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LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 LM5008A 100v, 350 ma constant on-time buck switching regulator check for samples: LM5008A 1 features ? precision internal reference ? low bias current 2 ? operating input voltage range: 6v to 95v ? intelligent current limit ? integrated 100v, n-channel buck switch ? thermal shutdown ? internal start-up regulator ? no loop compensation required typical applications ? ultra-fast transient response ? non-isolated telecommunication buck ? on time varies inversely with input voltage regulator ? operating frequency remains constant with ? secondary high voltage post regulator varying line voltage and load current ? +42v automotive systems ? adjustable output voltage from 2.5v ? highly efficient operation description the LM5008A is a functional variant of the lm5008 cot buck switching regulator. the functional differences of the LM5008A are: the minimum input operating voltage is 6 volts, the on-time equation is slightly different, and the requirement for a minimum load current is removed. the LM5008A step down switching regulator features all of the functions needed to implement a low cost, efficient, buck bias regulator. this high voltage regulator contains an 100 v n-channel buck switch. the device is easy to implement and is provided in the msop-8 and the thermally enhanced llp-8 packages. the regulator is based on a control scheme using an on time inversely proportional to v in . this feature allows the operating frequency to remain relatively constant. the control scheme requires no loop compensation. an intelligent current limit is implemented with forced off time, which is inversely proportional to vout. this scheme ensures short circuit control while providing minimum foldback. other features include: thermal shutdown, v cc under-voltage lockout, gate drive under-voltage lockout, max duty cycle limiter, and a pre-charge switch. package ? msop - 8 ? llp - 8 (4mm x 4mm) typical application, basic step-down regulator 1 please be aware that an important notice concerning availability, standard warranty, and use in critical applications of texas instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. 2 all trademarks are the property of their respective owners. production data information is current as of publication date. copyright ? 2009 ? 2011, texas instruments incorporated products conform to specifications per the terms of the texas instruments standard warranty. production processing does not necessarily include testing of all parameters. rt/sd fb vin sw rtn bst 6v - 95v input LM5008A shutdown rcl vcc v in gnd r cl r t c1 c3 c4 l1 d1 r fb2 r fb1 r3 c2 gnd v out
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com connection diagram figure 1. top view 8-lead llp figure 2. top view 8-lead msop pin functions pin descriptions pin name description application information 1 sw switching node power switching node. connect to the output inductor, re-circulating diode, and bootstrap capacitor. 2 bst boost pin (boot ? strap capacitor an external capacitor is required between the bst and the sw pins. a 0.01 f input) ceramic capacitor is recommended. an internal diode charges the capacitor from v cc during each off-time. 3 rcl current limit off time set pin a resistor between this pin and rtn sets the off-time when current limit is detected. the off-time is preset to 35 s if fb = 0v. 4 rtn ground pin ground for the entire circuit. 5 fb feedback input from regulated this pin is connected to the inverting input of the internal regulation comparator. output the regulation threshold is 2.5v. 6 rt/sd on time set pin a resistor between this pin and vin sets the switch on time as a function of v in . the minimum recommended on time is 400 ns at the maximum input voltage. this pin can be used for remote shutdown. 7 vcc output from the internal high voltage this regulated voltage provides gate drive power for the internal buck switch. series pass regulator. an internal diode is provided between this pin and the bst pin. a local 0.47 f decoupling capacitor is required. the series pass regulator is current limited to 9 ma. 8 vin input voltage input operating range: 6v to 95v. ep exposed pad the exposed pad has no electrical contact. connect to system ground plane for reduced thermal resistance. these devices have limited built-in esd protection. the leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the mos gates. 2 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A 12 3 4 5 87 6 fb vin vcc rt/sd sw bst rcl rtn 87 6 5 4 3 2 1 fb vin exposed pad on bottom connect to ground vcc rt/sd sw bst rcl rtn
