L4620
CIRCUIT OPERATION
The L4620 liquid level alarm is designed to operate
with a variety of sensor types which change imped-
ance depending on whether the sensor is above or
below the level of a liquid. If the impedancevariation
of ther liquid itself is sensed, a very simple sensor
(two electrodes) can be used. The output stage
drives directly the alarm indicator with a 300mArec-
tangularwave signal, the duty cycle of which is pro-
grammable.
SENSOR INTERFACE.
As shownin the applicationcircuit, the sensoris con-
nected so that it varies the attenuation of a square-
wave signal between pin 2 and pin 3 where its posi-
tive halfcycle is comparedwiththe referencethresh-
old (with hysteresis).
This frequency, generated internally by a 50% duty
cycle oscillator, is 50Hz in the typical application
(R
osc
= 180K鈩?C
osc
= 4.7nF).
The threshold of the sensor inputis a function of the
outputvoltageat pin 2. The hysteresisis providedby
a Schmitt trigger comparator. As shown in figure 1,
this gives hysteresis with either threshold polarity
selected.
The AC driving of the level sensor allows the use of
a capacitive filter (C
A
, C
B
, C
C
in block diagram)
which acts as a bandpass filter at the frequency
used.Theresistor R
C
in theapplicationcircuit biases
the sensor input stage. In this way the interference
problems typical of automotive applications are re-
duced considerably. If, however, it is not necessary
to decouple and filter the sensor a simple resistive
network may be used, eliminating the capacitors.
SPURIOUS INDICATION PROTECTION.
To preventspurious alarm signals when the liquid is
agitated or in the presence of interference, the de-
vice includes two protection mechanism :
Firstly, the sensor interface which samples the posi-
tive halfcycleof the sensorsignalactivatesits output
only if there are four consecutive alarm conditionin-
dications. Secondly, the alarm output stage is only
activated after an externally programmable delay.
During this delay if the alarm condition ceases the
alarm output will not be activated.
Using the values C
osc
= 4.7nF and R
osc
= 180K鈩?
which give a typical oscillator frequency of 1.6KHz,
delaysof about 10 s (programmingpin 7 low)or 20s.
INTERNAL MEMORY.
When the alarm output has been activated an inter-
nal latch holds it in the active state until the power
supply is removed. This feature ensures that the
alarm will not be interrupted if the sensor connection
breaks.
OUTPUT STAGE.
Throughpin 8 it is possibleto programthe duty cycle
of the alarm signal waveform (see figure 2). When
pin 8is high the outputsignalhasa dutycycle of50%
; if pin 8 is low the duty cycle is 1 : 64. The period of
the output signal is always 320ms using the compo-
nent values indicated in block diagram.
The outputstagecandeliver upto 300mAand ispro-
tected internally against overvoltages (by a zener).
A thermal shutdown circuit provides additional pro-
tection.
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