View LT1076CT06PBF_4759742.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

lt1074/lt1076 1 sn1074 1074fds , ltc and lt are registered trademarks of linear technology corporation. applicatio s u features descriptio u typical applicatio u step-down switching regulator the lt ? 1074 is a 5a (lt1076 is rated at 2a) monolithic bipolar switching regulator which requires only a few external parts for normal operation. the power switch, all oscillator and control circuitry, and all current limit com- buck converter with output voltage range of 2.5v to 50v tapped-inductor buck converter with 10a output at 5v positive-to-negative converter negative boost converter multiple output buck converter 5a onboard switch (lt1074) operates up to 60v input 100khz switching frequency greatly improved dynamic behavior available in low cost 5 and 7-lead packages only 8.5ma quiescent current programmable current limit micropower shutdown mode ponents, are included on the chip. the topology is a classic positive buck configuration but several design innova- tions allow this device to be used as a positive-to-negative converter, a negative boost converter, and as a flyback converter. the switch output is specified to swing 40v below ground, allowing the lt1074 to drive a tapped- inductor in the buck mode with output currents up to 10a. the lt1074 uses a true analog multiplier in the feedback loop. this makes the device respond nearly instanta- neously to input voltage fluctuations and makes loop gain independent of input voltage. as a result, dynamic behav- ior of the regulator is significantly improved over previous designs. on-chip pulse by pulse current limiting makes the lt1074 nearly bust-proof for output overloads or shorts. the input voltage range as a buck converter is 8v to 60v, but a self- boot feature allows input voltages as low as 5v in the inverting and boost configurations. the lt1074 is available in low cost to-220 or dd packages with frequency pre-set at 100khz and current limit at 6.5a (lt1076 = 2.6a). a 7-pin to-220 package is also available which allows current limit to be adjusted down to zero. in addition, full micropower shutdown can be programmed. see application note 44 for design details. a fixed 5v output, 2a version is also available. see lt1076-5. buck converter efficiency basic positive buck converter + + v sw v in v c gnd fb lt1074 5v 5a c1 500 f r1 2.8k 1% r2 2.21k 1% mbr745* 10v to 40v c3 ? 200 f r3 2.7k c2 0.01 f l1** 50 h (lt1074) 100 h (lt1076) use mbr340 for lt1076 coiltronics #50-2-52 (lt1074) #100-1-52 (lt1076) pulse engineering, inc. #pe-92114 (lt1074) #pe-92102 (lt1076) hurricane #hl-ak147qq (lt1074) #hl-ag210ll (lt1076) ripple current rating i out /2 * ** ? lt1074?a01 25v output load current (a) 0 efficiency (%) 60 70 80 90 1234 lt1074?tpc27 5 6 100 50 l = 50 h type 52 core diode = mbr735 v = 5v, v = 15v out in v = 12v, v = 20v out in lt1074
lt1074/lt1076 2 sn1074 1074fds absolute axi u rati gs w ww u package/order i for atio uu w (note 1) *assumes package is soldered to 0.5 in 2 of 1 oz. copper over internal ground plane or over back side plane. lt1076cq lt1076iq lt1076cr lt1076ir lt1076hvcr lt1076hvir lt1074ct7 lt1074hvct7 lt1074it7 lt1074hvit7 lt1076ct7 lt1076hvct7 lt1074ct lt1074hvct lt1074it lt1074hvit lt1076ct lt1076hvct lt1076it lt1076hvit lt1074ck lt1074hvck lt1074mk lt1074hvmk lt1076ck lt1076hvck lt1076mk lt1076hvmk input voltage lt1074/ lt1076 .................................................. 45v lt1074hv/lt1076hv ......................................... 64v switch voltage with respect to input voltage lt1074/ lt1076 .................................................. 64v lt1074hv/lt1076hv ......................................... 75v switch voltage with respect to ground pin (v sw negative ) lt1074/lt1076 (note 7) ..................................... 35v lt1074hv/lt1076hv (note 7) ........................... 45v feedback pin voltage ..................................... C2v, +10v shutdown pin voltage (not to exceed v in ) .............. 40v i lim pin voltage (forced) ............................................ 5.5v maximum operating ambient temperature range commercial ................................................. 0 c to 70 c industrial ................................................ C40 c to 85 c military (obsolete) ..................... C55 c to 125 c maximum operating junction temperature range commercial ............................................... 