lm3460-1.2, -1.5 precision controller for gtlp and gtl bus termination general description the lm3460 is a monolithic integrated circuit designed for precision control of gtlplus and gtl bus termination. this controller is available in a tiny sot23-5 package, and in- cludes an internally compensated op amp, a bandgap refer- ence, an npn output transistor, and voltage setting resistors. a trimmed precision bandgap voltage reference utilizes tem- perature drift curvature correction for excellent voltage stabil- ity over the operating range. the precision output control en- ables the termination voltage to maintain tight regulation, despite fast switching requirements on the bus. the lm3460 controller is designed to be used with a high current ( > 7a) npn pass transistor to provide the high cur- rent needed for the bus termination. the wide bandwidth of the feedback loop provides excellent transient response, and greatly reduces the output capacitance required, thus reducing cost and board space requirements. features n precision output (1%) n output voltage can be adjusted n extremely fast transient response in gtlp and gtl bus termination n tiny sot23-5 package n output voltage capability for gtl or gtlp n low temperature coefficient applications n gtl bus termination (1.2v output 7a) n gtlp bus termination (1.5v output 7a) n adjustable high-current linear regulator connection diagram and package information ordering information voltage order number package marking supplied as 1.5 lm3460m5-1.5 d06a 1000 unit increments on tape and reel 1.5 lm3460m5x-1.5 d06a 3000 unit increments on tape and reel 1.2 lm3460m5-1.2 d09a 1000 unit increments on tape and reel 1.2 lm3460m5x-1.5 d09a 3000 unit increments on tape and reel marking code: the first letter 2 d 2 identifies the part as a driver, and the next two numbers define the voltage for the part. the fourth letter indicates the grade, with 2 a 2 designating the prime grade of product. availability: the sot23-5 package is only available in quantity of 1000 on tape and reel (designated by the letters 2 m5 2 in the part number), or in quantity of 3000 on tape and reel (indicated by the letters 2 m5x 2 in the part number). ds012603-1 * this resistor is not used on the lm3460-1.2. lm3460 functional diagram 5-lead outline package (m5) actual size ds012603-2 ds012603-3 * no internal connection, but should be soldered to pc board for best heat transfer. top view see ns package number mf05a july 2000 lm3460-1.2, -1.5 precision controller for gtlp and gtl bus termination ? 2000 national semiconductor corporation ds012603 www.national.com
typical applications ds012603-4 figure 1. 1.5v typical application (see application information section) ds012603-5 figure 2. 1.2v typical application (see application information section) lm3460 www.national.com 2
absolute maximum ratings (note 1) if military/aerospace specified devices are required, please contact the national semiconductor sales office/ distributors for availability and specifications. input voltage v in 20v output current 20 ma junction temperature 150c storage temperature ?65c to +150c lead temperature vapor phase (60 sec.) infared (15 sec.) +215c +220c power dissipation (t a = 25c) (note 2) 300 mw esd susceptibility (note 3) human body model 1500v see an-450 2 surface mounting methods and their effect on product reliability 2 for methods on soldering surface mount devices. operating ratings (note 1) , (note 2) ambient temperature range 0c t a +70c output current 1 ma electrical characteristics lm3460-1.5 specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating temperature range. unless otherwise specified, (+)in = v reg ,v out = 200 mv symbol parameter conditions typ (note 4) limit (note 5) units v reg regulated voltage i out = 1 ma 1.5 1.515/ 1.530 1.485/ 1.470 v (max) v (min) regulated voltage tolerance i out =1ma 1/ 2 % (max) i q quiescent current i out = 1 ma 85 125/ 150 a (max) g m transconductance d i out / d v reg 20a i out 1ma v out = 500 mv 3.3 1/ 0.5 ma/mv (min) v sat output saturation voltage(note 6) v in =v reg + 100 mv i out =1ma 0.8 0.95 v (max) i l output leakage current v in =v reg ? 100 mv v out =0v 0.1 0.5/ 1.0 a (max) r f internal feedback resistor (see functional diagram) 7.1 8.9 5.3 k w (max) k w (min) e n output noise voltage i out = 1 ma, 10 hz f 10khz 50 v (rms) electrical characteristics lm3460-1.2 specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating temperature range. unless otherwise specified, (+)in = v reg ,v out = 200 mv symbol parameter conditions typ (note 4) limit (note 5) units v reg regulated voltage i out = 1 ma 1.220 1.232/ 1.244 1.208/ 1.196 v (max) v (min) regulated voltage tolerance i out =1ma 1/ 2 % (max) i q quiescent current i out = 1 ma 85 125/ 150 a (max) g m transconductance d i out / d v reg 20a i out 1ma v out = 200 mv 3.3 1/ 0.5 ma/mv (min) v sat output saturation voltage(note 6) v in =v reg + 100 mv i out =1ma 0.8 0.95 v (max) i l output leakage current v in =v reg ? 100 mv v out =0v 0.1 0.5/ 1.0 a (max) r f internal feedback resistor (see functional diagram) 10 12.5 7.5 k w (max) k w (min) e n output noise voltage i out = 1 ma, 10 hz f 10khz 50 v (rms) lm3460 www.national.com 3
