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參數(shù)資料
| 型號： | L4620 |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | LIQUID LEVEL ALARM |
| 中文描述： | 液位報警 |
| 文件頁數(shù)： | 5/9頁 |
| 文件大?。?/td> | 61K |
| 代理商： | L4620 |
The L4620liquid levelalarm isdesignedto operate
with a variety of sensortypeswhichchangeimped-
ancedependingon whetherthesensoris aboveor
belowthelevelofaliquid.Iftheimpedancevariation
of ther liquid itself is sensed, a very simple sensor
(two electrodes) can be used. The output stage
drivesdirectlythealarmindicatorwitha 300mArec-
tangularwavesignal,the dutycycleof whichispro-
grammable.
SENSORINTERFACE.
Asshownintheapplicationcircuit,thesensoriscon-
nectedso thatit varies the attenuationof a square-
wavesignalbetweenpin 2and pin3 whereitsposi-
tivehalfcycleiscomparedwiththereferencethresh-
old(withhysteresis).
This frequency,generatedinternallyby a 50%duty
cycle oscillator, is 50Hz in the typical application
(R
osc
= 180K
C
osc
= 4.7nF).
Thethresholdofthesensorinputis afunctionofthe
outputvoltageatpin2.Thehysteresisisprovidedby
a Schmitttriggercomparator.As shownin figure1,
this gives hysteresis with either threshold polarity
selected.
The ACdrivingof thelevelsensor allowsthe useof
a capacitive filter (C
A
, C
B
, C
C
in block diagram)
which acts as a bandpass filter at the frequency
used.TheresistorR
C
intheapplicationcircuitbiases
the sensor input stage. In this way the interference
problemstypical of automotiveapplications are re-
ducedconsiderably.If,however,itis not necessary
to decoupleand filter the sensor a simple resistive
networkmaybe used,eliminating thecapacitors.
SPURIOUSINDICATION PROTECTION.
Topreventspuriousalarmsignalswhentheliquidis
agitatedor in the presence of interference,the de-
vice includestwoprotectionmechanism:
Firstly,thesensorinterfacewhichsamplestheposi-
tivehalfcycleofthesensorsignalactivatesitsoutput
onlyiftherearefourconsecutivealarmconditionin-
dications.Secondly, the alarm outputstage is only
activated after an externally programmable delay.
During this delay if the alarm conditionceases the
alarmoutputwillnot be activated.
Using the values C
osc
= 4.7nF and R
osc
= 180K
,
whichgive a typicaloscillator frequencyof 1.6KHz,
delaysofabout10s(programmingpin7low)or20s.
INTERNAL MEMORY.
Whenthealarm outputhasbeenactivatedan inter-
nal latch holds it in the active state until the power
supply is removed. This feature ensures that the
alarmwillnotbeinterruptedifthesensorconnection
breaks.
OUTPUTSTAGE.
Throughpin8itispossibletoprogramthedutycycle
of the alarm signal waveform (see figure2). When
pin8ishightheoutputsignalhasadutycycleof50%
; ifpin8 is low thedutycycle is 1 : 64.Theperiodof
theoutputsignalisalways320msusingthecompo-
nentvalues indicatedin block diagram.
Theoutputstagecandeliverupto300mAandispro-
tectedinternallyagainstovervoltages(bya zener).
A thermal shutdowncircuit providesadditionalpro-
tection.
CIRCUIT OPERATION
L4620
5/9
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