0 c to 125 c industrial .............................................. C40 c to 125 c military (obsolete) .................... C55 c to 150 c maximum storage temperature ............... C65 c to 150 c lead temperature (soldering, 10 sec) ...................... 300 c order part number order part number lt1076: jc = 4 c, ja = 30 c/w lt1076: jc = 4 c, ja = 30 c/w lt1074: jc = 2.5 c, ja = 50 c/w lt1076: jc = 4 c, ja = 50 c/w lt1074: jc = 2.5 c, ja = 35 c/w lt1076: jc = 4 c, ja = 35 c/w lt1074: jc = 2.5 c, ja = 50 c/w lt1076: jc = 4 c, ja = 50 c/w q package 5-lead plastic dd front view v in v sw gnd v c fb/sense 5 4 3 2 1 r package 7-lead plastic dd front view shdn v c fb/sense gnd i lim v sw v in 7 6 5 4 3 2 1 2 4 1 3 v c v in v sw case is gnd fb k package 4-lead to-3 metal can bottom view t package 5-lead plastic to-220 leads are formed standard for straight leads, order flow 06 v in v sw gnd v c fb front view 5 4 3 2 1 t7 package 7-lead plastic to-220 shdn v c fb gnd i lim v sw v in front view 7 6 5 4 3 2 1 tab is gnd tab is gnd tab is gnd tab is gnd consult ltc marketing for parts specified with wider operating temperature ranges. obsolete package consider the t5 package for alternate source
lt1074/lt1076 3 sn1074 1074fds the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. t j = 25 c, v in = 25v, unless otherwise noted. parameter conditions min typ max units switch on voltage (note 2) lt1074 i sw = 1a, t j 0 c 1.85 v i sw = 1a, t j < 0 c 2.1 v i sw = 5a, t j 0 c 2.3 v i sw = 5a, t j < 0 c 2.5 v lt1076 i sw = 0.5a 1.2 v i sw = 2a 1.7 v switch off leakage lt1074 v in 25v, v sw = 0 5 300 a v in = v max, v sw = 0 (note 8) 10 500 a lt1076 v in = 25v, v sw = 0 150 a v in = v max, v sw = 0 (note 8) 250 a supply current (note 3) v fb = 2.5v, v in 40v 8.5 11 ma 40v < v in < 60v 912 ma v shut = 0.1v (device shutdown) (note 9) 140 300 a minimum supply voltage normal mode 7.3 8 v startup mode (note 4) 3.5 4.8 v switch current limit (note 5) lt1074 i lim open 5.5 6.5 8.5 a r lim = 10k (note 6) 4.5 a r lim = 7k (note 6) 3 a lt1076 i lim open 2 2.6 3.2 a r lim = 10k (note 6) 1.8 a r lim = 7k (note 6) 1.2 a maximum duty cycle 85 90 % switching frequency 90 100 110 khz t j 125 c 85 120 khz t j > 125 c 85 125 khz v fb = 0v through 2k ? (note 5) 20 khz switching frequency line regulation 8v v in v max (note 8) 0.03 0.1 %/v error amplifier voltage gain (note 7) 1v v c 4v 2000 v/v error amplifier transconductance 3700 5000 8000 mho error amplifier source and sink current source (v fb = 2v) 100 140 225 a sink (v fb = 2.5v) 0.7 1 1.6 ma feedback pin bias current v fb = v ref 0.5 2 a reference voltage v c = 2v 2.155 2.21 2.265 v reference voltage tolerance v ref (nominal) = 2.21v 0.5 1.5 % all conditions of input voltage, output 1 2.5 % voltage, temperature and load current reference voltage line regulation 8v v in v max (note 8) 0.005 0.02 %/v v c voltage at 0% duty cycle 1.5 v over temperature C 4 mv/ c multiplier reference voltage 24 v shutdown pin current v sh = 5v 51020 a v sh v threshold ( ? 2.5v) 50 a shutdown thresholds switch duty cycle = 0 2.2 2.45 2.7 v fully shut down 0.1 0.3 0.6 v thermal resistance junction to case lt1074 2.5 c/w lt1076 4.0 c/w electrical characteristics
lt1074/lt1076 4 sn1074 1074fds note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: to calculate maximum switch on voltage at currents between low and high conditions, a linear interpolation may be used. note 3: a feedback pin voltage (v fb ) of 2.5v forces the v c pin to its low clamp level and the switch duty cycle to zero. this approximates the zero load condition where duty cycle approaches zero. note 4: total voltage from v in pin to ground pin must be 8v after start- up for proper regulation. note 5: switch frequency is internally scaled down when the feedback pin voltage is less than 1.3v to avoid extremely short switch on times. during testing, v fb is adjusted to give a minimum switch on time of 1 s. note 6: i lim (lt1074) , i lim (lt1076). note 7: switch to input voltage limitation must also be observed. note 8: v max = 40v for the lt1074/76 and 60v for the lt1074hv/76hv. note 9: does not include switch leakage. r lim C 1k 2k r lim C 1k 5.5k electrical characteristics 6v regulator and bias + C -power shutdown 10 a 320 a + C 2.35v 0.3v current limit shutdown 4.5v 10k s + C current limit comp c2 15 400 + C c1 r/s latch r r q x y z + C a1 error amp fb v 100khz oscillator analog multiplier xy z freq shift sync g1 pulse width comparator 24v (equivalent) 2.21v switch output (v ) 0.04 250 500 input supply shutdown* i * 6v to all circuitry lim ? ? sw ? ? v in c lt1074 ? bd01 100 switch output (v ) 0.1 ? sw ? 3v(p-p) lt1076 lt1074 *available on packages with pin counts greater than 5. block diagra w