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 absolute maximum ratings (1) v in to gnd -0.3v to 100v bst to gnd -0.3v to 114v sw to gnd (steady state) -1v esd rating (2) human body model 2kv bst to v cc 100v bst to sw 14v v cc to gnd 14v all other inputs to gnd -0.3 to 7v lead temperature (soldering 4 sec) 260 c storage temperature range -55 c to +150 c (1) absolute maximum ratings are limits beyond which damage to the device may occur. operating ratings are conditions under which operation of the device is intended to be functional. for guaranteed specifications and test conditions, see the electrical characteristics. (2) the human body model is a 100pf capacitor discharged through a 1.5k ? resistor into each pin. the esd rating for pin 2, pin 7, and pin 8 is 1 kv. operating ratings (1) v in 6v to 95v operating junction temperature ? 40 c to + 125 c (1) absolute maximum ratings are limits beyond which damage to the device may occur. operating ratings are conditions under which operation of the device is intended to be functional. for guaranteed specifications and test conditions, see the electrical characteristics. copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 3 product folder links: LM5008A
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com electrical characteristics specifications with standard typeface are for t j = 25 c, and those with boldface type apply over full operating junction temperature range . v in = 48v, unless otherwise stated (1) . symbol parameter conditions min typ max units vcc supply vcc reg vcc regulator output vin = 48v 6.6 7 7.4 v vin ? vcc 6v < vin < 8.5v 100 mv vcc bypass threshold vin increasing 8.5 v vcc bypass hysteresis 300 mv vcc output impedance vin =6v 100 ? vin = 10v 8.8 ? vin = 48v 0.8 ? vcc current limit vin = 48v 9.2 ma vcc uvlo vcc increasing 5.3 v vcc uvlo hysteresis 190 mv vcc uvlo filter delay 3 s iin operating current fb = 3v, vin = 48v 550 750 a iin shutdown current rt/sd = 0v 110 176 a switch characteristics buckswitch rds(on) itest = 200 ma 1.25 2.57 ? gate drive uvlo vbst ? vsw rising 2.8 3.8 4.8 v gate drive uvlo hysteresis 490 mv pre-charge switch voltage at 1 ma 0.8 v pre-charge switch on-time 150 ns current limit current limit threshold 0.41 0.51 0.61 a current limit response time i switch overdrive = 0.1a time 350 ns to switch off t off-1 off time generator fb=0v, r cl = 100k 35 s t off-2 off time generator fb=2.3v, r cl = 100k 2.56 s on time generator t on - 1 vin = 10v 2.15 2.77 3.5 s ron = 200k t on - 2 vin = 95v 200 300 420 ns ron = 200k remote shutdown threshold rising 0.40 0.70 1.05 v remote shutdown hysteresis 35 mv minimum off time minimum off timer fb = 0v 300 ns regulation and ov comparators fb reference threshold internal reference 2.445 2.5 2.550 v trip point for switch on fb over-voltage threshold trip point for switch off 2.875 v fb bias current 100 na thermal shutdown tsd thermal shutdown temp. 165 c thermal shutdown hysteresis 25 c thermal resistance (1) all limits are guaranteed. all electrical characteristics having room temperature limits are tested during production with t a = t j = 25 c. all hot and cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control. 4 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 electrical characteristics (continued) specifications with standard typeface are for t j = 25 c, and those with boldface type apply over full operating junction temperature range . v in = 48v, unless otherwise stated (1) . symbol parameter conditions min typ max units ja junction to ambient mua package 200 c/w sdc package 40 c/w copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 5 product folder links: LM5008A
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com typical performance characteristics efficiency vs. vcc load current and v in vs. (circuit of figure 6 ) vin on-time current limit off-time vs vs. input voltage and r t v fb and r cl maximum frequency i cc current vs. vs. v out and v in applied v cc voltage 6 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A 0 0.5 1.0 1.5 2.0 2.5 0 5 10 15 20 25 30 35 current limit off time ( p s) v fb (v) 300k r cl = 500k 100k 50k
