Obsolete Product(s) - Obsolete Product(s)
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L9705
The L9705 circuit compares the input current with the current through the internal reference resistor. The
device is designed to work with an external input series resistor of R
IN1-8
= 1KΩ. With this input resistor
the contact current, when the contact is closed and the device activated (EN =LOW) is:
, for GND contacts, (1)
, fro V
BAT
contacts, (2)
For this calculation the limit value of the V
S
to V
IN
and V
IN
saturation voltage of 2V was considered so that
the lowest limit value of I
IN
is calculated in (1) and (2)
Figure 4. The output voltage as a function of the input resistance at the corresponding sense input.
The function of the circuit can be demonstrated with the transfer characteristics, showing the output status
as a function of the input resistor R
I
, shown in figure 4. The input resistor is a sum of the R
IN
and the con-
tact resistance R
CON
or R
COFF
, for the closed contact:
R
I
= R
IN
+ R
CON
, (3)
and for the open contact:
R
I
= R
IN
+ R
COFF
, (4)
The output goes HIGH when the input resistance increases above 5.3KΩ (GND contacts) or 6.5KΩ
(V
BAT
contacts) and goes LOW, when the input resistance decreases below 4KΩ (GND contacts) or
4.8KΩ (V
BAT
contacts); these values are typical values for the switching thresholds. The limit values of R
I
= 1.8KΩ (GND contacts) and R
I
= 1.8KΩ (V
BAT
contacts) for LOW and R
I
= 20KΩ (GND contacts) and
29KΩ (V
BAT
contacts) for HIGH implies that a contact with R
CON
= 100Ω ( at I
IN
= 10mA) will be recognized
as ON = LOW and a contact with R
COFF
= 19KΩ (GND contacts) or 28KΩ (V
BAT
contact) will be recognized
as OFF = HIGH.
These limits are valid within the supply voltage range 6V ≤ V
S
≤ 16V, the ground potential difference of
∆V
GND
= 0.1V
S
, the battery voltage potential difference of ∆V
BAT
< 0.1V
BAT
and the variation of the re-
verse battery protection diode D1 voltage from 0.5V to 1V.
The internal clamping diodes at the contact monitoring inputs together with the external contacts series
resistors R
IN
allows to withstand the transients at the contact connection.The contact series resistor R
IN
limits the input current at the transient.
The dynamic behaviour of the circuit is defined with the times t
do
and t
dTS
. When the contact is open, the
input capacitor C
IN
must be charged through the resistor R
IN
. In this case the total delay time tdo may be
influenced also with the time constant R
IN
C
IN
.
The delay time t
dTS
, when disabling the device, is defined only with the internal circuitry. In both cases,
output external capacitance less than 50pF is assumed, the internal output capacitance of the tristate buff-
ers are less than 5pF.
I
IN
V
S
2V–
1kΩ
---------------------=
I
IN
V
BAT
V
BAT
2V–∆+
1kΩ
----------------------------------------------------=