lt1074/lt1076 5 sn1074 1074fds a switch cycle in the lt1074 is initiated by the oscillator setting the r/s latch. the pulse that sets the latch also locks out the switch via gate g1. the effective width of this pulse is approximately 700ns, which sets the maximum switch duty cycle to approximately 93% at 100khz switch- ing frequency. the switch is turned off by comparator c1, which resets the latch. c1 has a sawtooth waveform as one input and the output of an analog multiplier as the other input. the multiplier output is the product of an internal reference voltage, and the output of the error amplifier, a1, divided by the regulator input voltage. in standard buck regulators, this means that the output voltage of a1 required to keep a constant regulated output is indepen- dent of regulator input voltage. this greatly improves line transient response, and makes loop gain independent of input voltage. the error amplifier is a transconductance type with a g m at null of approximately 5000 mho. slew current going positive is 140 a, while negative slew current is about 1.1ma. this asymmetry helps prevent overshoot on start-up. overall loop frequency compensa- tion is accomplished with a series rc network from v c to ground. switch current is continuously monitored by c2, which resets the r/s latch to turn the switch off if an overcurrent condition occurs. the time required for detection and switch turn off is approximately 600ns. so minimum switch on time in current limit is 600ns. under dead shorted output conditions, switch duty cycle may have to be as low as 2% to maintain control of output current. this would require switch on time of 200ns at 100khz switch- ing frequency, so frequency is reduced at very low output voltages by feeding the fb signal into the oscillator and creating a linear frequency downshift when the fb signal drops below 1.3v. current trip level is set by the voltage on the i lim pin which is driven by an internal 320 a current source. when this pin is left open, it self-clamps at about 4.5v and sets current limit at 6.5a for the lt1074 and 2.6a for the lt1076. in the 7-pin package an external resistor can be connected from the i lim pin to ground to set a lower current limit. a capacitor in parallel with this resistor will soft-start the current limit. a slight offset in c2 guarantees that when the i lim pin is pulled to within 200mv of ground, c2 output will stay high and force switch duty cycle to zero. the shutdown pin is used to force switch duty cycle to zero by pulling the i lim pin low, or to completely shut down the regulator. threshold for the former is approximately 2.35v, and for complete shutdown, approximately 0.3v. total supply current in shutdown is about 150 a. a 10 a pull-up current forces the shutdown pin high when left open. a capacitor can be used to generate delayed start- up. a resistor divider will program undervoltage lockout if the divider voltage is set at 2.35v when the input is at the desired trip point. the switch used in the lt1074 is a darlington npn (single npn for lt1076) driven by a saturated pnp. special patented circuitry is used to drive the pnp on and off very quickly even from the saturation state. this particular switch arrangement has no isolation tubs connected to the switch output, which can therefore swing to 40v below ground. block diagra w descriptio u
lt1074/lt1076 6 sn1074 1074fds supply current shutdown pin characteristics v c pin characteristics v c pin characteristics feedback pin characteristics shutdown pin characteristics i lim pin characteristics typical perfor a ce characteristics uw 0 C150 current (ma) 0 50 100 150 200 1234 lt1074?tpc01 C200 C100 5 6 789 voltage (v) C50 v 2v fb slope 400k ? v adjusted for i = 0 at v = 2v fb cc 0 C1.5 current (ma) C0.5 0 0.5 1.0 1.5 2.0 1234 lt1074?tpc02 C2.0 C1.0 5 6 789 v 2.5v fb voltage (v) 0 C400 current ( a) C200 0 1 234 lt1074?tpc03 C500 C300 5 6 78 10 9 C100 100 200 300 400 500 voltage (v) start of frequency shifting voltage (v) 10 C40 current ( a) 20 40 lt1074?tpc04 30 50 60 70 80 C30 C20 C10 0 10 20 30 40 v in = 50v this point moves in with v details of this area shown in other graph 0 voltage (v) 0 C40 current ( a) 1.0 2.0 lt1074?pc05 1.5 2.5 3.0 3.5 4.0 C35 C30 C25 C20 C15 C10 C5 0 shutdown threshold current flows out of shutdown pin = 25 c t j 0.5 C2 C350 current ( a) C250 C150 0 12 lt1074?tpc06 C400 C300 3 4 5 C200 C100 C50 0 50 100 voltage (v) C1 6 7 8 t = 25 c j input voltage (v) 0 0 input current (ma) 4 6 10 20 30 60 lt1074?tpc11 10 40 50 2 8 12 14 16 18 20 v c = 1v device not switching
