Voltage Converters

8/9/2019 Voltage Converters

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RAY MAWSTON

TH IS ARTICLE EXPLAINS TH E DE-

sign of a many different low-power electronic voltage con-verters. These circuits generatehigher-value supply voltagesfrom low-voltage DC sourcesand negative DC voltages frompositive DC voltage sources.

earn how to build a v riety o voltageintegrated circuits and put them

he circuits in t h g article in-clude the popuiar 5 55 industry- to 200 milliamperes when set Figures 2, 3 4 an d 5 are volt-sta nd ard timer IC an d the Har- u p a s a free- runn ing squ are - age-converter circuits based onr i s S e m i c o n d u c t o r ICL7660 wave genera tor. the 5 55 . The 55 5 is configuredvoltage converter I C devicesthat simplify the design andcons tru ct ion of a variety of volt-age converter circuits.

Low voPt@e generation.The easiest way to increase

th e voltage level of a DC source

voltage or reverse its polarity isto apply that DC voltage to afree-running , squarewave gen-erato r whose ou tput is fed to amultisection capacitor-diodevoltage-multiplier network. Fig-ure 1 show s block diagrams forposit ive an d negative outp utvoltage converters.

If a positive outpu t voltage isdes i red , the mul t ip l ie r mustp rov ide a non inve r t i ng r e -sponse, as shown in Fig. l-a.

But if a negative output is re-quir ed, th; multiplier m us tf u n c t i o n a s a n i nv er te r, a ssho wn in Fig. l-b.

Voltage converters are basedon a variety of multivibrator cir-cuits . These converters can in-c l u de f r e e - r u n n i n g , s q u a r e -w a v e- g e n e r a t i n g m u l t i -vibrators based on bipolar orfield-effect FET) trans istor s orCMOS or TTL integrated cir-cuits. In general, the square-wave generators should oscil-

FIG. 1 VOLTAGE CON VERT ER BLOCK DIAG RAM S: positive output a) and negative

output b).

late a? frequencies between 1 FIG. 2 DC VOLTAGE DOUBLER circuit based on the 555 timer IC.kHz and 10 ~ zo that the mul-tiplier section can operate effi-ciently. Th is frequency rangepermits the use of multiplyingcapacitor s with low values.

An easy way to build a voltageconverter is to design it aro unda n industry-standard 555 timerIC. Th is versatile device a n dmany practical circuits that in-clude it were discussed in theNovember and December issuesof Electronics Now, starting o npages 61 and 62, respectively.)The 555 can source or sink up FIG. 3 DC VOLTAGE TRIPLER circuit based on the timer


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onversion circuits from popui rwork in your own circuit designs.

a s a free-running astable multi-vibrator (squarewave gene rator)operating at about 3 kHz in all ofthese circuits. The 3-kHz fre-quency is determined by the val-ues of resistors R1 and R2 inseries with capacitor C2. Sup-ply-line ca pacit or C1 preventsthe 3-kHz output of the 555from feeding back into the de-v ic e , a n d c a p a c i t o r C 3 e n -hances circuit stability.

The circuit in Fig. 2 is a DCvoltage-doubler th at generates aDC output voltage with a valuetha t is approximately twice thatof the supply voltage. The 555 soutput is fed to a voltage-dou-bler network made up of capaci-tors C4 and C5 and diodes Dland D2. The network producesthe approximate two times sup-ply voltage output when thatoutput is unloaded.

FIG. 4 DC VOLTAGE QUADRUPLER circuit based on the 555 timer.

FIG. S D C NEGATIVE VOLTAGE generator based on the 555 timer.

The ~ r e c is e u t ~ u t alue is:2 G v eak vf:l + vfd2

Where: is the peak out-put voltage of the squarewavegenerato;vfd, and v are theforward voltage drops (about600 millivolts) of multiplier di-odes Dl a nd D2.

The out pu t voltage of the cir-cuit decreases when the outp utis loaded.

The Fig. 2 voltage doubler cir-cuit will work with any DC sup -ply of 5 to 15 volts. Because of itsvoltage-doubling respo nse, i tcan provide voltage outputsover a range of about 10 to 30volts. Higher voltage ca n be ob-tained by adding more multi-plier stages to the circuit.

