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LTM8040 1 8040p typical application features applications description 36v, 1a module led driver the ltm ? 8040 is a ? xed frequency 1a step-down dc/dc module? designed to operate as a constant current source. internal circuitry monitors the output current to provide accurate current regulation, which is 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 over an output volt- age range of 2.4v to 13v. unique pwm circuitry allows a dimming range of 400:1, avoiding the color shift normally associated with led current dimming. with its wide input range of 4v to 36v, the LTM8040 regu- lates a broad array of power sources, from 4-cell batteries and 5v logic rails to unregulated wall transformers, lead acid batteries and distributed power supplies. the LTM8040 is packaged in a thermally enhanced, com- pact (15mm 9mm) and low pro? le (2.82mm) molded land grid array (lga) package suitable for automated assembly by standard surface mount equipment. the LTM8040 is pb-free and rohs compliant. n true color pwm? delivers constant color with 400:1 dimming ratio n wide input range: 4v to 36v n up to 1a led current n adjustable control of led current n high output current accuracy is maintained over a wide range from 35ma to 1a n open led and short-circuit protection n grounded cathode connection n small footprint, low pro? le (15mm 9mm 2.82mm) surface mount lga package n automotive and avionic lighting n architectural detail lighting n display backlighting n constant current sources , lt, ltc and ltm are registered trademarks of linear technology corporation. module is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. gnd LTM8040 8040 ta01 v in shdn v in * 4v to 36v 1f 215k 650khz *running voltage. see application information for start-up requirements leda lpwr two white leds 6v to 9v 1a adj pwm rt bias ef? ciency 1a led driver module output current (ma) 0 0 efficiency (%) 10 30 40 50 100 70 200 400 8040 ta01b 20 80 90 60 600 800 1000 v in = 12v 3.3v at 1a leds electrical specifications subject to change
LTM8040 2 8040p absolute maximum ratings v in ............................................................................36v bias ..........................................................................25v bias + v in .................................................................51v leda .........................................................................15v pwm .........................................................................10v adj .............................................................................6v shdn ........................................................................36v shdn above v in .........................................................6v bias current ...............................................................1a internal operating temperature (note 2).... ?40 to 125c storage temperature range ....................... ?45 to 125c (note 1) pin configuration lead free finish part marking* package description temperature range (note 3) LTM8040ev#pbf LTM8040v 60-lead 15mm 9mm lga package 0c to 125c LTM8040iv#pbf LTM8040v 60-lead 15mm 9mm lga package ?40c 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/ electrical characteristics 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, bias = 0v, leda = 0v 0.01 2 a leda current adj open r adj = 5.11k l l 0.98 0.965 0.49 0.481 1 0.5 1.02 1.035 0.51 0.525 a a a a adj bias current adj = 0v, current ? ows out of pin 24.5 a adj pull-up resistor 5 5.11 5.22 k switching frequency rt open 470 500 530 khz shdn threshold v ih v il 2.60 1 v v pwm threshold v ih v il 0.4 1.2 v v 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, bias = lpwr, v out = 4v, adj open, rt open, v pwm = 5v, unless otherwise noted (note 3). order information v in bank 3 leda bank 1 gnd bank 2 pwm shdn adj rt bias gnd lpwr t jmax = 125c, 9mm 2.82mm) 6.35cm 4 layer pcb
LTM8040 3 8040p 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 module 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. 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, bias = lpwr, v out = 4v, adj open, rt open, v pwm = 5v, unless otherwise noted (note 3). parameter conditions min typ max units leda clamp voltage 13.2 14.5 v minimum bias voltage 2.0 2.6 v note 3: the LTM8040e is guaranteed to meet performance speci? cations from 0c to 125c internal. speci? cations over the full C40c to 125c internal operating temperature range are assured by design, characterization and correlation with statistical process controls. the LTM8040i is guaranteed to meet speci? cations over the full C40c to 125c internal operating temperature range. note that the maximum internal temperature is determined by speci? c operating conditions in conjunction with