application note AN254/1088 for automotive electronics by s. ciscato low drop voltage regulators low drop linear voltage regulators are low voltage (5 to 12v) regulators which are able to provide ef- fective stabilization of the output voltage even when the difference between input voltage and output voltage is less than 2v. thissituation can arise accidentallyfor a brief period when the main supply source is overloaded. it may also result from a deliberate design decision aimed at reducing the power dissipated in the supply - for example, when the device is used as a post regula- tor in portable instruments. low drop regulators are used widely in automotive applications,a field where integratedcircuits have to be particularlyrugged. for thisreason most low drop devices include protection functions not found in standard regulators. before describing the sgs thomson family of low drop regulatorswe will the- refore begin with a brief description of the automo- tive electrical environment. automotive environment in addition to the battery voltage drop during star- ting, the automotivefield presents a number of other serious problems concerning the regulator input voltage : positive and negative high energy / high voltage transients (load dump and field decay), po- sitive and negative low energy/very-high-voltage spikes (switching spikes), battery reversal and bat- tery voltage doubling. all of these hazards must be withstood by the regu- lator without damage over an ambient temperature range very close to military standards ( 40 to +125 c for underhood devices ; 40 to + 85 cfor other devices). moreover, an output voltage preci- sion of 4% to 2% is required over the whole tem- perature range and in all conditions of input voltage and load current. battery voltage drop during motor starting the battery is overloaded by a peak current of up to 100a drawn by the starter mo- tor. in this condition, which persists for 20-30ms, the battery voltage drops to about 6v in very cold wea- ther (figure 1). figure 1 : cold starting supply voltage drop. using standard regulators with a dropout of 1.7v to 2.1v the minimum 4.75v supply necessary for es- sential functions such as ignition, injection and elec- tronicenginecontrolcannot be guaranteed.another unfortunateconsequenceis the loss of ram memo- ry contents in car radios and trip computers. a voltage regulator with a voltage drop of less than 1.2v is therefore necessary. battery voltage doubling to aid cold weather starting with a partially flat bat- tery, sometimes two batteries are used in series, doubling the voltage. regulators must therefore withstand input voltages of 24-26v without distur- bing operation. battery reversal voltage regulators must be protected internally against negativeinput voltages to guard against ac- cidental battery reversal. load dump transients load dump transients are high voltage, high energy positive transients. linear voltage regulators with an input-output voltage drop of less than 2v are used to ensure continuity of the stabilized output in applications where a battery supply is used. this note describes the characteristics and operation of these devices. 1/9
the response time of the output voltage of an alter- nator to load variations is very long because of the long time constant of the excitation winding and me- chanical inertia. when the load is reduced instantaneously (by tur- ning off lights, cooling fans and so on) the output voltage of the alternator tends to present a positive peak, the amplitude of which depends on the speed of rotation and the excitation current. during normal operation this does not cause pro- blems because of the high capacity of the battery which, connected in parallel with the alternator out- put, is able to absorb the transient energy without a significant increase in voltage. however, motor manufactures impose the standard that electronic devices must be protected against load dump transients because it is possible for the connectionbetween battery and alternator to break. the worst case voltage peak occurs when the bat- tery-alternatorcable is disconnectedwith the battery discharged and the motor running at its fastest ro- tation speed. in this case, the load variation is at a maximum and the voltage peak reaches a value comparable with the no-load output of the alternator running at maximum speed with the maximum ex- citation current. figure 2 shows a typical load dump waveform. motor manufacturers require that voltage regulators are able to protect themselves and the load against peak voltages of 60 - 100v with an equivalent series resistance of 0.1 to 1 w , depending on the type of alternator and external protection device used. figure 2 : load drump transient. field decay transients field decay transients are