 1.50 0.090 2.9 mos6020-332 3.3 1.80 0.046 2.0 mos6020-472 10 1.50 0.050 2.0 do3316p-103 10 3.9 0.038 5.2 do3316p-153 15 3.1 0.046 5.2 the optimum inductor for a given application may differ from the one indicated by this simple design guide. a larger value inductor provides a higher maximum load current, and reduces the output voltage ripple. if your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor, or one with a lower dcr resulting in higher ef? ciency. in addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. for details of maximum output current and discontinuous mode operation, see linear technologys application note 44. finally, for duty cycles greater than 50% (v out /v in > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: l min = (v out + v f ) 800khz f v sw 20v/div i l 1a/div v out 500mv/div (ac coupled) 3475 f04 figure 4. operation above v in(max) . output ripple and peak inductor current increases inductor selection and maximum output current a good ? rst choice for the inductor value is: l = (v out + v f ) 1.2mhz f where v f is the voltage drop of the catch diode (~0.4v), f is the switching frequency and l is in h. with this value the maximum load current will be above 1.6a at all duty cycles. the inductors rms current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. for highest ef? ciency, the series resistance (dcr) should be less than 0.15. table 2 lists several vendors and types that are suitable. for robust operation at full load and high input voltages (v in > 30v), use an inductor with a saturation current higher than 3.2a. applications information downloaded from: http:///
lt3475/lt3475-1 11 3475fb the current in the inductor is a triangle wave with an average value equal to the load current. the peak switch current is equal to the output current plus half the peak-to-peak inductor ripple current. the lt3475 limits its switch cur- rent in order to protect itself and the system from overload faults. therefore, the maximum output current that the lt3475 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. when the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. this gives the peak-to-peak ripple current in the inductor i l = 1? dc () v out + v f () l?f () where f is the switching frequency of the lt3475 and l is the value of the inductor. the peak inductor and switch current is i sw pk () = i lpk () = i out + i l 2 to maintain output regulation, this peak current must be less than the lt3475s switch current limit i lim . i lim is at least 2.3a at low duty cycles and decreases linearly to 1.8a at dc = 0.9. the maximum output current is a function of the chosen inductor value: i out max () = i lim ? i l 2 = 2.3a? 1?0.25?dc () ? i l 2 choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. one approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals. then use these equations to check that the lt3475 will be able to deliver the required output current. note again that these equations assume that the inductor current is continu- ous. discontinuous operation occurs when i out is less than i l /2. input capacitor selection bypass the input of the lt3475 circuit with a 4.7f or higher ceramic capacitor of x7r or x5r type. a lower value or a less expensive y5v type will work if there is additional bypassing provided by bulk electrolytic capaci- tors or if the input source impedance is low. the following paragraphs describe the input capacitor considerations in more detail. step-down regulators draw current from the input supply in pulses with very fast rise and fall times. the input ca- pacitor is required to reduce the resulting voltage ripple at the lt3475 input and to force this switching current into a tight local loop, minimizing emi. the input capacitor must have low impedance at the switching frequency to do this effectively, and it must have an adequate ripple current rat- ing. with two switchers operating at the same frequency but with different phases and duty cycles, calculating the input capacitor rms current is not simple. however, a conservative value is the rms input current for the channel that is delivering most power (v out ? i out ): c inrms = i out v out (v in ?v out ) v in < i out 2 and is largest when v in = 2v out (50% duty cycle). as the second, lower power channel draws input current, the input