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lt3474/lt3474-1 1 3474fd the lt ? 3474/lt3474-1 are ? xed frequency step-down dc/dc converters designed to operate as constant-current sources. an internal sense resistor monitors the output current allowing accurate current regulation, ideal for driving high current leds. high output current accuracy is maintained over a wide current range, from 35ma to 1a, allowing a wide dimming range. unique pwm circuitry allows a dimming range of 400:1, avoiding 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 16-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 their wide input range of 4v to 36v, the lt3474/ lt3474-1 regulate a broad array of power sources, from 5v logic rails to unregulated wall transformers, lead acid batteries and distributed power supplies. 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 n true color pwm? delivers constant color with 400:1 dimming range n wide input range: 4v to 36v n up to 1a led current n adjustable 200khzC2mhz switching frequency n adjustable control of led current n integrated boost diode n high output current accuracy is maintained over a wide range from 35ma to 1a n open led (lt3474) and short-circuit protection n high side sense allows groundedcathode connection n uses small inductors and ceramic capacitors n lt3474-1 drives led strings up to 26v n compact 16-lead tssop thermally enhanced surface mount package typical application description step-down 1a led driver step-down 1a led driver l , lt, ltc and ltm are registered trademarks of linear technology corporat ion. true color pwm is a trademark of linear technology corporation. all othe r trademarks are the property of their respective owners. patent pending features applications ef? ciency v in bias out pwm led led1 dimming*control lt3474 0.1f v in 5v to 36v 2.2f 0.22f 10h 3474 ta01a 80.6k *see applications section for details shdn ref boost sw v adj r t v c gnd 2.2f led current (ma) 0 efficiency (%) 70 75 80 600 1000 3474 g02 65 60 55 200 400 800 85 90 95 two series connected white 1a leds one white 1a led v in = 12v downloaded from: http:///
lt3474/lt3474-1 2 3474fd pin configuration absolute maximum ratings v in pin ........................................................(C0.3v), 36v bias pin ....................................................................25v boost pin voltage ...................................................51v boost above sw pin ...............................................25v out, led pins (lt3474) ............................................15v out, led pins (lt3474-1) .........................................26v pwm pin ...................................................................10v v adj pin .....................................................................6v v c , ref, r t pins ..........................................................3v shdn pin ...................................................................v in bias pin current .........................................................1a maximum junction temperature (note 2)............. 125c operating temperature range (note 3) lt3474e, lt3474e-1 ............................ C40c to 85c lt3474i, lt3474i-1 ............................ C40c to 125c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) fe package 16-lead plastic tssop 12 3 4 5 6 7 8 top view 1615 14 13 12 11 10 9 dnc* out led v in sw boost bias gnd dnc*gnd pwm v adj v c refshdn r t 17 jc = 8c/w, ja = 40c/w exposed pad (pin 17) is gnd, must be soldered to pcb *do not connect external circuitry to these pins. order information lead free finish tape and reel part marking package descriptio n temperature range lt3474efe#pbf lt3474efe#trpbf 3474efe 16-lead tssop C40c to 85c lt3474ife#pbf lt3474ife# trpbf 3474ife 16-lead tssop C 40c to 125c lt3474efe-1#pbf lt3474efe-1#trpbf 3474efe-1 16-lead tssop C40c to 85c lt3474ife-1#pbf lt3474ife-1# trpbf 3474ife-1 16-lead tssop C 40c to 125c consult ltc marketing for parts speci? ed with wider operating temp erature ranges. consult ltc marketing for information on non-standard lead based ? ni sh 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/ parameter conditions min typ max units minimum input voltage l 3.5 4 v input quiescent current not switching 2.6 4 ma shutdown current shdn = 0.3v, v boost = 0v, v out = 0v 0.01 2 a led pin current v adj tied to v ref v adj tied to v ref /5 ll 0.98 0.9680.193 0.186 1 0.2 1.02 1.0250.207 0.210 aa a a ref voltage l 1.23 1.25 1.265 v electrical characteristics the l 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). downloaded from: http:///
