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L9705

DOUBLE QUAD CONTACT INTERFACE CIRCUIT

STMicroelectronics 

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 input resistor

the contact current, when the contact is closed and the device activated (EN =LOW) is:

IIN

=

V-----S----–-----2----V--

1kΩ

, for GND contacts, (1)

IIN

=

V-----B---A----T----+-----∆----V----B----A---T-----–----2----V---

1kΩ

, fro VBAT 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 IIN is calculated in (1) and (2)

Figure 4. The output voltage as a function of the input resistance at the corresponding sense input.

te Product(s) The function of the circuit can be demonstrated with the transfer characteristics, showing the output status

le as a function of the input resistor RI, shown in figure 4. The input resistor is a sum of the RIN and the con-

so tact resistance RCON or RCOFF, for the closed contact:

b RI = RIN + RCON, (3)

O and for the open contact:

) - RI = RIN + RCOFF, (4)

t(s The output goes HIGH when the input resistance increases above 5.3KΩ (GND contacts) or 6.5KΩ

c (VBAT contacts) and goes LOW, when the input resistance decreases below 4KΩ (GND contacts) or

u 4.8KΩ (VBAT contacts); these values are typical values for the switching thresholds. The limit values of RI

d = 1.8KΩ (GND contacts) and RI = 1.8KΩ (VBAT contacts) for LOW and RI = 20KΩ (GND contacts) and

ro 29KΩ (VBAT contacts) for HIGH implies that a contact with RCON = 100Ω ( at IIN = 10mA) will be recognized

P as ON = LOW and a contact with RCOFF = 19KΩ (GND contacts) or 28KΩ (VBAT contact) will be recognized

as OFF = HIGH.

lete These limits are valid within the supply voltage range 6V ≤ VS ≤ 16V, the ground potential difference of

∆VGND = 0.1VS, the battery voltage potential difference of ∆VBAT < 0.1VBAT and the variation of the re-

so verse battery protection diode D1 voltage from 0.5V to 1V.

bThe internal clamping diodes at the contact monitoring inputs together with the external contacts series

Oresistors RIN allows to withstand the transients at the contact connection.The contact series resistor RIN

limits the input current at the transient.

The dynamic behaviour of the circuit is defined with the times tdo and tdTS. When the contact is open, the

input capacitor CIN must be charged through the resistor RIN. In this case the total delay time tdo 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 buff-

ers are less than 5pF.

5/8

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