lt1074/lt1076 7 sn1074 1074fds reference shift with ripple voltage feedback pin frequency shift supply current (shutdown) reference voltage vs temperature switch ?n?voltage error amplifier phase and g m switching frequency vs temperature current limit vs temperature* typical perfor a ce characteristics uw input voltage (v) 0 0 input current ( a) 50 100 150 200 250 300 30 50 60 lt1074?tpc13 10 20 40 junction temperature ( c) C50 2.19 voltage (v) 2.20 2.21 2.22 2.23 2.24 2.25 C25 0 25 50 lt1074?tpc14 2.18 2.17 75 100 125 150 0 on voltage (v) 3.0 2.5 2.0 1.5 1.0 0.5 123 lt1074?tpc28 4 56 switch current (a) t = 25 c j lt1074 lt1076 square wave peak-to-peak ripple at fb pin (mv) 0 C80 change in reference voltage (mv) C70 C60 C50 C40 C30 20 20 40 60 80 lt1074?tpc16 100 120 140 160 180 200 C20 C10 0 10 tri wave frequency (hz) ( mho) 8k 1k 100k 1m 10m 0 10k transconductance 1k 2k 3k 4k 5k 6k 7k 200 150 100 50 0 C50 phase ( ) m g C100 C150 C200 lt1074?tpc17 junction temperature ( c) C50 80 frequency (khz) 95 100 105 110 115 120 C25 0 25 50 lt1074?tpc18 90 85 75 100 125 150 feedback pin voltage (v) 0 0 switching frequency (khz) 1.0 1.5 3.0 lt1074?tpc19 0.5 2.0 2.5 20 160 40 60 80 100 120 140 C55 c 25 c 150 c junction temperature ( c) 1 2 3 4 5 6 7 lt1074?tpc22 8 0 C50 C25 0 25 50 75 100 125 150 output current limit (a) *multiply currents by 0.4 for lt1076 i pin open lim r = 10k ? lim r = 5k ? lim
lt1074/lt1076 8 sn1074 1074fds figure 1. input capacitor ripple figure 2. proper ground pin connection v in pin the v in pin is both the supply voltage for internal control circuitry and one end of the high current switch. it is important, especially at low input voltages , that this pin be bypassed with a low esr, and low inductance capacitor to prevent transient steps or spikes from causing erratic operation. at full switch current of 5a, the switching transients at the regulator input can get very large as shown in figure 1. place the input capacitor very close to the regulator and connect it with wide traces to avoid extra inductance. use radial lead capacitors. feedback pin the feedback pin is the inverting input of an error amplifier which controls the regulator output by adjusting duty cycle. the noninverting input is internally connected to a trimmed 2.21v reference. input bias current is typically 0.5 a when the error amplifier is balanced (i out = 0). the error amplifier has asymmetrical g m for large input sig- nals to reduce startup overshoot. this makes the amplifier more sensitive to large ripple voltages at the feedback pin. 100mvp-p ripple at the feedback pin will create a 14mv offset in the amplifier, equivalent to a 0.7% output voltage shift. to avoid output errors, output ripple (p-p) should be less than 4% of dc output voltage at the point where the output divider is connected. see the error amplifier section for more details. frequency shifting at the feedback pin the error amplifier feedback pin (fb) is used to downshift the oscillator frequency when the regulator output voltage is low. this is done to guarantee that output short-circuit descriptio s u pi u l p = total inductance in input bypass connections and capacitor. spike height (di/dt ? l p ) is approximately 2v per inch of lead length for lt1074 and 0.8v per inch for lt1076. step for esr = 0.05 ? and i sw = 5a is 0.25v. ramp for c = 200 f, t on = 5 s, and i sw = 5a, is 0.12v. input current on the v in pin in shutdown mode is the sum of actual supply current ( 140 a, with a maximum of 300 a), and switch leakage current. consult factory for special testing if shutdown mode input current is critical. ground pin it might seem unusual to describe a ground pin, but in the case of regulators, the ground pin must be connected properly to ensure good load regulation. the internal reference voltage is referenced to the ground pin; so any error in ground pin voltage will be multiplied at the output; lt1074?pd01 () i sw () esr step = () i sw () t on ramp = () () l p dl dt c fb gnd lt1074 lt1074?pd02 r2 high current return path negative output node where load regulation will be measured ? ? v vv out gnd out = ()() 221 . to ensure good load regulation, the ground pin must be connected directly to the proper output node, so that no high currents flow in this path. the output divider resistor should also be connected to this low current connection line as shown in figure 2.