high energy, high voltage negative transients. if the ignition switch is turned off while current is flo- wing in inductive loads (electric motors, alternator field coil and so on) a negative voltage transient ap- pears on the supply rail. the peak value in modulo of this transient is of the same order of magnitude as a load dump transient. in this case, too, the re- gulator must protect itself and the load. switching spikes windscreen wiper motors, lamp flashers and igni- tion sparks behave as high frequency noise gener- ators with an equivalent series resistance of 50 to 500 w . the energy associated with these transients is much lower than load dump or field decay tran- sients but the negative and positive peakscan reach 200v. figure 3 shows the voltage waveform which the regulators must withstand. figure 3 : switching spikes. regulator design dropout the dropoutvoltage of a linear voltage regulator can be defined for a given output current, i o , as the mi- nimum difference between input and output voltage below which the output voltage is 100mv lower than the voltage measured at i o with the nominal input voltage. the current i o must be specified since the dropout voltage increases as the load current in- creases. to obtaina dropout voltage of 0.05 to 1v with an out- put current of 10 to 50ma, the regulator types l387a, l487, l47xx, l48xx, l4920, l4921, lm2930a and lm2931a are configured with a pnp series-pass transistor as shownin figure 4. the pnp transistor is connected in the common emitter con- figuration and can therefore operate in saturation, yielding the low dropout voltage desired. for higher dropout values an npn series-pass ele- ment in emitter follower configuration may be used. this approach, shown in figure 5, is used in the l2600 series regulators which have a maximum dropout voltage of 1.9v at 500ma. application note 2/9
figure 4 : pnp series pass transistor in com- mon emitter configuration for very low drop out voltage regulators. figure 5 : npn series pass transistor in emitter follower configuration. current consumption/quiescent cur- rent the circuit configurations shown in figures 4 and 5 behave differently as far as concerns the current consumed by the device but not delivered to the load. in the case of figure 5, this current is that ne- cessary for the functioning of the auxiliary circuitry of the regulator (voltage reference, op amp and so on). the base current of the output transistor flows into the load. in the figure 4 circuit, in contrast, the base current of the output transistor does not flow through the load and, particularly in saturation, depends heavily on the load current. normally lateral pnp transistors are chosen for ics becausethey can withstand high positive and nega- tive overvoltages. when negative overvoltages at the input do not occur, or are eliminated by external protection devices, vertical pnp transistors can be used in place of lateral types. since vertical pnp transistors have higher gain the current consumed in the regulator is significantly re- duced. vertical pnp transistors will be used in future designs. voltage reference the wide operating range of input voltage (6 to 26v) and ambient temperature( 40 to 125 c) over which high outputvoltage precision is required means that a well stabilized voltage reference must be used. all low drop regulators use bandgap type voltage references (see figure 6). in this structure the two transistors q 2 and q 1 have an emitter area ratio of 10 and carry equal collector currents imposed by the current mirror q 3 ,q 4 ,q 5 . in these conditions the base-emitter voltages of q 1 and q 2 differ (at 25 c) by : kt a(q 2 ) v be = 1n = 60 mv q a(q 1 ) a(q 2 ) where = 10 (emitter area ratio) a(q 1 ) kt =26mv q k = boltzmann's constant t = temperature in kelvin q = charge on an electron figure 6 : bandgap voltage reference circuit in low drop voltage regulators. application note 3/9
the rejection of v ref to variations in the supply volt- age is improved by supplying the reference circuit from a stabilized voltage. this is achieved in the l26xx, l48xx, l4920, l4921, lm2930a and lm2931a regulators by means of a preregulator. in the l487, analysing the figure 6 circuit gives : r1 v ref =v be (q 1 )+2 d v be (q 2 ,q 1 ) r2 to maintain v ref constant as temperature varies it dv ref is necessary that = 0 which implies choo s- ing dt r2 2r2 60 dv be (q 1 ) so that ? +=0 r1 r1 t(25 )dt where t(25 ) = 298 k dv be (q1) = negative temperature coefficient of dt the base-emitter voltage. in l387a and l47xxregulators, in contrast, the sup- ply to the bandgap is switched from the input to the output as soon as the nominal output voltage is rea- ched (figures 7,8, 9). the variation in output voltage with temperature is shown in figure 10. figure 7 : block diagram of sgs l2600 series regulators. figure 8 : block diagram of l387a and l487 se- ries regulators. figure 9 : block diagram of lm2930a, lm2931a and l4800 series regulators. figure 10 : outputs voltage vs. temperature. application note 4/9