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 block diagram functional description the LM5008A step down switching regulator features all the functions needed to implement a low cost, efficient, buck bias power converter. this high voltage regulator contains a 100 v n-channel buck switch, is easy to implement and is provided in the msop-8 and the thermally enhanced llp-8 packages. the regulator is based on a control scheme using an on-time inversely proportional to v in . the control scheme requires no loop compensation. current limit is implemented with forced off-time, which is inversely proportional to v out . this scheme ensures short circuit control while providing minimum foldback. the LM5008A can be applied in numerous applications to efficiently regulate down higher voltages. this regulator is well suited for 48 volt telecom and the new 42v automotive power bus ranges. features include: thermal shutdown, v cc under-voltage lockout, gate drive under-voltage lockout, max duty cycle limit timer, intelligent current limit off timer, and a pre-charge switch. control circuit overview the LM5008A is a buck dc-dc regulator that uses a control scheme in which the on-time varies inversely with line voltage (v in ). control is based on a comparator and the on-time one-shot, with the output voltage feedback (fb) compared to an internal reference (2.5v). if the fb level is below the reference the buck switch is turned on for a fixed time determined by the line voltage and a programming resistor (r t ). following the on period the switch will remain off for at least the minimum off-timer period of 300ns. if fb is still below the reference at that time the switch will turn on again for another on-time period. this will continue until regulation is achieved. the LM5008A operates in discontinuous conduction mode at light load currents, and continuous conduction mode at heavy load current. in discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. the next on- time period starts when the voltage at fb falls below the internal reference - until then the inductor current remains zero. in this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. therefore at light loads the conversion efficiency is maintained, since the switching losses reduce with the reduction in load and frequency. the discontinuous operating frequency can be calculated as follows: (1) where r l = the load resistance in continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. in this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively constant with load and line variations. the approximate continuous mode operating frequency can be calculated as follows: copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 7 product folder links: LM5008A q q fb vin vcc sw rtn on timer driver vin bst level shift thermal shutdown finish start rcl finish start rt l1 r vcc uvlo fb d1 uvlo regulation comparator sd finish start regulator current limit off timer shutdown gd buck switch current sense gnd LM5008A 7v bias bypass switch r t 2.875 v v in sense q2 c1 c5 6v to 95v input rt/sd 0.7 v 300 ns min off timer clr r set s 0.51a c2 c3 v out r3 r fb1 fb2 r cl r cl 2.5 v over-voltage comparator charge pre - c4 f = v out 2 x l x 1.04 x 10 20 r l x (r t ) 2
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com (2) the output voltage (v out ) is programmed by two external resistors as shown in the block diagram. the regulation point can be calculated as follows: v out = 2.5 x (r fb1 + r fb2 ) / r fb1 (3) the LM5008A regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of esr for the output capacitor c2. a minimum of 25mv to 50mv of ripple voltage at the feedback pin (fb) is required for the LM5008A. in cases where the capacitor esr is too small, additional series resistance may be required (r3 in the block diagram). for applications where lower output voltage ripple is required the output can be taken directly from a low esr output capacitor, as shown in figure 3 . however, r3 slightly degrades the load regulation. figure 3. low ripple output configuration start-up regulator (v cc ) the high voltage bias regulator is integrated within the LM5008A. the input pin (vin) can be connected directly to line voltages between 6v and 95v, with transient capability to 100v. referring to the block diagram and the graph of v cc vs v in , when v in is between 6v and the bypass threshold (nominally 8.5v), the bypass switch (q2) is on, and v cc tracks v in within 100 mv to 150 mv. the bypass switch on-resistance is approximately 100 ? , with inherent current limiting at approximately 100 ma. when v in is above the bypass threshold q2 is turned off, and v cc is regulated at 7v. the v cc regulator output current is limited at approximately 9.2 ma. when the LM5008A is shutdown using the rt/sd pin, the v cc bypass switch is shut off regardless of the voltage at v in . when vin exceeds the bypass threshold, the time required for