electrical characteristics lm3460-1.2 (continued) note 1: absolute maximum ratings are limits beyond which damage to the device may occur. operating ratings indicate conditions for which the device is intend ed to be functional, but do not guarantee specific performance limits. for guaranteed specifications and test conditions, see the electrical characte ristics. the guaran- teed specifications apply only for the test conditions listed. some performance characteristics may degrade when the device is not operated under th e listed test con- ditions. note 2: the maximum power dissipation must be derated at elevated temperatures and is dictated by t jmax (maximum junction temperature), q ja (junction ot am- bient thermal resistance), and t a (ambient temperature). the maximum allowable power dissipation at any temperature is (p dmax =(t jmax ?t a )/ q ja ) or the number given in the absolute maximum ratings, whichever is lower. the typical thermal resistance q ja when soldered to a printed circuit board is approximately 330 c/w. note 3: the human body model is a 100 pf capacitor discharged through a 1.5 k w resistor into each pin. note 4: typical numbers are at 25c and represent the most likely parametric norm. note 5: limits are 100% production tested at 25c. limits over the operating temperature range are guaranteed through correlation using statistical qualit y control (sqc) methods. the limits are used to calculate national's average outgoing quality level (aoql). note 6: v sat =v reg ?v out , when the voltage at the in pin is forced 100mv above the nominal regulating voltage (v reg ). lm3460 www.national.com 4
product description the lm3460 is a shunt regulator designed for use as a pre- cision control element in a feedback loop. the regulated out- put voltage is sensed between the in pin and ground pin of the lm3460. the output of the lm3460 sources current whenever the voltage at the in pin reaches the regulated voltage. this current is used to cut off the drive to the external pass trnasistor, which provides the negative feedback to force the output voltage to be the same value as v reg . if the voltage on the in pin is forced above the v reg voltage, the maximum voltage applied to the in pin should not ex- ceed 20v. in addition, an external resistor may be required on the out pin to limit the maximum current to 20 ma. compensation the inverting input of the error amplifier is brought out to sim- plify closed-loop compensation. typically, compensation is provided by a single capacitor connected from the com- pensation pin to the out pin of the lm3460. applying a load pulse to the output of the regulator circuit and observing the output voltage response is a good method of verifying the stability of the control loop. if excessive ringing on the output waveform is observed, this usually indicates marginal stability resulting from insufficient phase margin. test circuit the test circuit shown in figure 3 can be used to measure various lm3460 parameters. test conditions are set by forc- ing the appropriate voltage at the v out set test point and se- lecting the appropriate r l or i out as specified in the electri- cal characteristics section. use a dvm at the 2 measure 2 test points to read the data. setting the output voltage if a regulated voltage is desired which is not available as a standard voltage, the output voltage may be adjusted by us- ing an external resistive divider (see figure 4 ): ds012603-9 v out set note: 0v to 500 mv for lm3460-1.5 0v to 200 mv for lm3460-1.2 figure 3. test circuit lm3460 www.national.com 5
