L9703 查看數據表（PDF） - STMicroelectronics

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L9703

OCTAL GROUND CONTACT MONITORING CIRCUIT

STMicroelectronics 

L9703

FUNCTIONAL DESCRIPTION

The L9703 circuit monitors the status of the contacts

connected to ground and through this series exter-

nal resistors RIN to the contact sense input pins. The

contacts equivalent circuit is supposed to be as

shown in fig. 2.

The L9703 circuit compares the input current with

the current through the internal reference resistor.

The device is designed to work with anexternal 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

VS - 2V

IIN = 1kΩ

(1)

For this calculation the limit value of the VS to IN

saturation 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 contact resistance RCON orRCOFF, 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 above5.3kΩ (typical value) and goes LOW,

when the input resistance decreases below 4kΩ

(typical value). The limit values of RI = 1.8KΩ for

LOW and RI = 20kΩ for HIGH implies that a contact

with RCON = 100Ω (at IIN = 10mA) will be recognized

as ON = LOW and a contact with RCOFF = 19kΩ will

be recognized as OFF = HIGH. These limits are

valid within the supply voltage range 6V ≤ VS ≤ 16V

and the ground potential difference of

∆VGND = 0,1V.

The internal clamping diodes at the contact moni-

toring inputs, togetherwith the external contacts se-

ries resistors RIN, allows the device to withstand

transientsat the contactconnection.The contactse-

ries resistor RIN limits the input current at the tran-

sient.

The dynamic behaviour of the circuit is defined by

the times tdo and tdTs. When the contact is open, the

input capacitor CIN must be charged through the re-

sistor RIN. In this case the total delay time may also

be influenced by the time constant RINCIN.

The delay time tdTs, when disabling the device, is de-

fined only by the internal circuitry. In both cases, an

external output capacitance less than 50pF is as-

sumed, the internal output capacitances of the

three-state buffers are less than 5pF.

Figure 2 : The Contact Sense Input Connection with the Contact Equivalent Circuit.

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