capacitors rms current actually decreases as the out-of-phase current cancels the current drawn by the higher power channel. considering that the maximum load current from a single channel is ~1.5a, rms ripple current will always be less than 0.75a. the high frequency of the lt3475 reduces the energy storage requirements of the input capacitor, so that the capacitance required is less than 10f. the combination of small size and low impedance (low equivalent series resistance or esr) of ceramic capacitors makes them the preferred choice. the low esr results in very low voltage ripple. ceramic capacitors can handle larger magnitudes of ripple current than other capacitor types of the same value. use x5r and x7r types. applications information downloaded from: http:///
lt3475/lt3475-1 12 3475fb an alternative to a high value ceramic capacitor is a lower value ceramic along with a larger electrolytic capacitor. the electrolytic capacitor likely needs to be greater than 10f in order to meet the esr and ripple current requirements. the input capacitor is likely to see high surge currents when the input source is applied. tanta- lum capacitors can fail due to an over-surge of current. only use tantalum capacitors with the appropriate surge current rating. the manufacturer may also recommend operation below the rated voltage of the capacitor. a ? nal caution is in order regarding the use of ceramic capacitors at the input. a ceramic input capacitor can combine with stray inductance to form a resonant tank circuit. if power is applied quickly (for example by plug- ging the circuit into a live power source) this tank can ring, doubling the input voltage and damaging the lt3475. the solution is to either clamp the input voltage or dampen the tank circuit by adding a lossy capacitor in parallel with the ceramic capacitor. for details, see application note 88. output capacitor selection for most leds, a 2.2f, 6.3v ceramic capacitor (x5r or x7r) at the output results in very low output voltage ripple and good transient response. other types and values will also work. the following discusses tradeoffs in output ripple and transient performance. the output capacitor ? lters the inductor current to generate an output with low voltage ripple. it also stores energy in order to satisfy transient loads and stabilizes the lt3475s control loop. because the lt3475 operates at a high frequency, minimal output capacitance is necessary. in addition, the control loop operates well with or without the presence of output capacitor series resistance (esr). ceramic capacitors, which achieve very low output ripple and small circuit size, are therefore an option. you can estimate output ripple with the following equation: v ripple = i l / (8 ? f ? c out ) for ceramic capacitors where i l is the peak-to-peak ripple current in the inductor. the rms content of this ripple is very low so the rms current rating of the output capacitor is usually not of concern. it can be estimated with the formula: i c(rms) = i l /12 the low esr and small size of ceramic capacitors make them the preferred type for lt3475 applications. not all ceramic capacitors are the same, however. many of the higher value capacitors use poor dielectrics with high temperature and voltage coef? cients. in particular y5v and z5u types lose a large fraction of their capacitance with applied voltage and at temperature extremes. because loop stability and transient response depend on the value of c out , this loss may be unacceptable. use x7r and x5r types. table 3 lists several capacitor vendors. table 3. low esr surface mount capacitors. vendor type series taiyo-yuden ceramic x5r, x7r avx ceramic x5r, x7r tdk ceramic x5r, x7r diode selectionthe catch diode (d3 from the block diagram) conducts current only during switch off time. average forward cur- rent in normal operation can be calculated from: i d(avg) = i out (v in C v out )/v in the only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. the diode cur- rent will then increase to one half the typical peak switch current limit. peak reverse voltage is equal to the regulator input voltage. use a diode with a reverse voltage rating greater than the input voltage. table 4 lists several schottky diodes and their manufacturers. diode reverse leakage can discharge the output capacitor during led off times while pwm dimming. if operating at high ambient temperatures, use a low leakage schottky for the widest pwm dimming range. applications information downloaded from: http:///
