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Fig.d3ddPhototransisterdindcommon-emitterdbias

Ic(on)mind=d1.5mAdatdIFd=d20mA,dVced=d5V,
Ic(on)mindclosedtod0.75mAdwhendIFd=10mA
(dThedInputdlowdcurrentdanddInputdhighdcurrentdofdCMOSdgatedaredverydlowdanddcoulddbedneglectedd)
ThendR2(min)d=d5Vd/d0.75mAd=d6.7Kdohms

Whendthedtargetdobjectd(dthedobjectdtodbeddetectedd)ddoesdnotdinterruptdthedlightdpathd(dthedslotdofdthedopticaldswitchd
)dthendphototransisterdturndon.dConsiderdtodmakedPhototransisterdintoddeepdsaturateddareadtodgetdadgooddlogicdlowd
leveldanddreservednecesssarydguarddbanddfordwholedcircuit,dthedsuitabledR2disd2dtimesdofdR2(min),dsodthedsuitabledR2dind
abovedapplicationdcircuitdfordLTH-301-19disd13K.dWhendthedtargetdobjectdinterruptsdthedlightdpath,dthedcurentdfowdind
thedloopdisdcauseddbydthedtwodpossibledreasons,dAmbientdlightdanddIceodleakagedcurrentdofdthedphototrsister.dIndthed
specifcations,dthedleakagedcurrentdofdphototransisterdisdlessdthand100nA,dsodthedmaximumdallowabledambientdlightd
currentdindabovedexampledcoulddbedcheckeddbydfollowingdwayd:

1.dAssumingdthedminimundlogicdhighdleveldisd3.2VdindVccd=d5V.
2.dThendthedmaximundtotaldcurrentdfowdfordlogicdhighd=d(d5V-d3.2Vd)d/d13KdOhms
d =1.8Vd/d13KdOhms
d =138.5 A
3.dSodthedmaximumdallowabledambientdlightdcurrentd=d138.5 Ad-d0.1 A
d =d138.4 A

Becausedthedhousingdofdthedopticaldswitchdisdopaquedtype,dthedambientdlightdonlydcandviedthedslitdindfrontdofdthedpho-
totransisterdtodcausedambientdlightdcurrentdandditdisdreallydunusualdtodgetdsuchdhighdambientdlightdcurrentdindadnormald
applicationdcondition.dThedobjectdtodbeddetecteddcandnotdcutdthedlightdfromdthedemitterdcompletelydisdthedanotherd
possibledreasondtodmakedunexpecteddcurrentdfowdindthedphototransisterdwhendthedobjetdinterruptsdthedslot.dDuedtod
thisdreasondtheredisdpartialdlightdpassdthroughdthedobjectdanddcausedunexpectdcurrentdindthedphototransisterdanddifditd
isdhighdenoughdtodmakedwrongdlogicdleveldthenditdisdnecessarydtodconsiderdanotherdsuitabledmaterialdfordtheddetectedd
object.

Resultdofdthedexampled:dThedsuitabledR1danddR2dvaluedfordLTH-301-19indabovedapplicationdciucuitdare,dR1d=d380dOhms,dd PTIR > Application Note
R2d=d13KdOhms

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