The Fig. 3 DC voltage triplercircu it ca n provide ou tput volt-ages from 15 to 45 volts. TheFF~ . 4 DC voltage quadruplercircu it ca n provide ou tput volt-ages from 2 0 to 6 0 volts.

Figure 5 i s the schematic for aDC negative-voltage generatorcapable of producing an outp utvoltage that is approximatelyequal in amplitude bu t oppositein polarity to th at of t he powersupplyIt also operates at 3 Hz,and it drives an output stageconsi sting of capacitors C4 an d

C5 and diodes Dl a nd D2. Thiscircuit s split-supply output isuseful for powering ICs tha t re-quire bo th positive an d negativep o w e r f r o m a s i n g l e - e n d e dsource.

High vol e generation.The generation of higher out -

pu t voltages with voltage multi-pliers is usually cost-effectiveonly when the values of the re-quired voltages a re less th an six

times the supply voltage. Wherevery large step-up ratios are re-quired (e.g., hundreds of voltsgenerated from a 12-volt sup-ply), other circuit designs arerecommended.

It is usually cost-effective toapply the ou tp ut of the low-volt-age oscillator or squarewavegenerator a s the inpu t to a suit-ab l e s t ep -u p vo lt age t r a ns -former. The r equir ed hig h ACout put voltages appear acros sth e t r ans former s s econda rywinding.

Th is AC voltage can easilv beFIG. ~PIINE VOhT T~ 3QO \IOLT C to DC converter. converted backvto DC wiih a


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Figure 6 s a DC-to-DC con-verter circuit tha t c an generatea 300-volt DC output from a 9-volt DC power source. In thiscircuit, transistor Q1 and i ts as-sociated circuitry form a Hartleyinductive-capacitive (LC) os-cillator. The voltage on the p ri-m a r y w in d i n g t h a t s w i n g sbetween zero and volts is ap-plied to 250-volt transformerT1. The inductance of the pri-mary is the inductive element inthe LC oscillator, which is tu ne d

FIG. 7-HIGH-VOLTAGE GENERATOR that can also be a neon lamp driver. by capacitor C2.

FIG. 8-DC-TO-AC INVERTER based on the 555 IC.

FIG. FUNCTIONAL DIAGRAM for the iCL7660 CMO S voltage converter.

FIG. 10-PIN CONF lGURA TlON for anICL7660 in an eight-pin miniDlP.

simple rectifier-filter network, ifrequired. Figures 6, 7 and 8show schematics for low-power,

he supply voltage s steppedup to about 350 volts peak atthe secondary of TI. This out-put is half-wave rectified by di-ode Dl, a nd i t charges capacitorC3. Without a permanent loadon C3, th e capacitor ca n delivera powerful but non-lethal elec-trical shock. With a permanentload, the outpu t drops to about300 volts at a load cur re nt of afew milliamperes.

The Fig. 7 circuit can eitherdrive a neon lamp or generate alow-current DC voltage of u p toseveral hun dre d volts from a 5 -to 15-volt DC supply. The 555 isconfigured as a 3-kHz astablemultivibrator whose square-wave out put i s fed to th e inp utof transformer T1 through the

resistor R3.In this circuit, T1 is a smallaudio transformer with a t ur nsratio necessary to give the desir-ed output voltage. In this cir-cuit, a 10-volt supply and atransformer with a 1:20 turnsratio will give an unloaded DCoutput of 200 volts peak. ThisAC voltage can easilybe convert-ed to DC with a half-wave rec-

high-voltage gener ation cir- t if ier and fi l ter capacitor, a scui ts based on of this principle. show41 in Fig. 7


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FIG. 12 NEGATIVE VOLTAGE CONVERTER for a 3.5 to 6.5-volt supply.

FIG 6 U E T H O D FOR CASCADING voltage converter ICs for increased voltage.

Th e DC-to-AC inver ter c ircuit

in Fig. 8 will produce a n AC out-put at line frequency and volt-age. The 555 is configured a s a nas t ab l e mu l t i v ib r a to r whoseoutput f requency can be ad-justed over a range of 5 to 60Hz b y trimmer potentiometerR5.

The out pu t of the 555 is fed tothe paired NPN and RNP bipolart r a n s i s t o r s Q a n d Q 2 n dtheir output s fed in to t he low-voltage s ide of reverse-c on-

nected filament transformer TI .t ha s t he necessa ry s t ep -up

turns ratio. Capacitor C 4 andcoil El filter the input to ensure

M O S voltage e a n v e ~ e rT h e H a r r i s I C L 7 6 6 0 i s a

monolithic CMOS voltage con-verter that converts its supplyvoltage from positive to negativefor a n in pu t rang e of 1.5 voltsto 10 volts. Thi s results incomplementary ou tp ut voltagesof 1.5 volts to 10 volts. Forexample, if powered from a 5

volt sou rce, the device will gen-erate a 5-volt ou tpu t.The ICL7660 has a typical

open-circuit voltage conversionefficiencv of 99 .9 an d a m ic a 1power efficiency of 98 . - ent h e o u t p u t i s l oa d e d, t h eICL7660 ac ts l ike a voltagesource with an output imped-ance of about 70 ohms. It cansupply maximum currents ofabout 50 0 microamperes.

Only two external capacitors

a r e n e e de d t o p e r f or m t h echarge pum p a n d charge reser-voir fu ncti ons . The ICL7660ca n also be configured a s a volt-age doubler andit will generateou tp ut voltages u p to 18.6volts with a 4 10-volt inpu t.

FIG. 1SCAPA CBTOR SU BSTITUTION for reducing th voltage converter s oscillatorfrequency.

that the transformer inpu t is es- Figure 9 i s the func t iona lsentially a sinewave. block diagram of the XCL7660.


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It contains a n RC oscillator, di-vide-by-2 ci rc ui t, voltage-leveltran slato r, voltage regulator,four ou tpu t power MOS switch-es, and an unusual logic net-work. The logic network sensesthe most negative voltage in th edevice and ensures tha t the out-put N-channel switch source-subst rate junctions ar e not for-

ward biased. This is intended toIM HZ I ~ Z to* ens ure latchup-free operation.

OSCILLATOR FREQUENCY The unloaded oscillator oscil-FIG. 16-FREQUENCY OF OSCILLA- lates at a nominalTION as a function of externa l oscillator 1 kHz when t he in put supplycapacitor value V volts . voltage is 5.0 volts. This fre-

FIG. 17-EXTERNAL CLOCKING METHOD for reducing he internal oscillator frequen-cy of the ICL7660.

quency can be lowered by theaddition of an external capaci-tor to the osc terminal, or theoscillator can be overdriven bya n external clock.

The LV terminal can be tied toGROUND to bypass the internalseries regulator an d inprove lowvoltage LV) operation. At medi-um to high voltages + 3 . 5 to

10.0 volts), the LV pin is leftf l o a t i n g t o p r e v e n t d e v i c elatchup.

The ICL7660 is available ineight-pin miniDIP a nd small-outline IC SOIC) packages aswell a s TO-99 cases. Figure 1 isthe drawing of pin configura-tions for the eight-pin miniDIPpackage, th e least expensiveversion. Power dissipation ofthis package is 300 milliwatts.The ICL7660S, a n enhanced di-r e c t r e p l ac e m e n t f o r t h eICL7660, is now available andcan be sub stitu ted if desired.

The operation of the ICL7660is simila r to that of the oscillatoran d voltage-multiplier shown inFig. 2, bu t it performs that f unc-tion more efficiently. As statedearlier, th e use of conventionalsilicon multiplier diodes causesthe circuit s unloaded o utp utvoltage to drop by about 1.2

volts-the su m of th e forwardvoltage drops of the two diodes .The ICL7660 eliminates the

voltage drop by replacing th e di-odes with MOS power switches.These switches are driven bythe logic network so that eachswi tch automat ica l ly c loseswhen it is forward biased andope ns when it is reverse biased.This feature accounts for itshi gh operating efficiency

DO S and don tsThe manufac t u re r r ecom-mends some Do s and D o n t s toassure reliable device operation:1. DO not exceed t he device smaximum supply voltage.2. Do not COnnect any pins tovoltages greater than + V or lesstha n ground.3 . f the IC is operating in thepower SUPF~Y ange of 1.5 to 3.5volts, ground the ~=q sx in 6 ; atsupply values greater than 3.5volts, leave pin 6 open circuited.4. Do not short circuit th e out-put the V supply for volt-

Continued on page 62


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age s above 5. 5 volts for ex-tended periods.5. When installing polarized ca-pacitors, connect the positivelead of the capacitor to the CAP

pin 2 and the negative lead tothe CAP- pin 4 of the ICL7660.Connec t the positive terminal ofcapacitors to V pin toground.6. Be sure that pin 5 does notswing more positive than GNDpin 3 to prevent latchup7. f th e supply voltage is greaterthan 6.5 volts, put a protectivediode in series with pin 5.

Practical IICL7 8 Circuits.The most popular application

for th e ICL7660 is a s a negativevoltage converter. Figures 11,12, and 13 are three basic nega-tive converter circuits. In eachcircuit, capacitors C1 and C2are multiplier capacitors with avalue of 10 microfarads.

The negative voltage convert-er c ircuit in Fig. 11 is designedto be powered from a 1.5- to 3 . 5volt supply, and it requires onlytwo externa l electrolytic capaci-tors, C1 and C2. The circuit inFig. 12 is similar, but it is de-signed to be powered in t he 3.5-to 6.5-volt range an d i ts LV pin 6grounded.

The negative voltage convert-er c ircuit in Fig. 13 is designedfor a 6.5- to 10-volt supply. Di-ode Dl is in series with SVpin 5 to protect the IC from ex-cessive reverse biasing by C2when the power supply is re-

moved. Diode Dl , which shouldbe a Schottky diode, reduces theavailable o ut put voltage by V,

Up to ten cascading ICL7660swill give increased output volt-age. For example, if three ICs arecascaded, the i r f ina l outpu tvoltage will be 3 volts. Figure14 shows how to make the con-nections for cascading two ofthese stages. Connect all addi-tional stages in the same way a s

I shown i n the schematic.

Output frequency changeIn som e applicat ions there

might be a reason for reducingthe opera ting frequency of th e

IC s RC oscillator. This can bedone by inserting an externaloscillator c ap ac it or C, be-tween osc pin 7 and +Vpin 8as show n in Fig. 15.

The relationship between thevalue of C, in p icofarads andthe oscillator frequencyfoSc isshown in Fig. 16. For example, a

C of 100 pF reduces the oper-ating frequency by a factor often, from 10 kHz to 1 kHz.

However, it will be necessaryto alter the values of capacitorsC1 and C2 to compensate forthis 10 to 1 reduction of fre-quency if circuit efficiency is tobe kept high. Increase the val-ues of C1 and C2 (both 10 micro-far ads ) by th e inverse of t hereduction function. For exam-ple, if the oscillator frequency isto be 1 kHz, bot h CI an d C2should have values of about 100microfarads.

Oscillator frequency can alsobe reduced by connecting oscpin 7 as shown in Fig. 17 so th atan external clock input over-drives the internal oscillator.Feed the clock signal to pin 7th rough 1-ki lohm ser ie s re-s i s tor R1. T he c lock s igna lshould switch fully between t hetwo power su pply values. TheCMOS NAND gate is connected a san inverting buffer stage to en-sur e reliable switching betweenthose values.

The ICL7660 can also be apositive voltage multiplier. Itcan produce a positive outputvoltage th at is nearly double theini t ial supply voltage valuewhen set u p a s shown in Fig. 18.The oscillator output signal atCAP pin 2 drives a capacitor-

diode voltage-doubler network.It is the same network as theone shown in Fig. 2.

The two diodes, Dl and D2,reduce the available outp ut volt-age by an a mo un t equal to theircombined forward volt drops.Consequently, borh Dl and D2should be low-loss germaniumdiodes.

Figure 19 shows how the cir-cui ts of Figs. 42 3 and 18 canbe combined to form a positive

voltage multiplier a nd negativevoltage Wfiverter that providesdual outpue voltages. Each volt-age SourCZ has a n o ut pu t im-pedance of about 100 ohms. n

Voltage Converters

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