lt1074/lt1076 9 sn1074 1074fds figure 3. frequency shifting figure 4. shutdown pin characteristics current is well controlled even when switch duty cycle must be extremely low. theoretical switch on time for a buck converter in continuous mode is: t vv vf on out d in = + ? v d = catch diode forward voltage ( 0.5v) f = switching frequency at f = 100khz, t on must drop to 0.2 s when v in = 25v and the output is shorted (v out = 0v). in current limit, the lt1074 can reduce t on to a minimum value of 0.6 s, much too long to control current correctly for v out = 0. to correct this problem, switching frequency is lowered from 100khz to 20khz as the fb pin drops from 1.3v to 0.5v. this is accomplished by the circuitry shown in figure 3. q1 is off when the output is regulating (v fb = 2.21v). as the output is pulled down by an overload, v fb will eventu- ally reach 1.3v, turning on q1. as the output continues to drop, q1 current increases proportionately and lowers the frequency of the oscillator. frequency shifting starts when the output is 60% of normal value, and is down to its minimum value of ? 20khz when the output is ? 20% of normal value. the rate at which frequency is shifted is determined by both the internal 3k resistor r3 and the external divider resistors. for this reason, r2 should not be increased to more than 4k ? , if the lt1074 will be subjected to the simultaneous conditions of high input voltage and output short-circuit. descriptio s u pi u shutdown pin the shutdown pin is used for undervoltage lockout, micro- power shutdown, soft-start, delayed start, or as a general purpose on/off control of the regulator output. it controls switching action by pulling the i lim pin low, which forces the switch to a continuous off state. full micropower shutdown is initiated when the shutdown pin drops below 0.3v. the v/i characteristics of the shutdown pin are shown in figure 4. for voltages between 2.5v and v in , a current of 10 a flows out of the shutdown pin. this current in- creases to 25 a as the shutdown pin moves through the 2.35v threshold. the current increases further to 30 a at the 0.3v threshold, then drops to 15 a as the shutdown voltage fall below 0.3v. the 10 a current source is in- cluded to pull the shutdown pin to its high or default state when left open. it also provides a convenient pull-up for delayed start applications with a capacitor on the shut- down pin. when activated, the typical collector current of q1 in figure 5, is 2ma. a soft-start capacitor on the i lim pin will delay regulator shutdown in response to c1, by (5v)(c lim )/2ma. soft-start after full micropower shut- down is ensured by coupling c2 to q1. + C 2.21v +2v error amplifier v c fb r2 2.21k r1 v out external divider r3 3k q1 to oscillator lt1074?pd03 voltage (v) 0 C40 current ( a) 1.0 2.0 lt1074?pc05 1.5 2.5 3.0 3.5 4.0 C35 C30 C25 C20 C15 C10 C5 0 shutdown threshold current flows out of shutdown pin = 25 c t j 0.5
lt1074/lt1076 10 sn1074 1074fds figure 5. shutdown circuitry figure 6. undervoltage lockout figure 7. adding hysteresis descriptio s u pi u undervoltage lockout undervoltage lockout point is set by r1 and r2 in figure 6. to avoid errors due to the 10 a shutdown pin current, r2 is usually set at 5k, and r1 is found from: rr vv v tp sh sh 12 = ? () v tp = desired undervoltage lockout voltage v sh = threshold for lockout on the shutdown pin = 2.45v if quiescent supply current is critical, r2 may be increased up to 15k ? , but the denominator in the formula for r2 should replace v sh with v sh C (10 a)(r2). C + c1 10 a 300 a C + shutdown pin c2 v in i pin 6v q1 to total regulator shutdown 2.3v 0.3v lt1074?pd07 lim external c lim hysteresis in undervoltage lockout may be accomplished by connecting a resistor (r3) from the i lim pin to the shutdown pin as shown in figure 7. d1 prevents the shutdown divider from altering current limit. lt1074?pd08 r1 r2 5k v shut gnd in lt1074 lt1074?pd09 r1 r2 r3 v shut i lim in lt1074 optional current limit resistor d1* *1n4148 trip po v v r r tp int . == + ? ? ? ? ? ? 235 1 1 2 if r3 is added, the lower trip point (v in descending) will be the same. the upper trip point (v utp ) will be: vv r r r r v r r utp sh =+ ? ? ? ? ? ? ? ? ? ? ? ? ? 1 1 2 1 3 08 1 3 . if r1 and r2 are chosen, r3 is given by: r vvr vv r r sh utp sh 3 08 1 1 1 2 = ? ()() ? + ? ? ? ? ? ? . example: an undervoltage lockout is required such that the output will not start until v in = 20v, but will continue to operate until v in drops to 15v. let r2 = 2.32k. rk vv v k rk 1234 15 2 35 235 12 5 3 235 08 125 20 2 35 1 12 5 232 39 = () ? () = = ? ()() ? + ? ? ? ? ? ? = . . . . ... . . . .