lt3475/lt3475-1 13 3475fb table 4. schottky diodes v r (v) i ave (a) (a) v f at 1a (mv) v f at 2a (mv) on semiconductor mbr0540 40 0.5 620 mbrm120e 20 1 530 mbrm140 40 1 550 diodes inc b120 20 1 500 b130 30 1 500 b140hb 40 1 530 dfls140 40 1.1 510 b240 40 2 500 international recti? er 10bq030 30 1 420 boost pin considerations the capacitor and diode tied to the boost pin gener- ate a voltage that is higher than the input voltage. in most cases, a 0.22f capacitor and fast switching diode (such as the cmdsh-3 or mmsd914lt1) will work well. figure 5 shows three ways to arrange the boost circuit. the boost pin must be more than 2.5v above the sw pin for full ef? ciency. for outputs of 3.3v and higher, the standard circuit (figure 5a) is best. for outputs between 2.8v and 3.3v, use a small schottky diode (such as the bat-54). for lower output voltages, the boost diode can be tied to the input (figure 5b). the circuit in figure 5a is more ef? cient because the boost pin current comes from a lower voltage source. the anode of the boost diode can be tied to another source that is at least 3v. for example, if you are generating a 3.3v output, and the 3.3v output is on whenever the led is on, the boost pin can be connected to the 3.3v output. for lt3475-1 applications with higher output voltages, an additional zener diode may be necessary (figure 5d) to maintain pin voltage below the absolute maximum. in any case, be sure that the maximum voltage at the boost pin is both less than 60v and the voltage difference between the boost and sw pins is less than 30v. the minimum operating voltage of an lt3475 application is limited by the undervoltage lockout (~3.7v) and by the maximum duty cycle. the boost circuit also limits the minimum input voltage for proper start up. if the input voltage ramps slowly, or the lt3475 turns on when the output is already in regulation, the boost capacitor may not be fully charged. because the boost capacitor charges figure 5. generating the boost voltage v in boost gnd sw v in lt3475 (5a) d2 v out c3 v boost C v sw ? v out max v boost ? v in + v out v in boost gnd sw v in lt3475 (5b) d2 v out c3 v boost C v sw ? v in max v boost ? 2v in d2 v in boost gnd sw v in lt3475 (5c) 3475 f05? v out v boost C v sw ? v in2 max v boost ? v in2 + v in minimum value for v in2 = 3v v in2 > 3v c3 d2 v in boost gnd sw v in lt3475 (5d) 3475 f05? v out v boost C v sw C v z max v boost ? v in + v out C v z c3 applications information downloaded from: http:///
lt3475/lt3475-1 14 3475fb with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. the minimum load current generally goes to zero once the circuit has started. the typical performance char- acteristics section shows a plot of minimum load to start and to run as a function of input voltage. even without an output load current, in many cases the discharged output capacitor will present a load to the switcher that will allow it to start. the plots show the worst case, where v in is ramping very slowly. programming led current the led current can be set by adjusting the voltage on the v adj pin. for a 1.5a led current, either tie v adj to ref or to a 1.25v source. for lower output currents, program the v adj using the following formula: i led = 1.5a ? v adj /1.25v. voltages less than 1.25v can be generated with a voltage divider from the ref pin, as shown in figure 6. in order to have accurate led current, precision resistors are preferred (1% or better is recommended). note that the v adj pin sources a small amount of bias current, so use the following formula to choose resistors: r2 = v adj 1.25v ? v adj r1 + 50na to minimize the error from variations in v adj pin current, use resistors with a parallel resistance of less than 4k. use resistor strings with a high enough series resistance so as not to exceed the 500a current compliance of the ref pin. dimming control there are several different types of dimming control circuits. one dimming control circuit (figure 7) changes the voltage on the v adj pin by tying a low on resistance fet to the resistor divider string. this allows the se-lection of two different