protection against high energy tran- sients to protect the lm2930a, lm2931a, l4920, l4921 and l48xx regulators against high-voltage, high- energy positive transients the basic circuit shown in figure 11 is used. the zeners in this circuit limit the supply voltage to the maximum operatingvalue and turn off the output stage. the output transistor can thus withstand voltages up to the bv ces , break- down voltage. in the other regulators (l487, l387a, l47xx and l26xx) the supply to the internal circuits is also tur- ned off. the speed of intervention of these protection sche- mes is fast enough to ensure that the regulator can withstand high energy transients with a rising slope of 10v/ m s without problems, interrupting normal operation only momentarily. protection against negativetransients isprovided by the high series impedance of the possible current paths and the reverse bv beo breakdown voltage of the lateral pnp transistors (bv cbo ). the breakdown voltages bv ces and bv cbo depend on the technology therefore the transient capability is 60v, 80v or 100v for the various types. figure 11 : overvoltage protection circuit. protection against low energy overvoltages as shown in figure 3, the low energy overvoltages which the devices must resist have very brief rise time and can exceed the breakdown voltages. the protection schemes described above are therefore insufficient. however, since the energy associated with these transients is very low, the regulators can withstand them without problems. nevertheless it is advisable to place a capacitor of around 100nf at the input. all of the low drop regulators except the l26xx ty- pes need a compensation capacitor at the output. this capacitor also provides extra filtering for low energy transients because it has a low impedance at high frequencies. figure 12 : thermal protection circuit. thermal protection when the junction temperature exceeds the safe maximum for the device a thermal protection circuit (figure 12) holds the output transistor off until the overtemperature condition has passed. in the figure 12 circuit the resistors r1, r2 and r3 are calculated so that the base voltage of q 1 is 600mv, thus preventing the conduction of q 1 and q 2 . as the junction temperature increases the minimum v be for conduction of the two transistors fall until, at about 15 c, 2 v be = 600mv, the two transistorscon- duct and q 2 turns off the output transistor driver. current protection in the l487, l387aand l26xxregulators the output current is limited to its maximum value in the event of a short circuit. a special circuit acts on the base of the output transistor, preventing the output cur- rent from exceeding the limit set for the duration of the overload. application note 5/9
in the l4920, l4921, lm2930a, lm2931a and l48xx regulators a foldback circuit (figure 13) is used to limit the power dissipatedin both the devices and the load in short circuit conditions. the current is limited to a low value (i sc ) ofabout 200 ma as soon as it exceeds the maximum value. the output volt- age in this condition reaches a value corresponding to the current i sc flowing through the load. when the overload condition is removed the output voltage only returns to the nominal load value if the new static load line does not intersect the negative slope region of the curve in figure 13. if it does, the new operating point will be at the intersection. it is important to note that when power is applied, if the load line intersects the curve in the negative slo- pe region, the regulator will operate with a lower- than-nominal voltage. this can happen with a passive load greater than the normal load (even if it is less than the maximum load i m ) or with active lo- ads such as a current sinker which draw more than i sc even at low voltages (figure 13, curve 3). figure 13 : 1) acceptable load line for turn-on 2) unacceptable load line for turn- on. external compensation since the purpose of a voltage regulator is to supply a fixed output voltage in spite of supply and load va- riations, the open loop gain of the regulator must be very high at low frequencies. this may cause insta- bility as a result of the various poles present in the loop.to avoid this instability dominantpole compen- sation isused to reduce phase shifts due to otherpo- les at the unity gain frequency. the lower the frequency of these other poles, the greater must be the capacitor used to create the dominant pole for the same dc gain. where the output transistoris a lateralpnp type