board layout, the rated package thermal resistance and other environmental factors. typical performance characteristics ef? ciency - single 3.3v at 1a led ef? ciency - two 3.3v at 1a leds ef? ciency - (3 x 3.3v at 1a leds) ef? ciency - single 2.66v at 1a led ef? ciency - three 2.66v at 1a leds ef? ciency - four 2.66v at 1a leds output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g01 20 80 90 60 600 800 1000 24 v in 12 v in 5 v in output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g02 20 80 100 90 60 600 800 1000 24 v in 12 v in output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g03 20 80 100 90 60 600 800 1000 24 v in 12 v in output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g05 20 80 100 90 60 600 800 1000 24 v in 12 v in output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g04 20 80 90 60 600 800 1000 24 v in 12 v in 5 v in output current (ma) 0 0 efficiency (%) 10 30 40 50 70 200 400 8040 g06 20 80 100 90 60 600 800 1000 24 v in 12 v in
LTM8040 4 8040p typical performance characteristics minimum start voltage vs load 3.3v at 1a leds shdn current vs voltage bias current vs load current 12v in , single 2.7v at 1a led bias current vs load current 24v in , three 3.3v at 1a leds temp rise vs load 2.7v at 1a leds, 12v in minimum running voltage vs output voltage 2.7v at 1a leds minimum running voltage vs output voltage 3.3v at 1a leds minimum start voltage vs load 2.7v at 1a leds input current vs input voltage output short circuited output voltage 0 2 input voltage 4 8 10 12 5 8040 g07 6 14 10 15 1a load 0.5a load 0.1a load output voltage 0 2 input voltage 4 8 10 12 4 2 8040 g08 6 14 8 6 12 10 1a load 0.5a load 0.1a load i load (ma) 0 2 input voltage 4 8 10 12 400 200 8040 g09 6 14 800 600 1000 single led 2 leds 3 leds 4 leds i load (ma) 0 2 input voltage 4 8 10 12 400 200 8040 g10 6 14 800 600 1000 single led 2 leds 3 leds shutdown voltage 0 0 shutdown current (ma) 10 10 5 8040 g11 60 50 40 30 20 20 15 25 30 35 40 load current (ma) 0 0 bias current (ma) 2 6 8 10 400 200 8040 g12 4 12 800 600 1000 v bias = 5v v bias = 3.2v v bias = 12v load current (ma) 0 0 bias current (ma) 2 6 8 10 12 14 16 18 400 200 8040 g13 4 20 800 600 1000 v bias = 5v v bias = 3.2v v bias = 12v input voltage (v) 0 0 input current (ma) 20 10 5 8040 g14 120 100 80 60 40 20 15 25 30 35 40 i load (ma) 0 2 temperature rise (c) 7 400 200 8040 g15 32 27 22 17 12 800 600 1000 single led 2 leds 3 leds 4 leds
LTM8040 5 8040p pin functions leda (bank 1): this pin is the regulated current source of the LTM8040. connect the anode of the led string to this pin. this voltage must be at least 2.4v for accurate current regulation. shdn (pin l4): the shdn pin is used to shut down the switching regulator and the internal bias circuits. the 2.65v switching threshold can function as an accurate undervoltage lockout. pull below 0.3v to shut down the LTM8040. pull above 2.65v to enable the LTM8040. tie to v in if the shdn function is unused. bias (pin l5): the bias pin connects through an internal schottky diode to provide power to internal housekeeping circuits. connect to a voltage source (usually lpwr or v in ) greater than 3.2v. note that this pin is adjacent to the lpwr pin to ease layout. if this pi n is powered by an external power source, a decoupling cap may be necessary. lpwr (pin k5): this is the output of the buck regulator that sources the led current. if the leda voltage is above 3.2v, connect this pin to bias. it is pinned out primarily for the convenience of the user. if it is not used, leave this typical performance characteristics temp rise vs load 3.3v at 1a leds, 36v in led current vs adjust voltage temp rise vs load 2.7v at 1a leds, 36v in temp rise vs load 3.3v at 1a leds, 12v in i load (ma) 0 2 temperature rise (c) 7 400 200 8040 g16 47 32 37 42 27 22 17 12 800 600 1000 single led 2 leds 3 leds 4 leds i load (ma) 0 2 temperature rise (c) 7 400 200 8040 g18 37 22 27 32 17 12 800 600 1000 single led 2 leds 3 leds i load (ma) 0 2 temperature rise (c) 7 400 200 8040 g17 27 22 17 12 800 600 1000 single led 2 leds 3 leds adjust voltage 0 0 led current (ma) 200 400 200 8040 g19 1200 1000 800 600 400 800 600 1000 1200