setting the output voltage (continued) the simplest way to calculate the resistor values is to as- sume a value for r a and then solve the equation shown for r b . to assure best output voltage accuracy, the value selected for r a should be < 500 w , and 1% tolerance resistors should be used. as the ohmic value of r a is increased, the internal resistive divider inside the lm3460 will cause the output voltage to deviate from the value predicted by the formula shown. app circuit technical information figure 1 and figure 2 highlight two applications of the lm3460. this section provides details of circuit function. 1.5v/7a typical application figure 1 shows the schematic of a wide-bandwidth linear regulator which provides a regulated 1.5v output at up to 7a of load current from a 3v-3.6v input. the pass element of the regulator (which supplies the load current) is made up of a three-transistor complimentary dar- lington composed of q2, q3, and q4. the bias current flow- ing through r1 will drive the pass element on, until such time as q1 pulls down and takes the drive away from the base of q2. the circuit regulates the output to 1.5v using the lm3460 precision controller, which sources current from its output whenever the voltage at the in pin reaches 1.5v. when the lm3460 sources current from its output, it turns on q1 (stealing the base drive for q2) which reduces the cur- rent from the 1.5v regulated output. in this way, a negative feedback loop is established which locks the output at 1.5v. c1 and c2 are used for compensation, and should be ce- ramic capacitors. c4 is required for regulator stability, and both c3 and c4 af- fect transient response. circuit performance should be care- fully evaluated if substitutions are made for these two com- ponents. performance data all data taken at 20c ambient: load/line regulation: the output voltage changed < 0.1 mv as the load was increased from 0-7a, and the input voltage was varied from 3.0v-3.6v. dropout voltage: the dropout voltage (which is de- fined as the minimum input-output voltage differential re- quired to maintain a regulated output) was measured at 7a and found to be 1.4v. this means that a minimum input volt- age of 2.9v is required to keep the 1.5v output in regulation. transient response: transient response was tested using a 0.2 w power resistor connected to the output using a mechanical contact to provide a 0-7a load current step. when the load was applied, the change in output voltage was seen to be < 5 mv with a total recovery time of about 30 s (see figure 5 ). heatsinking/component selection heatsinking: as with any linear regulator, the power dissi- pated in the pass transistor (q4) is approximately: p=(v in ?v out )xi load q4 must be provided with adequate heatsinking so that the junction temperature never exceeds 150c. figure 6 shows the maximum allowable values of thermal resistance (from heatsink-to-ambient) that must be provided for various values of the load current. these values are calculated assuming a maximum ambient of 50c, 3.3v input, and a to-220 power transistor mounted using thermal grease and a mica insulator. ds012603-10 for best results: select r a < 500 w figure 4. setting the output voltage ds012603-11 figure 5. output transient response ds012603-12 figure 6. q4 heatsink requirements for circuit shown in figure 1 lm3460 www.national.com 6
app circuit technical information (continued) a given thermal resistance can be obtained by using differ- ent combinations of heatsink and airflow (refer to heatsink manufacturers datasheets). the design tradeoff here is that heatsinks which are smaller, lighter, and cheaper require more airflow to get the desired value of thermal resistance. transient response: if the regulator is to respond quickly to changes in load current demand, the input and output capacitors must be selected carefully. the output capacitor c4 is most critical, as it must supply current to the load in the time it takes the regulator loop to sense the output voltage change and turn on the pass tran- sistor. a sanyo oscon type (or equivalent) will give the best performance here. the input capacitor c3 is also important, as it provides an energy reservoir from which the regulator sources current to force the output back up to the nominal value. a good, low esr electrolytic such as a panasonic hfq type is a good choice for c3. layout tips: in order to optimize performance, parasitic inductance due to connecting traces must be minimized. all paths shown as heavy lines on the schematic must be made by traces which are wide and short as possible (component placement should be optimized for minimum lead length). power transistor and driver: the power transistor used at q4 must have very good current gain at 7a, and wide bandwidth (high f t ) for this circuit to work as specified. the d44h8 is an excellent choice for cost and performance. the current gain of q4 dictates the power dissipation in its driver (q3) which must supply the base current to q4. if the gain of q4 is lowered, q3 must source more current into its base (and the power dissipation in q3 goes up proportion- ately). the d44h8 has a guaranteed minimum gain of 40 @ 4a, with typical