led currents. for reliable op- eration program an led current of no less than 50ma. the maximum current dimming ratio (i ratio ) can be calculated from the maximum led current (i max ) and the minimum led current (i min ) as follows: i max /i min = i ratio another dimming control circuit (figure 8) uses the pwm pin and an external nfet tied to the cathode of the led. an external pwm signal is applied to the pwm pin and the gate of the nfet (for pwm dimming ratios of 20 to 1 or less, the nfet can be omitted). the average led current is proportional to the duty cycle of the pwm signal. when the pwm signal goes low, the nfet turns off, turning off the led and leaving the output capacitor charged. the pwm pin is pulled low as well, which disconnects the v c pin, storing the voltage in the capacitor tied there. use the c-rc string shown in figure 8 and figure 9 tied to the v c pin for proper operation during startup. when the pwm pin goes high again, the led current returns rapidly to its previous on state since the compensation and output capacitors are at the correct voltage. this fast settling time allows the pwm led gnd lt3475 3475 f08 pwm 100hz to 10khz v c 10k 0.1f 3.3nf figure 8. dimming using pwm signal ref v adj gnd lt3475 3475 f07 r1r2 dim figure 7. dimming with a mosfet and resistor divider figure 6. setting v adj with a resistor divider ref v adj gnd lt3475 3475 f06 r1 r2 applications information downloaded from: http:///
lt3475/lt3475-1 15 3475fb figure 10. recommended component placement 12 3 4 5 6 7 8 9 10 2019 18 17 16 15 14 13 12 11 3475 f10 v in pwm2 shdn pwm1 via to local gnd plane r t v c gnd lt3475 3475 f09 1m r t 10k 220pf 0.1 f pwm1 3.3nf figure 9. extending the pwm dimming range lt3475 to maintain diode current regulation with pwm pulse widths as short as 7.5 switching cycles (12.5s for f sw = 600khz). maximum pwm period is determined by the system and is unlikely to be longer than 12ms. using pwm periods shorter than 100s is not recommended. the maximum pwm dimming ratio (pwm ratio ) can be calculated from the maximum pwm period (t max ) and minimum pwm pulse width (t min ) as follows: t max /t min = pwm ratio total dimming ratio (dim ratio ) is the product of the pwm dimming ratio and the current dimming ratio.example: i max = 1a, i min = 0.1a, t max = 9.9ms t min = 3.3s (f sw = 1.4mhz) i ratio = 1a/0.1a =10:1 pwm ratio = 9.9ms/3.3s = 3000:1 dim ratio = 10 ? 3000 = 30000:1 to achieve the maximum pwm dimming ratio, use the circuit shown in figure 9. this allows pwm pulse widths as short as 4.5 switching cycles (7.5s for f sw = 600khz). note that if you use the circuit in figure 9, the rising edge of the two pwm signals must align within 100ns. layout hintsas with all switching regulators, careful attention must be paid to the pcb layout and component placement. to maximize ef? ciency, switch rise and fall times are made as short as possible. to prevent electromagnetic interfer- ence (emi) problems, proper layout of the high frequency switching path is essential. the voltage signal of the sw and boost pins have sharp rise and fall edges. minimize the area of all traces connected to the boost and sw pins and always use a ground plane under the switching regulator to minimize interplane coupling. in addition, the ground connection for frequency setting resistor r t and capacitors at v c1 , v c2 pins (refer to the block diagram) should be tied directly to the gnd pin and not shared with the power ground path, ensuring a clean, noise-free connection. applications information downloaded from: http:///
lt3475/lt3475-1 16 3475fb boost1 boost2 sw1 d1 led 1 led 2 d2 d3 d4 sw2 out1 out2 led1 led2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in shdn l2 10 h l1 10 h c30.22 f 6.3v c4 0.22 f 6.3v r21k r3 2k r124.3k c22.2 f 6.3v c5 2.2 f 6.3v c6 0.1 f c7 0.1 f 3475 ta02 c1 to c5: x5r or x7rd1, d2: dfls140 d3, d4: mbr0540 led current = 1a v in 5v to 36v c14.7 f 50v f sw = 600khz boost1 boost2 sw1 led 1 1.5a led current 1.5a ledcurrent led 2 d2 d3 d4 sw2 out1 out2 led1 led2 pwm1 pwm2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in shdn d1 l2 10 h l1 10 h c3 0.22 f 6.3v c20.22 f 6.3v c42.2 f 6.3v c5 2.2 f 6.3v c63.3nf c80.1 f c90.1 f c8220p c7 3.3nf 3475 ta03 d1, d2: b260d3, d4: mbr0540 c1 to c5: x5r or x7r m1, m2: si2302ads m3: 2n7002l v in 6v to 36v c14.7 f 50v m1 m2 m3 r124.3k r310k r410k 1m r2 pwm1 pwm2 f sw = 600khz dual step-down 1a led driver dual step-down 1.5a led driver with 1200 : 1 true color pwm dimming typical applications downloaded from: http:///