q2 to shut off is approximately 2 - 3 s. the capacitor at vcc (c3) must be a minimum of 0.47 f to prevent the voltage at vcc from rising above its absolute maximum rating in response to a step input applied at v in . c3 must be located as close as possible to the vcc and rtn pins. in applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. an auxiliary voltage of between 7.5v and 14v can be diode connected to the vcc pin to shut off the v cc regulator, thereby reducing internal power dissipation. the current required into the vcc pin is shown in the graph ? i cc current vs. applied v cc voltage ? . internally a diode connects vcc to vin requiring that the auxiliary voltage be less than v in . the turn-on sequence is shown in figure 4 . during the initial delay (t1) vcc ramps up at a rate determined by its current limit and c3 while internal circuitry stabilizes. when v cc reaches the upper threshold of its under-voltage lock-out (uvlo, typically 5.3v) the buckswitch is enabled. the inductor current increases to the current limit threshold (i lim ) and during t2 v out increases as the output capacitor charges up. when v out reaches the intended voltage the average inductor current decreases (t3) to the nominal load current (i o ). 8 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A fb sw l1 c2 r fb2 v out2 r3 LM5008A r fb1 f = v out 1.385 x 10 -10 x r t
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 figure 4. startup sequence regulation comparator the feedback voltage at fb is compared to an internal 2.5v reference. in normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at fb falls below 2.5v. the buck switch will stay on for the on-time, causing the fb voltage to rise above 2.5v. after the on-time period, the buck switch will stay off until the fb voltage again falls below 2.5v. during start-up, the fb voltage will be below 2.5v at the end of each on- time, resulting in the minimum off-time of 300 ns. bias current at the fb pin is nominally 100 na. over-voltage comparator the feedback voltage at fb is compared to an internal 2.875v reference. if the voltage at fb rises above 2.875v the on-time pulse is immediately terminated. this condition can occur if the input voltage, or the output load, change suddenly. the buck switch will not turn on again until the voltage at fb falls below 2.5v. on-time generator and shutdown the on-time for the LM5008A is determined by the r t resistor, and is inversely proportional to the input voltage (vin), resulting in a nearly constant frequency as vin is varied over its range. the on-time equation for the LM5008A is: t on = 1.385 x 10 -10 x r t / v in (4) r t should be selected for a minimum on-time (at maximum v in ) greater than 400 ns, for proper current limit operation. this requirement limits the maximum frequency for each application, depending on v in and v out . the LM5008A can be remotely disabled by taking the r t /sd pin to ground. see figure 5 . the voltage at the r t /sd pin is between 1.5 and 3.0 volts, depending on vin and the value of the r t resistor. copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 9 product folder links: LM5008A uvlo 7 v sw pin inductor current vin t 2 i lim v cc v in t1 t3 i o 0v v out
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com figure 5. shutdown implementation current limit the LM5008A contains an intelligent current limit off timer. if the current in the buck switch exceeds 0.51a the present cycle is immediately terminated, and a non-resetable off timer is initiated. the length of off-time is controlled by an external resistor (r cl ) and the fb voltage (see the graph current limit off-time vs. v fb and r cl ). when fb = 0v, a maximum off-time is required, and the time is preset to 35 s. this condition occurs when the output is shorted, and during the initial part of start-up. this amount of time ensures safe short circuit operation up to the maximum input voltage of 95v. in cases of overload where the fb voltage is above zero volts (not a short circuit) the current limit off-time will be less than 35 s. reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and the start-up time. the off-time is calculated from the following equation: (5) the current limit sensing circuit is blanked for the first 50-70ns of each on-time so it is not falsely tripped by the current surge which occurs at turn-on. the current surge is required by the re-circulating diode (d1) for its turn- off recovery. n - channel buck switch and driver the LM5008A integrates an n-channel buck switch and associated floating high voltage gate driver. the gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. a 0.01 f ceramic capacitor (c4) connected between the bst pin and sw pin provides the voltage to the driver during the on-time. during each off-time, the sw pin is at approximately 0v, and the bootstrap capacitor charges from vcc through the internal diode. the minimum off timer, set to 300ns, ensures a minimum time each cycle to recharge the bootstrap capacitor. the internal pre-charge switch at the sw pin is turned on for ? 