lt3474/lt3474-1 3 3474fd electrical characteristics 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 lt3474e and lt3474e-1 are guaranteed to meet performance speci? cations from 0c to 70c. speci? cations over the C40c to 85c the l 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 reference voltage line regulation 5v < v in < 36v 0.01 %/v reference voltage load regulation 0 < i ref < 250a 0.0002 %/a v adj pin bias current (note 4) l 20 400 na switching frequency r t = 80.6k l 470450 500 530 540 khzkhz maximum duty cycle r t = 80.6k r t = 10k r t = 232k l 90 95 7698 %% % foldback frequency r t = 80.6k, v out = 0v 70 khz shdn threshold (to switch) 2.6 2.65 2.7 v shdn pin current (note 5) v shdn = shdn threshold 8.3 10.3 12.3 a pwm threshold 0.4 0.9 1.2 v v c switching threshold 0.8 v v c source current v c = 1v 100 a v c sink current v c = 1v 100 a led to v c current gain 1.5 a/ma led to v c transresistance 1 v/ma v c to switch current gain 2a / v v c clamp voltage 1.9 v v c pin current in pwm mode v c = 1v, v pwm = 0.3v l 0.01 1 a out pin clamp voltage (lt3474) 13.2 13.8 14.5 v out pin current in pwm mode v out = 4v, v pwm = 0.3v l 0.1 10 a switch current limit (note 6) C40c to 85c lt3474i, lt3474i-1 at 125c l 1.6 1.5 2.1 3.2 3.2 aa switch v cesat i sw = 1a 380 500 mv boost pin current i sw = 1a 30 50 ma switch leakage current 0.01 1 a minimum boost voltage (note 7) 1.9 2.5 v boost diode forward voltage i dio = 100ma 600 mv operating temperature range are assured by design, characterization and correlation with statistical process controls. the lt3474i and lt3474i-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. note 7: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch. downloaded from: http:///
lt3474/lt3474-1 4 3474fd typical performance characteristics current limit vs duty cycle switch current limit vs temperature current limit vs output voltage oscillator frequency vs temperature oscillator frequency vs r t oscillator frequency foldback led current vs v adj led current vs temperature switch voltage drop v adj (v) 0 led current (ma) 600 800 1000 1 3474 go3 400 200 0 0.25 0.5 0.75 1.25 t a = 25c temperature (c) C50 led current (ma) 800 1000 1200 25 75 3474 g04 600 400 C25 0 50 100 125 200 0 v adj = v ref v adj = v ref /5 switch current (ma) 0 700600 500 400 300 200 100 0 3474 g05 500 1000 1500 switch voltage drop (mv) t a = 25c duty cycle (%) 0 current limit (a) 1.5 2 2.5 80 3474 g06 1 0.5 0 20 40 60 100 typical minimum (125c) minimum (85c) temperature (c) C50 C25 0 current limit (a) 1 2.5 0 50 75 3474 g07 0.5 2 1.5 25 100 125 v out (v) 0 0 current limit (a) 0.5 1 1.5 2 2.5 2 468 3474 g08 10 12 t a = 25c r t (k) 10 100 oscillator frequency (khz) 1000 100 3474 g09 t a = 25c temperature (c) C50 400 oscillator frequency (khz) 450 500 550 600 C25 0 25 50 3474 g10 75 100 125 r t = 80.6k v out (v) 0 0 oscillator frequency (khz) 100 200 300 400 500 600 0.5 1 1.5 2 3474 g11 2.5 t a = 25c r t = 80.6k downloaded from: http:///
lt3474/lt3474-1 5 3474fd typical performance characteristics schottky reverse leakage schottky forward voltage drop open-circuit output voltage and input current minimum input voltage, one white luxeon iii star minimum input voltage, two series connected white luxeon iii stars boost pin current quiescent current reference voltage temperature (c) C50 0 reverse current (a) 5 10 15 20 C25 0 25 50 3474 g15 75 100 125 v r = 5v v in (v) 0 0 output voltage (v) input current (ma) 10 20 30 40 50 60 0 1 2 3 4 5 6 87 10 20 30 40 3474 g16 t a = 25c output voltage input current lt3474 lt3474-1 lt3474-1 lt3474 led current (ma) 0 0 v in (v) 1 2 3 4 5 6 200 400 600 800 3474 g17 1000 t a = 25c to start led voltage to run led current (ma) 0 v in (v) 8 9 10 800 3474 g18 7 6 5 200 400 600 1000 to run t a = 25c to start led voltage forward voltage (mv) 0 forward current (ma) 300 400 500 800 3474 g19 200 100 0 200 400 600 1000 t a = 25c switch current (ma) 0 0 boost pin current (ma) 10 20 30 40 60 250 500 750 1000 3473 g12 1250 1500 50 t a = 25c v in (v) 0 0 input current (ma) 0.5 1.0 1.5 2.0 3.0 6 12 18 24 3474 g13 30 36 2.5 t a = 25c temperature (c) C50 C25 1.235 v ref (v) 1.245 1.260 0 50 75 3474 g14 1.240 1.255 1.250 25 100 125 downloaded from: http:///
