L9705 Datasheet, PDF (5/8 Pages) STMicroelectronics – DOUBLE QUAD CONTACT INTERFACE CIRCUIT

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 RIN1-8 = 1KΩ. With this in-
put resistor the contact current, when the contact
is closed and the device activated (EN =LOW) is:
IIN
=
V_S − 2V
1KΩ
,for
GND
contacts,
(1)
I_IN
=
V_BAT
+
∆V_BAT
1KΩ
−
2V,
for
V_BAT
contacts,
(2)
For this calculation the limit value of the VS to VIN
and VIN 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 out-
put status as a function of the input resistor RI,
shown in figure 4. The input resistor is a sum of
the R_IN and the contact resistance R_CON or
R_COFF, for the closed contact:
R_I = R_IN + R_CON, (3)
and for the open contact:
RI = RIN + RCOFF, (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 (VBAT contacts); these values are typical
values for the switching thresholds. The limit val-
ues 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 (VBAT contacts) for
HIGH implies that a contact with RCON = 100Ω ( at
I_IN = 10mA) will be recognized as ON = LOW and
a contact with RCOFF = 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 differ-
ence of ∆VGND = 0.1VS, the battery voltage poten-
tial difference of ∆VBAT < 0.1VBAT and the vari-
ation of the reverse battery protection diode D1
voltage from 0.5V to 1V.
The internal clamping diodes at the contact moni-
toring inputs together with the external contacts
series resistors RIN allows to withstand the tran-
sients at the contact connection.The contact se-
ries resistor R_IN limits the input current at the tran-
sient.
The dynamic behaviour of the circuit is defined
with the times tdo and tdTS. When the contact is
open, the input capacitor C_IN must be charged
through the resistor R_IN. In this case the total de-
lay time t_do may be influenced also with the time
constant RINCIN.
The delay time tdTS, 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 buffers are less than 5pF.
5/8