150 ns during the minimum off-time period, ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. this feature helps prevent operating problems which can occur during very light load conditions, involving a long off-time, during which the voltage across the bootstrap capacitor could otherwise reduce below the gate drive uvlo threshold. the pre- charge switch also helps prevent startup problems which can occur if the output voltage is pre-charged prior to turn-on. after current limit detection, the pre-charge switch is turned on for the entire duration of the forced off- time . 10 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A stop run r t LM5008A r t /sd vin input voltage
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 thermal protection the LM5008A should be operated so the junction temperature does not exceed 125 c during normal operation. an internal thermal shutdown circuit is provided to shutdown the LM5008A in the event of a higher than normal junction temperature. when activated, typically at 165 c, the controller is forced into a low power reset state by disabling the buck switch. this feature prevents catastrophic failures from accidental device overheating. when the junction temperature reduces below 140 c (typical hysteresis = 25 c) normal operation is resumed. applications information selection of external components a guide for determining the component values will be illustrated with a design example. refer to the block diagram. the following steps will configure the LM5008A for: ? input voltage range (vin): 12v to 95v ? output voltage (v out1 ): 10v ? load current (for continuous conduction mode): 100 ma to 300 ma r fb1 , r fb2 : v out = v fb x (r fb1 + r fb2 ) / r fb1 , and since v fb = 2.5v, the ratio of r fb2 to r fb1 calculates as 3:1. standard values of 3.01 k ? and 1.00 k ? are chosen. other values could be used as long as the 3:1 ratio is maintained. f s and r t : the recommended operating frequency range for the LM5008A is 50 khz to 1.1 mhz. unless the application requires a specific frequency, the choice of frequency is generally a compromise since it affects the size of l1 and c2, and the switching losses. the maximum allowed frequency, based on a minimum on-time of 400 ns, is calculated from: f max = v out / (v inmax x 400 ns) (6) for this exercise, fmax = 263 khz. from equation 1, r t calculates to 274 k ? . a standard value 324 k ? resistor will be used to allow for tolerances in equation 1, resulting in a frequency of 223 khz. l1: the main parameter affected by the inductor is the output current ripple amplitude. the choice of inductor value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple current occurs at maximum vin. a) minimum load current: to maintain continuous conduction at minimum io (100 ma), the ripple amplitude (i or ) must be less than 200 ma p-p so the lower peak of the waveform does not reach zero. l1 is calculated using the following equation: (7) at vin = 95v, l1(min) calculates to 200 h. the next larger standard value (220 h) is chosen and with this value i or calculates to 182 ma p-p at vin = 95v, and 34 ma p-p at vin = 12v. b) maximum load current: at a load current of 300 ma, the peak of the ripple waveform must not reach the minimum guaranteed value of the LM5008A ? s current limit threshold (410 ma). therefore the ripple amplitude must be less than 220 ma p-p, which is already satisfied in the above calculation. with l1 = 220 h, at maximum vin and io, the peak of the ripple will be 391 ma. while l1 must carry this peak current without saturating or exceeding its temperature rating, it also must be capable of carrying the maximum guaranteed value of the LM5008A ? s current limit threshold (610 ma) without saturating, since the current limit is reached during startup. the dc resistance of the inductor should be as low as possible. for example, if the inductor ? s dcr is one ohm, the power dissipated at maximum load current is 0.09w. while small, it is not insignificant compared to the load power of 3w. c3: the capacitor on the v cc output provides not only noise filtering and stability, but its primary purpose is to prevent false triggering of the v cc uvlo at the buck switch on/off transitions. c3 should be no smaller than 0.47 f. c2, and r3: when selecting the output filter capacitor c2, the items to consider are ripple voltage due to its esr, ripple voltage due to its capacitance, and the nature of the load. copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 11 product folder links: LM5008A l1 = v out x (v in - v out ) i or x f s x v in