lt1074/lt1076 11 sn1074 1074fds figure 8. i lim pin circuit figure 9. foldback current limit descriptio s u pi u i lim pin the i lim pin is used to reduce current limit below the preset value of 6.5a. the equivalent circuit for this pin is shown in figure 8. q1 r1 8k to limit circuit v in 320 a d1 d3 6v i lim 4.3v d2 lt1047?pd12 when i lim is left open, the voltage at q1 base clamps at 5v through d2. internal current limit is determined by the current through q1. if an external resistor is connected between i lim and ground, the voltage at q1 base can be reduced for lower current limit. the resistor will have a voltage across it equal to (320 a)(r), limited to 5v when clamped by d2. resistance required for a given current limit is: r lim = i lim (2k ? ) + 1k ? (lt1074) r lim = i lim (5.5k ? ) + 1k ? (lt1076) as an example, a 3a current limit would require 3a(2k) + 1k = 7k ? for the lt1074. the accuracy of these formulas is 25% for 2a i lim 5a (lt1074) and 7a i lim 1.8a (lt1076), so i lim should be set at least 25% above the peak switch current required. foldback current limiting can be easily implemented by adding a resistor from the output to the i lim pin as shown in figure 9. this allows full desired current limit (with or without r lim ) when the output is regulating, but reduces current limit under short-circuit conditions. a typical value for r fb is 5k ? , but this may be adjusted up or down to set the amount of foldback. d2 prevents the output voltage from forcing current back into the i lim pin. to calculate a value for r fb , first calculate r lim , the r fb : r ir rk i rink fb sc l lsc l = ? ()() ?? () ? ? () 044 05 1 .* .* *change 0.44 to 0.16, and 0.5 to 0.18 for lt1076. example: i lim = 4a, isc = 1.5a, r lim = (4)(2k) + 1k = 9k r k kk k fb = ? () ? () ? () ? ? () 15 044 9 059 1 15 38 .. .. . error amplifier the error amplifier in figure 10 is a single stage design with added inverters to allow the output to swing above and below the common mode input voltage. one side of the amplifier is tied to a trimmed internal reference voltage of 2.21v. the other input is brought out as the fb (feed- back) pin. this amplifier has a g m (voltage in to current out) transfer function of 5000 mho. voltage gain is determined by multiplying g m times the total equivalent output loading, consisting of the output resistance of q4 and q6 in parallel with the series rc external frequency compensation network. at dc, the external rc is ignored, and with a parallel output impedance for q4 and q6 of 400k ? , voltage gain is 2000. at frequencies above a few hertz, voltage gain is determined by the external compen- sation, r c and c c . v out i lim fb r lim r fb d2 1n4148 lt1074?pd13 lt1074
lt1074/lt1076 12 sn1074 1074fds figure 10. error amplifier descriptio s u pi u a g fc at mid frequencies a g r at high frequencies v m c vmc = ?? =? 2 phase shift from the fb pin to the v c pin is 90 at mid frequencies where the external c c is controlling gain, then drops back to 0 (actually 180 since fb is an inverting input) when the reactance of c c is small compared to r c . the low frequency pole where the reactance of c c is equal to the output impedance of q4 and q6 (r o ), is: f rc rk pole o o = ?? ? 1 2 400 although f pole varies as much as 3:1 due to r o variations, mid-frequency gain is dependent only on g m , which is specified much tighter on the data sheet. the higher frequency zero is determined solely by r c and c c . f rc zero cc = ?? 