LTM8040 6 8040p block diagram v in shdn pwm gnd rt adj 80.6k 5.11k internal 1.25v 0.1f 4.7f 8.2h sense resistor lpwr leda bias internal compensation current mode controller 8040 bd pin functions pin ? oating. please refer to the applications information section for details. pwm (pin l7): input pin for pwm dimming control. a pwm signal above 0.9v (on threshold) turns the on output current source, while a pwm signal below 0.5v shuts it down. if the application does not require pwm dimming, then the pwm pin can be left either open (an internal 10a source current pulls pwm high) or pulled up to a voltage source between 1.2v and 10v. v in (bank 3): the v in pin supplies cur-rent to the LTM8040s internal power converter and other circuitry. it must be lo- cally bypassed with a high quality (low esr) capacitor. adj (pin l3): use the adj pin to scale the leda output current below 1a by either applying a voltage source or by connecting a resistor to gnd. this pin is internally pulled up to a 1.25v reference through a 5.11k resistor, so ensure that the voltage source can drive a 5.11k impedance. if applying a voltage to adj, the leda current follows the formula: i led = 1a ? adj/1.25v. if connecting a resistor to gnd, the resistor value should be r = 5.11 ? i led /(1amp C i led ), where r is in k and i led is the desired current out of leda in amps. make sure that the voltage at leda is at least 2.4v. rt (pin l2): the rt pin is used to set the internal oscillator frequency. an 80.6k resistor has already been installed inside the LTM8040 to default switching frequency to 500khz. if no modi? cation of the switching frequency is necessary, leave this pin ? oating. otherwise, a parallel resistor applied from rt to gnd will raise the switching frequency. see table 2 for details. gnd (bank 2): tie all gnd pins directly to a local ground plane. these pins serve as both signal and power return to the LTM8040 module, as well as providing the primary thermal path for heat dissipation within the unit. see the applications information section for more information about heat-sinking and printed circuit board layout.
LTM8040 7 8040p operation the LTM8040 is a constant frequency, current mode converter capable of generating a constant 1a output intended to drive leds or other applications where a constant current is required. operation can be best understood by referring to the block diagram. the power stage is step down converter that regulates the output current by reading the voltage across a power sense resistor that is in series with the output. if the shdn pin is tied to ground, the LTM8040 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. when the shdn pin exceeds 2.65v, the switching regulator will begin to operate. there are two means to dim a led with the LTM8040. the ? rst is to adjust the current on the leda output via a voltage on the adj pin. the adj pin is internally pulled up to a precision 1.25v reference through a 1% 5.11k resistor. leaving the adj pin ? oating sets the led pin current to 1a. reducing the voltage below 1.25v on the adj pin proportionally reduces the current ? owing out of leda. this can be accomplished by connecting a resistor from the adj pin to gnd, forming a divider network with the internal 5.11k resistor. led pin current can also be programmed by tying the adj pin directly to a voltage source. for proper operation, make sure that leda is at least 2.4v at the desired operating point. the other means by which the LTM8040 can dim a led is with pulse width modulation using the pwm pin and an optional external nfet. if the pwm pin is unconnected or pulled high, the part operates nominally. if the pwm pin is pulled low, the LTM8040 stops switching and the internal control circuitry is held in its present state. cir- cuitry drawing current from the lpwr pin is also disabled. this way, the LTM8040 remembers the current sourced from the leda output 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 rt pin allows programming of the switching fre- quency. the LTM8040 is shipped with 80.6k on this pin to gnd, yielding a default switching frequency of 500khz. for applications requiring a faster switching frequency, apply another resistor in parallel, from rt to gnd. refer to table 2 for the frequencies that correspond to the applied external resistor values. an external voltage is required at the bias pin to power internal circuitry. for proper operation, bias must be at least 3.2v. for many applications, bias should be tied to lwpr; if lwpr is below 3.2v, bias may be tied to v in or some other voltage source. the switching regulator performs frequency foldback during overload conditions. an ampli? er senses when lwpr is less than 2v and begins decre asing the oscillator frequency down from full frequency to 20% of the nominal frequency when v out = 0v. the lpwr pin is less than 2v during startup, short circuit, and overload conditions, so the bias pin will be below the speci? ed limit for ef? cient operation if the two pins are tied together. frequency foldback helps limit internal power and thermal stresses under these conditions. the LTM8040 is equipped with thermal protection that reduces the output led current if the internal operating temperature is too high. this thermal protection is active above the 125c temperature rating of the LTM8040, so continuous operation under this operating condition may impair reliability.