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com esr and r3: a low esr for c2 is generally desirable so as to minimize power losses and heating within the capacitor. however, the regulator requires a minimum amount of ripple voltage at the feedback input for proper loop operation. for the LM5008A the minimum ripple required at pin 5 is 25 mv p-p, requiring a minimum ripple at v out of 100 mv. since the minimum ripple current (at minimum vin) is 34 ma p-p, the minimum esr required at v out is 100 mv/34 ma = 2.94 ? . since quality capacitors for smps applications have an esr considerably less than this, r3 is inserted as shown in the block diagram. r3 ? s value, along with c2 ? s esr, must result in at least 25 mv p-p ripple at pin 5. generally, r3 will be 0.5 to 3.0 ? . r cl : when current limit is detected, the minimum off-time set by this resistor must be greater than the maximum normal off-time, which occurs at maximum input voltage. using equation 2, the minimum on-time is 472 ns, yielding an off-time of 4 s (at 223 khz). due to the 25% tolerance on the on-time, the off-time tolerance is also 25%, yielding a maximum off-time of 5 s. allowing for the response time of the current limit detection circuit (350 ns) increases the maximum off-time to 5.35 s. this is increased an additional 25% to 6.7 s to allow for the tolerances of equation 3. using equation 3, r cl calculates to 325 k ? at v fb = 2.5v. a standard value 332 k ? resistor will be used. d1: the important parameters are reverse recovery time and forward voltage. the reverse recovery time determines how long the reverse current surge lasts each time the buck switch is turned on. the forward voltage drop is significant in the event the output is short-circuited as it is only this diode ? s voltage which forces the inductor current to reduce during the forced off-time. for this reason, a higher voltage is better, although that affects efficiency. a good choice is a schottky power diode, such as the dfls1100. d1 ? s reverse voltage rating must be at least as great as the maximum vin, and its current rating be greater than the maximum current limit threshold (610 ma). c1: this capacitor ? s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at vin, on the assumption that the voltage source feeding vin has an output impedance greater than zero. at maximum load current, when the buck switch turns on, the current into pin 8 will suddenly increase to the lower peak of the output current waveform, ramp up to the peak value, then drop to zero at turn-off. the average input current during this on-time is the load current (300 ma). for a worst case calculation, c1 must supply this average load current during the maximum on-time. to keep the input voltage ripple to less than 2v (for this exercise), c1 calculates to: (8) quality ceramic capacitors in this value have a low esr which adds only a few millivolts to the ripple. it is the capacitance which is dominant in this case. to allow for the capacitor ? s tolerance, temperature effects, and voltage effects, a 1.0 f, 100v, x7r capacitor will be used. c4: the recommended value is 0.01 f for c4, as this is appropriate in the majority of applications. a high quality ceramic capacitor, with low esr is recommended as c4 supplies the surge current to charge the buck switch gate at turn-on. a low esr also ensures a quick recharge during each off-time. at minimum vin, when the on- time is at maximum, it is possible during start-up that c4 will not fully recharge during each 300 ns off-time. the circuit will not be able to complete the start-up, and achieve output regulation. this can occur when the frequency is intended to be low (e.g., r t = 500k). in this case c4 should be increased so it can maintain sufficient voltage across the buck switch driver during each on-time. c5: this capacitor helps avoid supply voltage transients and ringing due to long lead inductance at v in . a low esr, 0.1 f ceramic chip capacitor is recommended, located close to the LM5008A. final circuit the final circuit is shown in figure 6 . the circuit was tested, and the resulting performance is shown in figure 7 and figure 8 . pc board layout the LM5008A regulation and over-voltage comparators are very fast, and as such will respond to short duration noise pulses. layout considerations are therefore critical for optimum performance. the components at pins 1, 2, 3, 5, and 6 should be as physically close as possible to the ic, thereby minimizing noise pickup in the pc tracks. the current loop formed by d1, l1, and c2 should be as small as possible. the ground connection from d1 to c1 should be as short and direct as possible. 12 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A c1 = i x t on ' v 0.3a x 3.74 p s 2.0v = = 0.56 p f