1 2 the error amplifier has asymmetrical peak output current. q3 and q4 current mirrors are unity-gain, but the q6 mirror has a gain of 1.8 at output null and a gain of 8 when the fb pin is high (q1 current = 0). this results in a maximum positive output current of 140 a and a maxi- mum negative (sink) output current of ? 1.1ma. the asym- metry is deliberateit results in much less regulator output overshoot during rapid start-up or following the release of an output overload. amplifier offset is kept low by area scaling q1 and q2 at 1.8:1. amplifier swing is limited by the internal 5.8v supply for positive outputs and by d1 and d2 when the output goes low. low clamp voltage is approximately one diode drop ( 0.7v C 2mv/ c). note that both the fb pin and the v c pin have other internal connections. refer to the frequency shifting and synchro- nizing discussions. 140 a q1 lt1074 ? pd11 q2 fb v 50 a 90 a q6 300 c q4 5.8v c c r c q3 50 a 2.21v external frequency compensation 90 a d1 90 a ? d2 x1.8 all currents shown are at null condition
lt1074/lt1076 13 sn1074 1074fds tapped-inductor buck converter positive-to-negative converter with 5v output typical applicatio s u + + v sw v in v c gnd fb lt1074hv r1 2.8k r2 2.21k d2 35v 5w c3 200 f 50v r3 1k c2 0.2 f l1* lt1074 ?ta02 d1** d3 1n5819 31 c1 4400 f (2 ea 2200 f, 16v) l2 5 h c4 390 f 16v 0.01 f + pulse engineering #pe 65282 * ** ? motorola mbr2030ctl if input voltage is below 20v, maximum output current will be reduced. see an44 v in 20v ? to 35v v out 5v, 10a ? r3* 2.74k v in v sw v gnd r4 1.82k* c2 1000 f 10v l1 25 h 5a ?? + + d1 ? mbr745 r1** 5.1k v c c3 0.1 f c4** 0.01 f + r2** 10k c1 220 f 50v * = 1% film resistors d1 = motorola-mbr745 c1 = nichicon-upl1c221mrh6 c2 = nichicon-upl1a102mrh6 l1 = coiltronics-ctx25-5-52 v 4.5v to 40v in C5v,1a*** + 200 f 10v C optional filter 5 h lower reverse voltage rating may be used for lower input voltages. lower current rating is allowed for lower output current. see an44. r1, r2, and c4 are used for loop frequency compensation with low input voltage, but r1 and r2 must be included in the calculation for output voltage divider values. lower current rating may be used for lower output current. see an44. r3 = C2.37 (k ? ) r1 = (r3) (1.86) r2 = (r3) (3.65) v out maximum output current of 1a is determined by minimum input voltage of 4.5v. higher minimum input voltage will allow much higher output currents. see an44. lt1074 ? ta03 lt1074 fb for higher output voltages, increase r1, r2, and r3 proportionately. for input voltage > 10v, r1, r2, and c4 can be eliminated, and compensation is done totally on the v pin. c ? ?? ** **
lt1074/lt1076 14 sn1074 1074fds q package 5-lead plastic dd pak (reference ltc dwg # 05-08-1461) k package 4-lead to-3 metal can (reference ltc dwg # 05-08-1311) package descriptio u k4(to-3) 1098 72 18 0.490 C 0.510 (12.45 C 12.95) r 0.470 tp p.c.d. 0.167 C 0.177 (4.24 C 4.49) r 0.151 C 0.161 (3.84 C 4.09) dia 2 plc 0.655 C 0.675 (16.64 C 19.05) 1.177 C 1.197 (29.90 C 30.40) 0.038 C 0.043 (0.965 C 1.09) 0.060 C 0.135 (1.524 C 3.429) 0.320 C 0.350 (8.13 C 8.89) 0.420 C 0.480 (10.67 C 12.19) 0.760 C 0.775 (19.30 C 19.69) q(dd5) 1098 0.028 C 0.038 (0.711 C 0.965) 0.143 +0.012 C0.020 () 3.632 +0.305 C0.508 0.067 (1.70) bsc 0.013 C 0.023 (0.330 C 0.584) 0.095 C 0.115 (2.413 C 2.921) 0.004 +0.008 C0.004 () 0.102 +0.203 C0.102 0.050 0.012 (1.270 0.305) 0.059 (1.499) typ 0.045 C 0.055 (1.143 C 1.397) 0.165 C 0.180 (4.191 C 4.572) 0.330 C 0.370 (8.382 C 9.398) 0.060 (1.524) typ 0.390 C 0.415 (9.906 C 10.541) 15 typ 0.300 (7.620) 0.075 (1.905) 0.183 (4.648) 0.060 (1.524) 0.060 (1.524) 0.256 (6.502) bottom view of dd pak hatched area is solder plated copper heat sink obsolete package