the- re is a pole in the regulation loop at a frequency too low to be compensated by a capacitor which can be integrated. for the l487, l47xx, l48xx, l387a, lm2930a and lm2931a external compensation is therefore necessary so a very high value capacitor must be connected from the output to ground. the parassitic equivalent series resistance of the capacitor used adds a zero to the regulation loop. this zero may compromise the stability of the sy- stem since its effect tends to cancel the effect of the pole added. in regulatorsthis esr must be less than 3 w and the minimum capacitor value is 47 f (100 m f for l4800 series). in the l2600, which uses an npn power transistor, the stabilization capacitor is small enough to be in- tegrated so no output capacitor is needed. indeed, if an output capacitoris used it may cause oscillation unless it is greater than 100 m f, in which case it would itself be the dominant pole. if an electrolytic capacitor of more than 100 m f is used, a small ca- pacitor must not be added in parallel or with the esr of the electrolytic it would from another pole, worse- ning the stability of the system. turn-on with capacitive loads a load which presents a significant capacity bet- ween the output and ground (including the external compensation capacitor) will be seen by the regula- tor as a short circuit when power is applied. the re- gulator therefore delivers the short circuit current until the load capacitor has been charged to the no- minal value. this factor is extremely important for the dimensio- ning of the power source. evena very small dc load can in such cases behave like a maximum load and the power drained from the supply is the sum of the short circuit current delivered to the load and the ma- ximum current consumed in the regulator. moreover, as explained above,in regulatorswith fol- dback protection the static load line must not cross the negative slope region of figure 13 or the output voltage will not reach the nominal value when power is applied. special f unctions reset the l387a and l487 include a power on/off reset function which inhibits the operation of circuits sup- plied by the regulator when the output voltage is too low (4.75v) to guarantee correct operation of logic (figure 14). to avoid malfunctions a delay is also in- troduced so that the enable signal is only issued some time after the safe output voltage has been reached. application note 6/9
figure 14 : reset timing waveforms. the reset circuitry (figure 15) consists of : - a comparator connected between the voltage reference and a tap of the output divider, the voltage of which is higher than the feedback voltage ; - an scr to memorize any brief glitches in the output voltage that can cause some trouble with the logic. - a delay circuit with an external capacitor char- ged by an internal current source this function has been integrated into the voltage regulator to exploit the basic advantageof taking in- formation at the source. the use of double calibra- tions can thus be avoided. for the correct operation of the reset function, two basic relations must be satisfied in all cases v res max 4.75 v (2) wherev res max /v res min are maximum/minimum value for the reset signal going high-low. (1) means that the reset signalmust be high when the device is regulating (2) means that the reset signal must be low when the output voltage goes under 95 % of the nomi- nal (5v). expressions (1) and (2) can be rewritten as : (v res max v res min )+(v nom v out min ) (3) figure 16 : the l4920 and l4921 are structurally identical to l48xx series regulators except that the volt- age divider in the feedback loop is available externally. variable output voltage the l4920 and l4921 are structurally identical to l48xx series regulators except that the voltage di- vider in the feedback loop is available externally (fig- ure 16). the output voltage can therefore be varied from 1.25v (the reference voltage) to 20v. it should be noted, however, that the minimum input voltage is 5.1v for operation with outputvoltages below 4.5v (otherwisethe internal circuits will notwork). for out- put voltages above 4.5v the input voltage must be at least equal to the output voltage plus the dropout voltage. the l4920 and l4921 are therefore low dropout regulators only for voltages above 4.5v. avalueof6k w is recommended for r2 to match the internal circuitry. application note 8/9
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifica- tions mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information pre- viously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - all rights reserved sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thaliand - united kingdom - u.s.a. application note 9/9
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