LTM8040 8 8040p for most applications, the design process is straight forward, summarized as follows: 1. look at table 1 and ? nd the row that has the desired input voltage range led string voltage range and output current. 2. apply the recommended c in , r t and r adj values. 3. connect bias as indicated. 4. connect leda to the anode of the led string. 5. connect the remaining pins as needed by the system requirements. while these component combinations have been tested for proper operation, it is incumbent upon the user to verify proper operation over the intended systems line, load and environmental conditions. open led protection the LTM8040 has internal open led circuit protection. if the led is absent or fails open, the LTM8040 clamps the voltage on the leda pin to 14v. the switching regulator then skips cycles to limit the input current. the input current and output voltage during an open led condition is shown in the typical performance characteristics section. undervoltage lockout u n d e r v o l t a g e l o c k o u t ( u v l o) i s t y p i c a l l y u s e d i n s i t u a t i o n s 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 this might occur. an internal comparator w i l l f o r c e t h e p a r t i n t o s h u t d o w n w h e n v in falls below 3.5v. an adjustable uvlo threshold is also realized through the shdn pin, as the internal comparator that performs this function has a 2.65v threshold. an internal resistor pulls 10.3a to ground from the shdn pin at the uvlo threshold in order to provide hysteresis. choose resistors according to the following formula: r2 = 2.65v v uvlo < 2.65v r1 ? 10.3 a where v uvlo is the desired uvlo threshold suppose that the output needs to stay off until the input is above 8v. v th = 8v let r1 = 100k r2 = 2.65v 8v < 2.65v 100k ? 10.3a = 61.9 application information figure 1. undervoltage lockout gnd LTM8040 8040 f01 v in v in c1 r1 r2 shdn keep the connections from the resistors to the shdn pin short. if high resistance values are used, the shdn pin should be bypassed with a 1nf capacitor to prevent cou- pling problems from switching nodes. setting the switching frequency the LTM8040 uses a constant frequency architecture that can be programmed over a 500khz to 2mhz range with a single external timing resistor from the rt pin to ground. the current that ? ows into the timing resistor is used to charge an internal oscillator capacitor. the LTM8040 is con? gured such that the default frequency is 500khz without adding any resistor. many applications use this value. if another frequency is desired, a graph for selecting
LTM8040 9 8040p the value of rt for a given operating frequency is shown in the typical performance characteristics section. table 2 shows suggested rt selections for a variety of switch- ing frequencies. table 2. rt vs frequency r t (k) frequency (mhz) 13.0 2.00 16.0 1.84 18.7 1.70 24.9 1.50 29.4 1.37 35.8 1.25 54.9 1.00 75.0 0.90 88.7 0.85 137.0 0.75 175.0 0.68 215.0 0.65 487.0 0.57 open 0.50 gnd 3.3v white led LTM8040 8040 f02a v in shdn v adj pwm rt v in 4v to 36v c1 2.2f leda lpwr bias figure 2a. tie bias to lpwr if it is greater than 2.6v gnd 2.7v red led LTM8040 8040 f02b v in shdn v adj pwm rt v in 5.5v c1 2.2f leda lpwr bias gnd 3.3v 2.7v red led LTM8040 8040 f02c v in shdn v adj pwm rt v in 4v to 36v c1 2.2f leda lpwr bias figure 2b. bias may be tied to xv in if lpwr is below 2.6v figure 2c. tie bias to an external power source if neither v in nor lpwr are suitable application information bias pin considerations for proper operation, the bias pin must be powered by at least 3.2v. figure 2 shows three ways to arrange the circuit. for outputs of 3.2v or higher, the standard circuit (figure 2a) is best, because the circuits ef? ciency is better for lower voltages above 3.2v. for output voltages below 3.2v, the bias pin can be tied to the input (figure 2b) at the cost of some ef? ciency. finally, the bias pin can be tied to another source that is at least 3.2v (figure 2c). 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. in all cases, be sure that the maximum voltage at the bias pin is both less than 25v and the sum of v in and bias is less than 51v. if bias is powered by a source other than lpwr, a local decoupling capacitor may be necessary. the value of the decoupling cap is dependent upon the source and pcb layout. programming led current the led current can be set by adjusting the voltage on the adj pin. the adj pin is internally pulled up to a pre-
LTM8040 10 8040p cision 1.25v voltage source through a 5.11k 1% resistor. this resistor makes it easy to adjust the led current with a single external resistor. for a 1a led current, leave the adj pin ? oating. for lower output currents, apply a re- sistor from adj to gnd as shown in figure 3, using the following formula: r adj = 5.11 ? i led /(1amp C i led ), where r adj is in k and i led is the desired current out of leda. in order to have accurate led current, a precision 1% resistor is recommended. 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 5) 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, dis- connecting the led from the internal current source and freezing the state of LTM8040 internal control and drive circuitry, but leaving the output capacitor of the internal step down converter charged. when the pwm pin goes high again, the led current returns rapidly to its previous on state. this fast settling time allows the LTM8040 to maintain led current regulation with pwm pulse widths as short as 40s. it is also possible to not use an external nfet, but the output capacitor of the internal regulator will be discharged by the led(s) and have to be charged up again when the current source turns back on. this will lengthen the minimum dimming pulse width, in turn reducing the pwm dimming frequency. figure 4. dimming with an nfet and resistor gnd LTM8040 8040 f04 ref r2 v adj dim figure 5. dimming using pwm signal gnd LTM8040 8040 f05 pwm pwm 60hz to 10khz leda application information the leda voltage must be at least 2.4v for proper current regulation. dimming control there are several different types of dimming control circuits. one dimming control circuit (figure 4) changes the voltage on the adj pin by tying a low on-resis- tance 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 (iratio) 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 figure 3. setting adj with a resistor gnd LTM8040 8040 f03 ref r adj adj