lt3475/lt3475-1 17 3475fb boost1 boost2 sw1 d2 d3 d4 sw2 out1 out2 led1led2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in shdn d1 l2 10 h l1 10 h c3 0.22 f 6.3v c20.22 f 6.3v c42.2 f 6.3v c5 2.2 f 6.3v 3475 ta04 d1, d2: b240ad3, d4: mbr0540 c1 to c5: x5r or x7r v in 5v to 36v c14.7 f 50v r124.3k c70.1 f c6 0.1 f 3a ledcurrent led 1 f sw = 600khz boost1 boost2 sw1 d2 d3 d4 sw2 out1led1 out2 led2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in shdn d1 l2 15 h l1 15 h c3 0.22 f 10v c20.22 f 10v c4 2.2 f 10v c52.2 f 10v 3475 ta05 d1, d2: b240ad3, d4: mmsd4148t1 c1 to c5: x5r or x7r v in 10v to 36v c14.7 f 50v r124.3k c70.1 f c6 0.1 f led 4 led 3 1.5a ledcurrent led 2 1.5a led current led 1 f sw = 600khz step-down 3a led driver dual step-down led driver with series connected leds typical applications downloaded from: http:///
lt3475/lt3475-1 18 3475fb boost1 boost2 sw1 d2 d3 sw2 out1led1 out2 led2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475 v in shdn d1 l2 10 h l1 10 h c3 0.22 f 35v c20.22 f 35v c4 2.2 f 6.3v c52.2 f 6.3v 3475 ta06 d1, d2: b240ad3, d4: mmsd4148t1 c1 to c5: x5r or x7r v in 5v to 28v c14.7 f 35v r124.3k c70.1 f c6 0.1 f led 2 led 1 1.5a ledcurrent 1.5a led current f sw = 600khz d4 dual step-down 1.5a red led driver typical applications downloaded from: http:///
lt3475/lt3475-1 19 3475fb information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. fe20 (cb) tssop 0204 0.09 ? 0.20 (.0035 ? .0079) 0 ? 8 0.25 ref recommended solder pad layout 0.50 ? 0.75 (.020 ? .030) 4.30 ? 4.50* (.169 ? .177) 134 5 6 7 8910 11 12 14 13 6.40 ? 6.60* (.252 ? .260) 3.86 (.152) 2.74 (.108) 20 1918 17 16 15 1.20 (.047) max 0.05 ? 0.15 (.002 ? .006) 0.65 (.0256) bsc 0.195 ? 0.30 (.0077 ? .0118) typ 2 2.74 (.108) 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 3.86 (.152) millimeters (inches) *dimensions do not include mold flash. mold flash shall not exceed 0.150mm (.006") per side note:1. controlling dimension: millimeters 2. dimensions are in 3. drawing not to scale see note 4 4. recommended minimum pcb metal size for exposed pad attachment 6.40 (.252) bsc fe package 20-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation cb package description downloaded from: http:///
lt3475/lt3475-1 20 3475fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2006 lt 1007 rev b printed in usa related parts part number description comments lt1618 constant-current, 1.4mhz, 1.5a boost converter v in(min) = 1.6v, v in(max) = 18v, v out(max) = 35v, analog/pwm, i sd < 1a, ms10 package lt3466 dual full function step-up led driver drivers up to 20 leds, v in : 2.7v to 24v, v out(max) = 40v, dfn, tssop16e packages lt3474 36v, 1a (i led ), 2mhz step-down led driver v in(min) = 4v, v in(max) = 36v, 400:1 true color pwm, i sd < 1a, tssop16e package lt3477 42v, 3a, 3.5mhz boost, buck-boost, buck led driver v in(min) = 2.5v, v in(max) = 25v, v out(max) = 40v, analog/pwm, i sd < 1a, qfn, tssop20e packages lt3479 3a, full-featured dc/dc converter with soft-start and inrush current protection v in(min) = 2.5v, v in(max) = 24v, v out(max) = 40v, analog/pwm, i sd < 1a, dfn, tssop packages lt3846 dual 1.3a, 2mhz, led driver v in : 2.5v to 24v, v out(max) = 36v, 1000:1 true color pwm tm dimmin, dfn, tssop16e packages boost1 boost2 sw1 sw2 out1 out2 led1 12v to 18v led voltage 12v to 18v led voltage led2 v c1 v c2 ref r t v adj1 v adj2 gnd lt3475-1 v in v in 21v to 36v shdn c2 0.22f 16v c14.7f 50v l1 33h r1 1k r6100k r410k r7 100k r510k l2 33h d3 d6 d4 r2 1k c30.22f 16v r324.3k c4 2.2f 25v c52.2f 25v d7 q1 d2 d1 d5 c7 0.1f c6 0.1f 3475 ta08 1.5a led current* 1.5a led current* f sw = 600khz d8 q2 d1, d4: 7.5v zener dioded2, d3: mmsd4148 d5, d6: b240a d7, d8: 22v zener diode r1, r2: use 0.5w resistor of two 2k 0.25w resistors in parallel q1, q2: mmbt3904 c1 to c5: x5r or x7r *derate led current at elevated ambient temperatures to maintain lt3475-1 junction temperature below 125 c. dual step-down 1.5a led driver with four series connected led output typical application downloaded from: http:///

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