L9704 datasheet(4/8 Pages) STMICROELECTRONICS

4 / 8 page

APPLICATION CIRCUIT DIAGRAM

Figure 1 : Typical application diagram for the L9704 circuit. The current flowing in the arrow direction is as-

sumed positive. The external capacitors CIN and COUT represent the total wiring capacitance at

the corresponding pins.

FUNCTIONAL DESCRIPTION

The L9704 circuit monitors the status of the contacts

connectedto battery and through the series external

resistors RIN to the contact sense input pins. The

contacts equivalent circuit is supposed to be as

shown in fig. 2.

The L9704 circuit compares the input current with

the current through the internal reference resistor.

The device is designed to work with an externalinput

series resistor of RIN1-8 =1k

Ω. With thisinputresistor

the contact current, when the contact is closed and

the device activated (EN = LOW) is

VBAT +

∆VBAT –2V

IIN =

Ω

(1)

For this calculation the limit value of the VIN (satura-

tion voltage of 2V) was considered so that the lowest

limit value of IIN is calculated in (1).

The function of the circuit can be demonstrated with

the transfer characteristics, showing the output

status as a function of the input resistor RI, shown

in figure 3. The input resistor is a sum of the RIN and

the contactresistance RCON or RCOFF, for the closed

contact :

RI =RIN +RCON,

(2)

and for the open contact :

RI =RIN +RCOFF.

(3)

The output goes HIGH when the input resistance in-

creases above 6.5k

Ω (typicalvalue) and goesLOW,

when the input resistance decreases below 4.8K

Ω

(typical value). The limit values of RI = 1.1K

Ω for

LOW and RI = 29k

Ω for HIGH implies that a contact

with RCON =100

Ω (at IIN = 10mA) will be recognized

as ON = LOW and a contact with RCOFF =28k

Ω will

be recognized as OFF = HIGH. These limits are

valid within the supply voltage range 5V

≤ Vs ≤ 16V,

the

battery

voltage

potential

difference

∆VBAT≤ 0.1VBAT andthe variation ofthereverse

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

L9704

4/8

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