gain much higher. assuming the gain of q4 is about 30% lower at 7a, it will still be > 28. therefore, to support 7a of load current, q3 must supply 250 ma to the base of q4 (worst case). the power dissipation in q3 (assuming 3.3v input) will never exceed approximately 250 mw, which is easily handled by 2n3906 in a to-92 case (which has a thermal resistance of about 180c/w), but could be a problem for a very small sur- face mount device. if substitutions are made for q3 or q4, careful attention must be paid to the current gain as well as the f t . transistor bandwidth: fast transient response that the regulator be able to respond quickly to any change in output voltage (which will occur if the current drawn by the load suddenly changes). all of the transistors specified in the schematic are very wide-band devices (have high f t values) which is necessary for fast response. if substitutions are made for any of the transistors, this specification must be considered. 1.2v/7a typical application the 1.2v @ 7a design in figure 2 is very similar in function to the design shown in figure 1 . most of the circuit descrip- tions previously detailed for that circuit apply unchanged to figure 2 , will not be repeated. detailed information will be presented in the areas which dif- fer between the two circuits. heatsinking the 1.2v design needs a little more heatsinking because the lower output voltage means more power dissipation in q4 at any value of load current. figure 7 shows the maximum allowable values of thermal resistance (from heatsink-to-ambient) that must be provided for various values of the load current. q1 drive circuitry in the circuit shown in figure 1 , the output of u1 drives the base of q1 with current when the voltage at v out reaches the regulation point. as q1 turns on, it steals drive from q2 which holds the loop in regulation. the circuit of figure 2 uses a different drive configuration for q1, required because of the lower voltage across u1. with only 1.2v across u1, the out pin of the lm3460 can- not swing up high enough in voltage to turn on the v be of q1. in the circuit of figure 2 , drive for q1 is provided by r7, but only when u1 sources current: the operation of the drive scheme is as follows: if the voltage at v out is below 1.2v, no current flows from the out pin of u1. q1 is held off as the current flowing down through r7 goes through d1 and r5 to ground. important: diode d1 is a 1n4001 because its v f must be much less than the v be of q1 (a signal diode like 1n4148 will not work here). when u1 is not sourcing current, the voltage at the out pin (and the cathode of d1) will be held at about 50 mv by the r7/d1/r5 divider. the current flowing to ground through these components is about 110 a. because d1 is a 1a power diode, the v f across d1 at this small value of current will be much less than the v be needed to turn on q1 (so q1 is held off by d1). when u1 begins to source current (to cut off the pass tran- sistor and regulate v out ) it forces the voltage at the cathode of d1 to rise. this action causes the current that was flowing through d1 to flow into the base of q1, turning it on and taking drive away from the base of q2. this action provides the negative feedback required to regu- late v out and allows the lm3460 to operate with only 1.2v of total supply voltage across the device. ds012603-13 figure 7. q4 heatsink requirements for circuit shown in figure 2 lm3460 www.national.com 7
physical dimensions inches (millimeters) unless otherwise noted life support policy national's products are not authorized for use as critical components in life support devices or systems without the express written approval of the president and general counsel of national semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. national semiconductor corporation americas tel: 1-800-272-9959 fax: 1-800-737-7018 email: support@nsc.com national semiconductor europe fax: +49 (0) 180-530 85 86 email: europe.support@nsc.com deutsch tel: +49 (0) 69 9508 6208 english tel: +44 (0) 870 24 0 2171 fran?ais tel: +33 (0) 1 41 91 8790 national semiconductor asia pacific customer response group tel: 65-2544466 fax: 65-2504466 email: ap.support@nsc.com national semiconductor japan ltd. tel: 81-3-5639-7560 fax: 81-3-5639-7507 www.national.com 5-lead small outline package (m5) order number, see ordering information table ns package number mf05a lm3460-1.2, -1.5 precision controller for gtlp and gtl bus termination national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the righ t at any time without notice to change said circuitry and specifications.
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