lt3474/lt3474-1 6 3474fd pin functions dnc (pins 1, 16): do not connect external circuitry to these pins, or tie them to gnd. leave the dnc pins ? oating. out (pin 2): the out pin is the input to the current sense resistor. connect this pin to the inductor and the output capacitor. led (pin 3): the led pin is the output of the current sense resistor. connect the anode of the led here. v in (pin 4): the v in pin supplies current to the internal circuitry and to the internal power switch and must be locally bypassed. sw (pin 5): the sw pin is the output of the internal power switch. connect this pin to the inductor and switching diode. boost (pin 6): the boost pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. bias (pin 7): the bias pin connects through a schottky diode to boost. tie to out. gnd (pins 8, 15, exposed pad pin 17): ground. tie both gnd pins 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. it must be soldered to the circuit board for proper operation. r t (pin 9): the r t pin is used to set the internal oscilla- tor frequency. tie an 80.6k resistor from r t to gnd for a 500khz switching frequency. shdn (pin 10): 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 under-voltage lockout. pull below 0.3v to shut down the lt3474/lt3474-1. pull above 2.65v to enable the lt3474/ lt3474-1. tie to v in if the shdn function is unused. ref (pin 11): the ref pin is the buffered output of the internal reference. either tie the ref pin to the v adj pin for a 1a output current, or use a resistor divider to generate a lower voltage at the v adj pin. leave this pin unconnected if unused.v c (pin 12): 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 adj (pin 13): 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 1a output current. for lower output cur- rents, program the v adj pin using the following formula: i led = 1a ? v adj /1.25v. pwm (pin 14): 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. downloaded from: http:///
lt3474/lt3474-1 7 3474fd block diagram bias v c v adj out 3474 bd c1 r t pwm gnd 2v ref r t driver shdn c c1 c c2 r c 1.25k d led1 q2 q1 1.25v osc slopecomp frequency foldback int reg and uvlo r q q s c in v in v in C + 0.1 100 led d1 l1 sw boost c2 pwm use with pwm dimming c1 g m 7 6 5 2 3 12 8 14 11 10 9 4 13 figure 1. block diagram downloaded from: http:///
lt3474/lt3474-1 8 3474fd applications information operation the lt3474 is a constant frequency, current mode regula- tor with an internal power switch capable of generating a constant 1a output. operation can be best understood by referring to the block diagram. if the shdn pin is tied to ground, the lt3474 is shut down and draws minimal current from the input source tied to v in . if the shdn pin exceeds 1.5v, the internal bias circuits turn on, including the internal regulator, reference, and oscillator. the switching regulator will only begin to operate when the shdn pin exceeds 2.65v. 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 1.9v limits the output current. the voltage on the v adj pin sets the current through the led pin. the npn q2 pulls a current proportional to the voltage on the v adj pin through the 100 resistor. the g m ampli? er servos the v c pin to set the current through the 0.1 resistor and the led pin. when the voltage drop across the 0.1 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 1a. tying a resistor divider to the ref pin allows the programming of led pin currents of less than 1a. led pin current can also be programmed by tying the v adj pin directly to a voltage source up to 1.25v. an led can be dimmed with pulse width modulation us- ing the pwm pin and an external nfet. if the pwm pin is unconnected or pulled high, the part operates nominally. if the pwm pin is pulled low, the v c pin is disconnected from the internal circuitry and draws minimal current from the compensation capacitor. circuitry drawing 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 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 very low 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 q2 can only operate correctly with suf? cient voltage at v out , around 1.7v. a comparator senses v out and forces the v c pin high until v out rises above 2v, and q2 is operating correctly. the switching regulator performs frequency foldback dur- ing overload conditions. an ampli? er senses when v out is less than 2v and begins decreasing the oscillator frequency down from full frequency to 20% 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. downloaded from: http:///
lt3474/lt3474-1 9 3474fd applications information the switch driver operates either from v in or from the boost pin. an external capacitor and internal 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. open circuit protection the lt3474 has internal open circuit protection. if the led is absent or fails open, the lt3474 clamps the voltage on the led pin at 14v. the switching regulator then skips cycles to limit the input current. the lt3474-1 has no internal open circuit protection. with the lt3474-1, be careful not to violate the absmax voltage of the boost pin; if v in > 25v, external open circuit protection circuitry (as shown in figure 2) 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 shutdown when v in falls below 3.5v. if an adjustable uvlo threshold is required, the shdn pin can be used. the threshold voltage of the shdn pin comparator is 2.65v. a internal resistor pulls 10.3a to ground from the shdn pin at the uvlo threshold. choose resistors according to the following formula: r2 = 2.65v v th ? 2.65v r1 ? 10.3 a v th = uvlo threshold example: switching should not start until the input is above 8v. v th = 8v r1 = 100k r2 = 2.65v 8v ? 2.65v 100k ? 10.3 a = 61.9k 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. gnd 10.3a 2.65v v in v c lt3474 r1 c1 r2 v in shdn 3474 f03 figure 3. undervoltage lockout figure 2. external overvoltage protection circuitry for the lt3474-1. v c 100k 10k 27v out 3474 f02 downloaded from: http:///
lt3474/lt3474-1 10 3474fd applications information setting the switching frequency the lt3474 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. the current that ? ows into the timing resistor is used to charge an internal oscillator capacitor. a graph for selecting the value of r t for a given operating frequency is shown in the typical performance characteristics section. table 1 shows suggested r t selections for a variety of switching frequencies. table 1. switching frequencies switching frequency (mhz) r t (k) 21 0 1.5 18.7 1 33.2 0.7 52.3 0.5 80.6 0.3 147 0.2 232 operating frequency selection the choice of operating frequency is determined by sev- eral factors. there is a tradeoff between ef? ciency and component size. 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 lt3474 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 lt3474s 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 vv vv v out f in sw 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 vv dc vv in min out f max fs w () = + + ? with dc max = 1Ct off(min) ? f where t 0ff(min) is equal to 200ns and f is the switching frequency. example: f = 500khz, v out = 4v dc ns khz v vv max in min = ? = = + () 1 200 500 0 90 404 0 ?. . . 99 04 04 49 ?. . . vvv += 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 vv dc vv in max out f min fs w () = + + ? with dc min = t on(min) ? f where t on(min) is equal to 160ns and f is the switching frequency. example: f = 500khz, v out = 2.5v dc ns khz v vv min in max == = + () 160 500 0 08 25 04 0 ?. .. . 008 04 04 36 ?. . vvv += 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 rating. downloaded from: http:///
lt3474/lt3474-1 11 3474fd applications information 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. be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. 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. see application note 19. 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 lt3474 limits its switch cur- rent in order to protect itself and the system from overload faults. therefore, the maximum output current that the lt3474 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 lt3474 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 inductor selection and maximum output currenta good first choice for the inductor value is lv v khz f out f =+ () 900 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 1.1a, independent of input voltage. 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 0.2. 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 2.5a. 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 1.1 0.063 1.8 cdrh4d28-3r3 3.3 1.57 0.049 3 cdrh4d28-4r7 4.7 1.32 0.072 3 cdrh5d28-100 10 1.3 0.048 3 cdrh5d28-150 15 1.1 0.076 3 cdrh73-100 10 1.68 0.072 3.4 cdrh73-150 15 1.33 0.13 3.4 coilcraft do1606t-332 3.3 1.3 0.1 2 do1606t-472 4.7 1.1 0.12 2 do1608c-332 3.3 2 0.08 2.9 do1608c-472 4.7 1.5 0.09 2.9 mos6020-332 3.3 1.8 0.046 2 mos6020-472 10 1.5 0.05 2 downloaded from: http:///
lt3474/lt3474-1 12 3474fd applications information to maintain output regulation, this peak current must be less than the lt3474s switch current limit i lim . for sw1, i lim is at least 1.6a (1.5a at 125c) at low duty cycles and decreases linearly to 1.15a (1.08a at 125c) at dc = 0.8. the maximum output current is a function of the chosen inductor value: i out max () = i lim ? i l 2 = 1.6a ? 1 ? 0.35 ?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 lt3474 will be able to deliver the required output current. note again that these equations assume that the inductor current is continuous. discontinuous operation occurs when i out is less than i l /2. input capacitor selection bypass the input of the lt3474 circuit with a 2.2f 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 sup- ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple at the lt3474 input and to force this switching current into a tight local loop, minnimizing emi. the input capacitor must have low impedance at the switching frequency to do this effectively, and it must have an adequate ripple current rating. the rms input is: ci vvv v i inrms out out in out in out = () < ? ? 2 and is largest when v in = 2v out (50% duty cycle). con- sidering that the maximum load current is 1a, rms ripple current will always be less than 0.5a the high switching frequency of the lt3474 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. an alternative to a high value ceramic capacitor is a lower value ceramic along with a larger electrolytic capaci- tor. 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. downloaded from: http:///
lt3474/lt3474-1 13 3474fd applications information 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 crms () = i l 12 the low esr and small size of ceramic capacitors make them the preferred type for lt3474 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 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 plugging the circuit into a live power source), this tank can ring, doubling the input voltage and damaging the lt3474. 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 lt3474s control loop. because the lt3474 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. downloaded from: http:///
lt3474/lt3474-1 14 3474fd applications information diode selectionthe catch diode (d1 from figure 1) conducts current only during switch off time. average forward current in normal operation can be calculated from: i ivv v davg out in out 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. peak reverse voltage is equal to the regulator input voltage. use a diode with a reverse voltage rating greater than the input voltage. if using the pwm mode of the lt3474, select a diode with low reverse leakage. table 4 lists several schottky diodes and their manufacturers. table 4. schottky diodes part number v r (v) i ave (a) v f at 0.5a (mv) v f at 1a (mv) on semiconductor mbr0520l 20 0.5 385 mbr0540 40 0.5 510 620 mbrm120e 20 1 530 mbrm140 40 1 550 diodes inc. b0530w 30 0.5 430 b120 20 1 500 b130 30 1 500 b140 hb 40 1 530 international recti? er 10bq030 30 1 420 downloaded from: http:///
lt3474/lt3474-1 15 3474fd applications information boost and bias pin considerationsthe capacitor and internal diode tied to the boost pin generate a voltage that is higher than the input voltage. in most cases, a 0.22f capacitor will work well. figure 4 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 2.8v or higher, the standard circuit (figure 4a) is best. for lower output voltages, the bias pin can be tied to the input (figure 4b). the circuit in figure 4a is more ef? cient because the boost pin current comes from a lower voltage source. the bias pin can be tied to another source that is at least 3v (figure 4c). for example, if a 3.3v source is on whenever the led is on, the bias pin can be connected to the 3.3v output. for lt3474-1 applications with higher output voltages, an additional zener diode may be necessary (figure 4d) to maintain the boost pin voltage below the absolute maximum. in any case, be sure that the maximum voltage at the boost pin is both less than 51v and the voltage difference between the boost and sw pins is less than 25v. programming led current the led current can be set by adjusting the voltage on the v adj pin. for a 1a 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 = 1a ? v adj 1.25v voltages less than 1.25v can be generated with a voltage divider from the ref pin, as shown in figure 5. figure 4. generating the boost voltage ref v adj gnd lt3474 3474 f04 r1 r2 figure 5. setting v adj with a resistor divider 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: r v vv r na adj adj 2 125 1 50 = + .? bias v in 3474 f04c boost v in v in2 > 3v sw gnd lt3474 (4c) v boost C v sw v in2 max v boost v in2 + v in minimum value for v in2 = 3v v out c3 bias v in 3474 f04b boost v in sw gnd lt3474 (4b) v boost C v sw v in max v boost 2v in v out c3 bias v in 3474 f04a boost v in sw gnd lt3474 (4a) v boost C v sw v out max v boost v in + v out v out c3 bias v in 3474 f04d boost v in sw gnd lt3474 (4d) v boost C v sw v out Cv z max v boost v in + v out Cv z v out c3 downloaded from: http:///
lt3474/lt3474-1 16 3474fd applications information to minimize the error from variations in v adj pin current, use resistors with a parallel resistance of less than 4k. use resistors with a series resistance of 5.11k or greater so as not to exceed the 250a current limit on the ref pin. dimming control there are several different types of dimming control cir- cuits. one dimming control circuit (figure 6) changes the voltage on the v adj pin by tying a low on-resistance fet to the resistor divider string. this allows the selection of two different led currents. for reliable operation, program an led current of no less than 35ma. the maximum current dimming ratio (i ratio ) can be calculated from the maxi- mum led current (i max ) and the minimum led current (i min ) as follows: i i i max min ratio = another dimming control circuit (figure 7) uses the pwm pin and an external nfet tied to the cathode of the led. when the pwm signal goes low, the nfet turns off, turn- ing 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 (tied to the v c pin) shown in figure 7 for proper operation during start-up. 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 lt3474 to maintain diode current regulation with pwm pulse widths as short as 40s. if the nfet is omitted and the cathode of the led is instead tied to gnd, use pwm pulse widths of 1ms or greater. the maximmum 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 t pwm max min 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 = 12ms, t min = 40s i ratio = 1a 0.1a = 10:1 pwm ratio = 12ms 40 s = 300:1 dim ratio = 10 ? 300 = 3000:1 ref v adj gnd lt3474 3474 f05 r1r2 di figure 6. dimming with an nfet and resistor divider pwm led 10k 3.3nf gnd lt3474 3474 f06 pwm 60hz to 10khz 0.1f figure 7. dimming using pwm signal downloaded from: http:///
lt3474/lt3474-1 17 3474fd applications information led voltage range the lt3474 can drive led voltages from 2.4v to 12v. the lt3474-1 can drive led voltages from 2.4v to 30v. be careful not to exceed the absmax rating of the out, led, or boost pins of the lt3474-1 since the internal output clamp is disabled. see the typical application section for an example of adding an external output clamp. if the led voltage can drift below 2.4v due to temperature or component variation, add extra series resistance to bring the overall voltage above 2.4v. layout hints as 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 v in gnd shdn pwm via to local gnd planevia to out figure 8. recommended component placement 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 (refer to figure 1) should be tied directly to the gnd pin and not shared with any other component, ensuring a clean, noise-free connection. downloaded from: http:///
lt3474/lt3474-1 18 3474fd v in bias out led pwm led1 lt3474 c4 0.1f v in 12v to 36v c22.2f 10v 1aled current c30.22f 10v d1 l1 10h 3474 ta02 r133.2k shdn ref boost sw v adj r t v c gnd c1 2.2f 50v led2 d1: mbrm 140 c1 to c3: x5r or x7r led current (ma) 0 efficiency (%) 70 75 80 600 1000 3474 g01 65 60 55 200 400 800 85 90 95 v in = 12v v in = 24v v in bias out pwm led led1 m1 d1 lt3474 c4 3.3nf v in 6v to 36v c22.2f 6.3v c30.22f 6.3v l1 10h 3474 ta01 r180.6k r210k c50.1 f pwm shdn ref boost sw v adj r t v c gnd c1 2.2f 50v d1: b140hbc1 to c3: x5r or x7r m1: si2302ads i led1 500ma/div v(pwm) 5v/div 1ms/div ef? ciency, two led output led current in pwm mode step-down 1a led driver with pwm dimming step-down 1a led driver with two series connected led output typical applications downloaded from: http:///
lt3474/lt3474-1 19 3474fd 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description fe package 16-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation ba fe16 (ba) tssop 0204 0.09 ? 0.20 (.0035 ? .0079) 0 ? 8 0.25 ref 0.50 ? 0.75 (.020 ? .030) 4.30 ? 4.50* (.169 ? .177) 134 5 6 7 8 10 9 4.90 ? 5.10* (.193 ? .201) 16 1514 13 12 11 1.10 (.0433) max 0.05 ? 0.15 (.002 ? .006) 0.65 (.0256) bsc 2.74 (.108) 2.74 (.108) 0.195 ? 0.30 (.0077 ? .0118) typ 2 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 recommended solder pad layout 3. drawing not to scale 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 2.74 (.108) 2.74 (.108) see note 4 4. recommended minimum pcb metal size for exposed pad attachment 6.40 (.252) bsc downloaded from: http:///
lt3474/lt3474-1 20 3474fd linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2005 lt 1008 rev d printed in usa related parts typical application part number description comments lt1618 constant current, 1.4mhz, 1.5a boost converter v in : 1.6v to 18v, v out(max) = 36v, i q = 1.8ma, i sd = <1a, ms10 package lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(max) = 1.20v, i q = 2.5ma, i sd = 25a, tssop16/e packages lt1956 60v, 1.2a (i out ), 500khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(max) = 1.20v, i q = 2.5ma, i sd = 25a, tssop16/e packages lt1961 1.5a (i sw ), 1.25mhz, high ef? ciency step-up dc/dc converter v in : 3v to 25v, v out(max) = 35v, i q = 0.9ma, i sd = 6a, ms8e package lt1976/lt1977 60v, 1.2a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converters with burstmode ? operation v in : 3.3v to 60v, v out(max) = 1.20v, i q = 100a, i sd = <1a, tssop16e package lt3430/lt3431 60v, 2.5a (i out ), 200khz, high ef? ciency step-down dc/dc converters v in : 5.5v to 60v, v out(max) = 1.20v, i q = 2.5a, i sd = <25a, tssop16/e packages lt3433 60v, 400ma (i out ), 200khz, high ef? ciency step-up/step-down dc/dc converters with burst mode operation v in : 4v to 60v, v out : 3.3v to 20v, i q = 100a, i sd = <1a, tssop16e package lt3434/lt3435 60v, 2.5a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converters with burst mode operation v in : 3.3v to 60v, v out(max) = 1.20v, i q = 100a, i sd = <1a, tssop16e package ltc3453 1mhz, 800ma synchronous buck-boost high power led driver v in : 2.7v to 5.5v, v out(max) = 5.5v, i q = 2.5ma, i sd = <6a, qfn package lt3467/lt3467a 1.1a (i sw ), 1.3mhz/2.1mhz, high ef? ciency step-up dc/dc converters with integrated soft-start v in : 2.4v to 16v, v out(max) = 40v, i q = 1.2ma, i sd = <1a, thinsot? package lt3477 3a, 42v, 3mhz step-up regulator with dual rail to rail cur rent sense v in : 2.5v to 2.5v, v out(max) = 40v, i q = 5ma, i sd = <1a, qfn, tssop16e packages lt3479 3a, full featured dc/dc converter with soft-start and inrush current protection v in : 2.5v to 24v, v out(max) = 40v, i q = 6.5ma, i sd = <1a, dfn and tssop packages burst mode is a registered trademark of linear technology corporatio n. thinsot is a trademark of linear technology corporation. v in bias out led pwm lt3474-1 c4 0.1f v in 21v to 36v c2 2.2f 25v r210k r3 100k 1a led current f sw = 500khz 12v to 18v led voltage c30.22f 16v d1 d3 q1 d2 l1 47h 3474 ta02a r180.6k shdn ref boost sw v adj r t v c gnd c1 2.2f 50v d1: mbrm 140d2: 7.5v zener diode d3: 22v zener diode q1: mmbt3904 c1 to c3: x5r or x7r step-down 1a led driver with four series connected led output downloaded from: http:///

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