lt1074/lt1076 15 sn1074 1074fds t package 5-lead plastic to-220 (standard) (reference ltc dwg # 05-08-1421) r package 7-lead plastic dd pak (reference ltc dwg # 05-08-1462) package descriptio u r (dd7) 1098 0.026 C 0.036 (0.660 C 0.914) 0.143 +0.012 C0.020 () 3.632 +0.305 C0.508 0.050 (1.27) bsc 0.013 C 0.023 (0.330 C 0.584) 0.095 C 0.115 (2.413 C 2.921) 0.004 +0.008 C0.004 () 0.102 +0.203 C0.102 0.050 0.012 (1.270 0.305) 0.059 (1.499) typ 0.045 C 0.055 (1.143 C 1.397) 0.165 C 0.180 (4.191 C 4.572) 0.330 C 0.370 (8.382 C 9.398) 0.060 (1.524) typ 0.390 C 0.415 (9.906 C 10.541) 15 typ 0.300 (7.620) 0.075 (1.905) 0.183 (4.648) 0.060 (1.524) 0.060 (1.524) 0.256 (6.502) bottom view of dd pak hatched area is solder plated copper heat sink t5 (to-220) 0399 0.028 C 0.038 (0.711 C 0.965) 0.067 (1.70) 0.135 C 0.165 (3.429 C 4.191) 0.700 C 0.728 (17.78 C 18.491) 0.045 C 0.055 (1.143 C 1.397) 0.095 C 0.115 (2.413 C 2.921) 0.013 C 0.023 (0.330 C 0.584) 0.620 (15.75) typ 0.155 C 0.195* (3.937 C 4.953) 0.152 C 0.202 (3.861 C 5.131) 0.260 C 0.320 (6.60 C 8.13) 0.165 C 0.180 (4.191 C 4.572) 0.147 C 0.155 (3.734 C 3.937) dia 0.390 C 0.415 (9.906 C 10.541) 0.330 C 0.370 (8.382 C 9.398) 0.460 C 0.500 (11.684 C 12.700) 0.570 C 0.620 (14.478 C 15.748) 0.230 C 0.270 (5.842 C 6.858) bsc seating plane * measured at the seating plane 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
lt1074/lt1076 16 sn1074 1074fds t7 package 7-lead plastic to-220 (standard) (reference ltc dwg # 05-08-1422) linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 1994 lt/cpi 0202 1.5k rev d ?printed in usa related parts package descriptio u typical applicatio u part number description comments lt1375/lt1376 1.5a, 500khz step-down switching regulators v in up to 25v, i out up to 1.25a, so-8 lt1374/lt1374hv 4.5a, 500khz step-down switching regulators v in up to 25v (32v for hv), i out up to 4.25a, so-8/dd lt1370 6a, 500khz high efficiency switching regulator 6a/42v internal switch, 7-lead dd/to-220 lt1676 wide input range, high efficiency step-down regulator v in from 7.4v to 60v, i out up to 0.5a, so-8 lt1339 high power synchronous dc/dc controller v in up to 60v, i out up to 50a, current mode lt1765 3a, 1.25mhz, step-down regulator v in = 3v to 25v, v f =1.2v, tssop-16e, so8 package negative boost converter 0.050 (1.27) 0.026 C 0.036 (0.660 C 0.914) t7 (to-220) 0399 0.135 C 0.165 (3.429 C 4.191) 0.700 C 0.728 (17.780 C 18.491) 0.045 C 0.055 (1.143 C 1.397) 0.165 C 0.180 (4.191 C 4.572) 0.095 C 0.115 (2.413 C 2.921) 0.013 C 0.023 (0.330 C 0.584) 0.620 (15.75) typ 0.155 C 0.195* (3.937 C 4.953) 0.152 C 0.202 (3.860 C 5.130) 0.260 C 0.320 (6.604 C 8.128) 0.147 C 0.155 (3.734 C 3.937) dia 0.390 C 0.415 (9.906 C 10.541) 0.330 C 0.370 (8.382 C 9.398) 0.460 C 0.500 (11.684 C 12.700) 0.570 C 0.620 (14.478 C 15.748) 0.230 C 0.270 (5.842 C 6.858) bsc seating plane *measured at the seating plane r1 12.7k v in v fb v gnd r2 2.21k c1 1000 f 25v l1 25 h + c3 200 f 15v + d1* r3 750 ? v c 0.01 f 100pf c2 1nf v out C15v** v in C5v to C15v mbr735 i out (max) = 1a to 3a depending on input voltage. see an44 lt1074 ? ta04 lt1074 sw * ** 100 f optional output filter 5 h +

▲Up To Search▲

Price & Availability of LT1076CT06PBF

	To Download LT1076CT06PBF Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .