201305.pdf 43279KB May 08 2013 11:07:04 PM

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designideas0.87VREFR 31MR 42MSUMMINGINTEGRATORR 21MR 12MV DD− 10IC 1½TS12011wave. The integrator-summing nodeis held at V REFby the feedback actionof the amplifier. Thus, a fixed positivereference current set by R 1is balancedby a larger-amplitude switchednegative current set by R 2. The lowercomparator block has an open-drainoutput; when its output is low, currentis pulled from the summing node via R 2:I R1=(0.87×V REF−0.58×V REF)/R 1and I R2(switched)=0.58×V REF/R 2. If I R2is setto 2×I R1, a symmetrical triangle waveresults.The frequency is set as follows:23101+470 pF4IC 2½TS12012− +2 3V DD0.9 TO2.5V0.1 µF41.5× REFGENERATOR10.58VREF0.87VREF1f=[1/I R1+1/I R2]×C×V ,where V is the difference between 0.87×V REFand 0.58×V REF. Here, f=850 Hz.Figure 2 shows the waveforms at thesawtooth and pulse outputs.The window comparator employs abuilt-in latch function of the TS12012to provide hysteresis. The latch functionhas a sly feature: When LHDETis pulled low, the comparator inputsare still active and sensing the inputEDNDI_PLANETANALOG Fig 1.eps7V REF8−½TS120116+8 5+6½TS12012−DIANE9LHDET(LATCHINPUT)9WINDOWCOMPARATORstate, until the inputscross. The comparatorin IC 2gets set when theramp crosses the lowerthreshold at 0.58×V REF,and reset when the rampcrosses 0.87×V REF. Thereset pulse is momentary,but puts the latchin a state where thecomparator inputs crossingcause it to set andlatch again (which happensdue to the switchedreference current causingthe integrator toramp negative). The netresult: No glue logic isneeded.The battery voltageranges down to0.9V with a miserlyV DDcurrent of 3.2 μA.Maximum operatingfrequency is limited bythe op-amp slew rateand prop delays to about3 kHz. DisconnectingR 1and driving it witha voltage source greaterthan 0.58×V REFgivesyou a VCO function.Editor’s note: ThisDesign Idea is courtesy of EDN.com’ssister site, Planet Analog: http://bit.ly/11IkNeK.EDN1-kHz SAWTOOTHOUTPUT1-kHz PULSEOUTPUTFigure 1 This low-voltage sawtooth generator uses only eight components and draws extremely lowpower.Figure 2 The waveforms at the sawtooth and pulse outputs are shown. The pulsetrain is used to reset the latching comparator.50 EDN | MAY 2013 [ www.edn.com]
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