LTM8040 11 8040p ceramic capacitors are also piezoelectric. while the LTM8040 is a ? xed frequency device, the internal regulators may skip cycles at light loads and extend the switching cycle on time as the input voltage falls towards the to output. under either of these conditions, the LTM8040 can excite a ceramic capacitor at audio frequencies, generating audible noise. if this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. the input capacitor can be a parallel combination of a 4.7f ceramic capacitor and a low cost electrolytic capacitor. a ? nal precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8040. a ceramic input capacitor combined with trace or cable inductance forms a high q (under damped) tank circuit. if the LTM8040 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possi- bly exceeding the devices rating. this situation is easily avoided by introducing a small series damping resistance into the circuit. this is most often taken care of by the presence of an electrolytic bulk capacitor in the board. high temperature considerations the internal operating temperature of the LTM8040 must be lower than 125c rating, so care should be taken in the layout of the circuit to ensure good heat sinking of the LTM8040. to estimate the junction temperature, ap- proximate the power dissipation within the LTM8040 by applying the typical ef? ciency stated in this datasheet to the desired output power, or, if you have and actual module, by taking a power measurement. then calculate the tem- perature rise of the LTM8040 junction above the surface of the printed circuit board by multiplying the modules power dissipation by the thermal resistance. the actual thermal resistance of the LTM8040 to the printed circuit board depends on the layout of the circuit board, but the thermal resistance given on page 2, which is based upon a 40.3cm 2 4 layer fr4 pc board, can be used a guide. the LTM8040 is equipped with thermal protection that reduces the output led current if the internal operating temperature is too high. this thermal protection is active above the 125c temperature rating of the LTM8040, so 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 = 1.0ms, t min = 25s i ratio = 1a 0.1a = 10:1 pwm ratio = 10ms 25s = 400:1 dim ratio = 10 ? 400 = 4000:1 minimum input voltage the LTM8040 is a step down converter, so a minimum amount of headroom is required to keep the output in regulation. for most applications at full load, the input needs to be at least 1.5v above the desired output. in addition, it takes more input voltage to initially start than is required for continuous operation. this start voltage is also dependent on whether turn-on is controlled by the LTM8040s shdn pin or uvlo (that is, the shdn pin is tied to v in ). see typical performance characteristics for details. capacitor selection considerations the c in and capacitor values in tables 1 and 2 are the minimum recommended values for the associated oper- ating conditions. applying capacitor values below those indicated in table 1 is not recommended, and may result in undesirable operation. using larger values is generally acceptable, and can yield improved performance, if it is necessary. again, it is incumbent upon the user to verify proper operation over the intended systems line, load and environmental conditions. ceramic capacitors are small, robust and have very low esr. however, not all ceramic capacitors are suitable. x5r and x7r types are stable over temperature and ap- plied voltage and give dependable service. other types, including y5v and z5u have very large temperature and voltage coef? cients of capacitance. in an application cir- cuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. application information
LTM8040 12 8040p figure 6. suggested layout leda led string gnd vias to gnd plane shdn lpwr bias adj rt pwm v in c in 8040 f06 application information continuous operation under this operating condition may impair reliability. layout hints most of the headaches associated with pcb layout have been alleviated or even eliminated by the high level of in- tegration of the LTM8040. the LTM8040 is nevertheless a switching power supply, and care must be taken to minimize emi and ensure proper operation. even with the high level of integration, you may fail to achieve speci? ed operation with a haphazard or poor layout. see figure 6 for a sug- gested layout. ensure that the grounding and heatsinking are acceptable. a few rules to keep in mind are: 1. place the c in capacitor as close as possible to the v in and gnd connection of the LTM8040. 2. connect all of the gnd co nnections to as large a cop- per pour or plane area as possible on the top layer. avoid breaking the ground connection between the external components and the LTM8040. 3. use vias to connect the gnd copper area to the boards internal ground plane. liberally distribute these gnd vias to provide both a good ground connection and thermal path to the internal planes of the printed circuit board. 4. if the application requires bias to be connected to the input voltage potential, tie bias to v in , but be careful not to break up the ground plane.