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 if the internal dissipation of the LM5008A produces excessive junction temperatures during normal operation, good use of the pc board ? s ground plane can help considerably to dissipate heat. the exposed pad on the bottom of the llp-8 package can be soldered to a ground plane on the pc board, and that plane should extend out from beneath the ic to help dissipate the heat. additionally, the use of wide pc board traces, where possible, can also help conduct heat away from the ic. judicious positioning of the pc board within the end product, along with use of any available air flow (forced or natural convection) can help reduce the junction temperatures. figure 6. LM5008A example circuit table 1. bill of materials item description part number value c1 ceramic capacitor tdk c4532x7r2a105m 1 f, 100v c2 ceramic capacitor tdk c4532x7r1e226m 22 f, 25v c3 ceramic capacitor kemet c1206c474k5rac 0.47 f, 50v c4 ceramic capacitor kemet c1206c103k5rac 0.01 f, 50v c5 ceramic capacitor tdk c3216x7r2a104m 0.1 f, 100v d1 schottky power diode diodes inc. dfls1100 100v, 1a l1 power inductor coiltronics dr125-221-r, or 220 h tdk slf10145t-221mr65 r fb2 resistor vishay crcw12063011f 3.01 k ? r fb1 resistor vishay crcw12061001f 1.0 k ? r3 resistor vishay crcw12063r00f 3.0 ? r t resistor vishay crcw12063243f 324 k ? r cl resistor vishay crcw12063323f 332 k ? u1 switching regulator national semiconductor LM5008A copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 13 product folder links: LM5008A fb vin sw rtn bst LM5008A 8 6 5 3 4 1 2 7 shutdown vcc rcl r t /sd 12 - 95v input 324k c1 1.0 p f r t r cl 332k r fb1 1.0k r fb2 3.01k r3 3.0 gnd c2 22 p f v out 10.0v l1 220 p h c3 0.47 p f c4 0.01 p f d1 c5 0.1 p f
LM5008A snvs583e ? march 2009 ? revised april 2011 www.ti.com figure 7. efficiency vs. load current and v in figure 8. efficiency vs. v in low output ripple configurations for applications where low output ripple is required, the following options can be used to reduce or nearly eliminate the ripple. a) reduced ripple configuration: in figure 9 , cff is added across r fb2 to ac-couple the ripple at v out directly to the fb pin. this allows the ripple at v out to be reduced to a minimum of 25 mvp-p by reducing r3, since the ripple at v out is not attenuated by the feedback resistors. the minimum value for cff is determined from: (9) where t on(max) is the maximum on-time, which occurs at v in(min) . the next larger standard value capacitor should be used for cff. figure 9. reduced ripple configuration 14 submit documentation feedback copyright ? 2009 ? 2011, texas instruments incorporated product folder links: LM5008A fb sw l1 LM5008A cff r fb2 r3 c2 v out r fb1 cff = 3 x t on (max) (r fb1 //r fb2 )
LM5008A www.ti.com snvs583e ? march 2009 ? revised april 2011 b) minimum ripple configuration: if the application requires a lower value of ripple ( < 10 mvp-p), the circuit of figure 10 can be used. r3 is removed, and the resulting output ripple voltage is determined by the inductor ? s ripple current and c2 ? s characteristics. ra and ca are chosen to generate a sawtooth waveform at their junction, and that voltage is ac-coupled to the fb pin via cb. to determine the values for ra, ca and cb, use the following procedure: calculate v a = v out - (v sw x (1 - (v out /v in(min) ))) (10) where v sw is the absolute value of the voltage at the sw pin during the off-time (typically 1v). va is the dc voltage at the ra/ca junction, and is used in the next equation. - calculate ra x ca = (v in(min) - v a ) x t on / v (11) where t on is the maximum on-time (at minimum input voltage), and v is the desired ripple amplitude at the ra/ca junction (typically 40-50 mv). ra and ca are then chosen from standard value components to satisfy the above product. typically ca is 1000 pf to 5000 pf, and ra is 10 k ? to 300 k ? . cb is then chosen large compared to ca, typically 0.1 f. figure 10. minimum output ripple using ripple injection c) alternate minimum ripple configuration: the circuit in figure 11 is the same as that in the block diagram, except the output voltage is taken from the junction of r3 and c2. the ripple at v out is determined by the inductor ? s ripple current and c2 ? s characteristics. however, r3 slightly degrades the load regulation. this circuit may be suitable if the load current is fairly constant. figure 11. alternate minimum output ripple copyright ? 