LTM8040 13 8040p application information table 1. recommended con? guration v in range c in led string voltage (leda) led string current (leda) rt optimal f optimal rt min f max radj bias connection 4.5-36v 1f 0805 50v 2.5-4v 35ma open 0.50m open 0.50m 154 2.8v to 25v source 6.5-36v 1f 0805 50v 4-6v 35ma open 0.50m open 0.50m 154 lpwr 9.5-36v 1f 0805 50v 6-9v 35ma open 0.50m open 0.50m 154 lpwr 12.5-36v 1f 0805 50v 8-12v 35ma open 0.50m open 0.50m 154 lpwr 4.5-36v 1f 0805 50v 2.5-4v 100ma open 0.50m open 0.50m 453 2.8v to 25v source 6.5-36v 1f 0805 50v 4-6v 100ma open 0.50m 165k 0.70m 453 lpwr 9.5-36v 1f 0805 50v 6-9v 100ma 487k 0.57m 137k 0.75m 453 lpwr 12.5-36v 1f 0805 50v 8-12v 100ma 487k 0.57m 88.7k 0.85m 453 lpwr 4.8-36v 1f 0805 50v 2.5-4v 350ma open 0.50m open 0.50m 2.87k 2.8v to 25v source 7-36v 1f 0805 50v 4-6v 350ma open 0.50m 165k 0.70m 2.87k lpwr 10.5-36v 1f 0805 50v 6-9v 350ma 137k 0.75m 54.9k 1.0m 2.87k lpwr 13.8-36v 1f 0805 50v 8-12v 350ma 75k 0.90m 29.4k 1.37m 2.87k lpwr 4.8-36v 1f 0805 50v 2.5-4v 500ma open 0.50m open 0.50m 5.11k 2.8v to 25v source 7-36v 1f 0805 50v 4-6v 500ma open 0.50m 165k 0.70m 5.11k lpwr 10.5-36v 1f 0805 50v 6-9v 500ma 137k 0.75m 54.9k 1.0m 5.11k lpwr 14.3-36v 1f 0805 50v 8-12v 500ma 75k 0.90m 29.4k 1.37m 5.11k lpwr 5-36v 1f 0805 50v 2.5-4v 700ma open 0.50m open 0.50m 11.8k 2.8v to 25v source 7.7-36v 1f 0805 50v 4-6v 700ma 487k 0.57m 165k 0.70m 11.8k lpwr 11-36v 1f 0805 50v 6-9v 700ma 165k 0.70m 54.9k 1.0m 11.8k lpwr 14.8-36v 1f 0805 50v 8-12v 700ma 75k 0.90m 29.4k 1.37m 11.8k lpwr 5.5-36v 1f 0805 50v 2.5-4v 1a open 0.50m open 0.50m open 2.8v to 25v source 8-36v 1f 0805 50v 4-6v 1a open 0.50m 137k 0.75m open lpwr 11.5-36v 1f 0805 50v 6-9v 1a 215k 0.65m 54.9k 1.0m open lpwr 15.5-36v 1f 0805 50v 8-12v 1a 137k 0.75m 29.4k 1.37m open lpwr
LTM8040 14 8040p typical applications step down 1a drive with single red or white led step down 350ma drive with three series red leds step down 1a drive with three white series leds gnd LTM8040 8040 ta02 v in * 5.5v to 25v 1f *running voltage. see application information for start-up requirements leda lpwr 2.5v to 4v 1a v in shdn bias adj pwm rt gnd LTM8040 8040 ta03 v in * 10.5v to 36v 1f 2.87k 137k 750khz *running voltage. see application information for start-up requirements leda lpwr bias 6v to 9v 350ma v in shdn adj pwm rt gnd LTM8040 8040 ta04 v in * 15.5v to 36v 1f 137k 750khz *running voltage. see application information for start-up requirements leda lpwr bias 8v to 12v 1a v in shdn adj pwm rt
LTM8040 15 8040p information furnished by linear technology corpor ation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- t i o n t h a t t h e i n t e r c o n n e c t i o n o f i t s c i r c u i t s a s d e s c r i b e d h e r e i n w i l l n o t i n f r i n g e o n e x i s t i n g p a t e n t r i g h t s . package description lga package 66-lead (15mm 9mm 2.82mm) (reference ltc dwg # 05-08-1810 rev ?) notes: 1. dimensioning and tolerancing per asme y14.5m-1994 2. all dimensions are in millimeters land designation per jesd mo-222, spp-010 and spp-020 5. primary datum -z- is seating plane 6. the total number of pads: 66 4 3 details of pad #1 identifier are optional, but must be located within the zone indicated. the pad #1 identifier may be either a mold or a marked feature symbol aaa bbb tolerance 0.15 0.10 9.000 bsc package top view lga 66 0407 rev ? 15.000 bsc 4 pad 1 corner 3 pads see notes x y aaa z aaa z 2.670 ? 2.970 detail a package side view detail a substrate mold cap 0.27 ? 0.37 2.40 ? 2.60 bbb z z 1.270 bsc 0.605 ? 0.665 0.605 ? 0.665 12.700 bsc 7.620 bsc pad 1 c (0.30) hba dc 6 7 5 1 2 3 4 e f package bottom view package in tray loading orientation g lj k 2.540 2.540 1.270 5.080 5.080 6.350 6.350 3.810 3.810 0.000 1.270 3.810 3.810 2.540 2.540 1.270 1.270 0.000 1.5875 0.9525 suggested pcb layout top view ltmxxxxxx module tray pin 1 bevel component pin 1 0.9525 1.5875
LTM8040 16 8040p linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com linear technology corporation 2008 lt 0808 ? printed in usa related parts typical application step down 1a drive with four series red leds gnd LTM8040 8040 ta05 v in * 15.5v to 36v 1f 137k 750khz *running voltage. see application information for start-up requirements leda lpwr bias 8v to 12v 1a v in shdn adj pwm rt part number description comments ltm4600 10a dc/dc module basic 10a dc/dc module, 15mm 15mm 2.8mm lga ltm4600hvmpv military plastic 10a dc/dc module C55c to 125c operation, 15mm 15mm 2.8mm lga ltm4601/ ltm4601a 12a dc/dc module with pll, output tracking/margining and remote sensing synchronizable, polyphase ? operation, ltm4601-1 version has no remote sensing ltm4602 6a dc/dc module pin-compatible with the ltm4600 ltm4603 6a dc/dc module with pll and output tracking/ margining and remote sensing synchronizable, polyphase operation, ltm4603-1 version has no remote sensing, pin-compatible with the ltm4601 ltm4604 4a low v in dc/dc module 2.375v v in 5v, 0.8v v out 5v, 9mm 15mm 2.3mm lga ltm4605 5a to 12a buck-boost module high ef? ciency, adjustable frequency, 4.5v v in 20v, 0.8v v out 16v, 15mm 15mm 2.8mm ltm4607 5a to 12a buck-boost module high ef? ciency, adjustable frequency, 4.5v v in 36v, 0.8v v out 25v, 15mm 15mm 2.8mm ltm4608 8a low v in dc/dc module 2.375v v in 5v, 0.8v v out 5v, 9mm 15mm 2.8mm lga ltm8020 36v, 200ma dc/dc module 4v v in 36v, 1.25v v out 5v, 6.25mm 6.25mm 2.3mm lga ltm8022 1a, 36v dc/dc module adjustable frequency, 0.8v v out 5v, 11.25mm 9mm 2.82mm, pin-compatible to the ltm8023 ltm8023 2a, 36v dc/dc module adjustable frequency, 0.8v v out 5v, 11.25mm 9mm 2.82mm, pin-compatible to the ltm8022 polyphase is a trademark of linear technology corporation

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