2009 ? 2011, texas instruments incorporated submit documentation feedback 15 product folder links: LM5008A fb sw l1 LM5008A r3 c2 v out r fb2 r fb1 sw fb LM5008A l1 c2 cb ca ra v out r fb2 r fb1
package option addendum www.ti.com 17-nov-2012 addendum-page 1 packaging information orderable device status (1) package type package drawing pins package qty eco plan (2) lead/ball finish msl peak temp (3) samples (requires login) LM5008Amm/nopb active vssop dgk 8 1000 green (rohs & no sb/br) cu sn level-1-260c-unlim LM5008Ammx/nopb active vssop dgk 8 3500 green (rohs & no sb/br) cu sn level-1-260c-unlim LM5008Asd/nopb active wson ngu 8 1000 green (rohs & no sb/br) cu sn level-1-260c-unlim LM5008Asdx/nopb active wson ngu 8 4500 green (rohs & no sb/br) cu sn level-1-260c-unlim (1) the marketing status values are defined as follows: active: product device recommended for new designs. lifebuy: ti has announced that the device will be discontinued, and a lifetime-buy period is in effect. nrnd: not recommended for new designs. device is in production to support existing customers, but ti does not recommend using this part in a new design. preview: device has been announced but is not in production. samples may or may not be available. obsolete: ti has discontinued the production of the device. (2) eco plan - the planned eco-friendly classification: pb-free (rohs), pb-free (rohs exempt), or green (rohs & no sb/br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. tbd: the pb-free/green conversion plan has not been defined. pb-free (rohs): ti's terms "lead-free" or "pb-free" mean semiconductor products that are compatible with the current rohs requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. where designed to be soldered at high temperatures, ti pb-free products are suitable for use in specified lead-free processes. pb-free (rohs exempt): this component has a rohs exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. the component is otherwise considered pb-free (rohs compatible) as defined above. green (rohs & no sb/br): ti defines "green" to mean pb-free (rohs compatible), and free of bromine (br) and antimony (sb) based flame retardants (br or sb do not exceed 0.1% by weight in homogeneous material) (3) msl, peak temp. -- the moisture sensitivity level rating according to the jedec industry standard classifications, and peak solder temperature. important information and disclaimer: the information provided on this page represents ti's knowledge and belief as of the date that it is provided. ti bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. efforts are underway to better integrate information from third parties. ti has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. ti and ti suppliers consider certain information to be proprietary, and thus cas numbers and other limited information may not be available for release. in no event shall ti's liability arising out of such information exceed the total purchase price of the ti part(s) at issue in this document sold by ti to customer on an annual basis.
tape and reel information *all dimensions are nominal device package type package drawing pins spq reel diameter (mm) reel width w1 (mm) a0 (mm) b0 (mm) k0 (mm) p1 (mm) w (mm) pin1 quadrant LM5008Amm/nopb vssop dgk 8 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 q1 LM5008Ammx/nopb vssop dgk 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 q1 LM5008Asd/nopb wson ngu 8 1000 178.0 12.4 4.3 4.3 1.3 8.0 12.0 q1 LM5008Asdx/nopb wson ngu 8 4500 330.0 12.4 4.3 4.3 1.3 8.0 12.0 q1 package materials information www.ti.com 17-nov-2012 pack materials-page 1
*all dimensions are nominal device package type package drawing pins spq length (mm) width (mm) height (mm) LM5008Amm/nopb vssop dgk 8 1000 203.0 190.0 41.0 LM5008Ammx/nopb vssop dgk 8 3500 349.0 337.0 45.0 LM5008Asd/nopb wson ngu 8 1000 203.0 190.0 41.0 LM5008Asdx/nopb wson ngu 8 4500 349.0 337.0 45.0 package materials information www.ti.com 17-nov-2012 pack materials-page 2

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