L9705 datasheet(5/8 Pages) STMICROELECTRONICS

5 / 8 page

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 =1K

Ω. With this in-

put resistor the contact current, when the contact

is closed and the device activated (EN =LOW) is:

IIN

VS − 2V

Ω

,for GND contacts, (1)

IIN

VBAT +∆VBAT − 2V

Ω

, for 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.

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 RIN and the contact resistance RCON or

RCOFF, for the closed contact:

RI =RIN +RCON, (3)

and for the open contact:

RI =RIN +RCOFF,(4)

The output goes HIGH when the input resistance

increases above 5.3K

Ω (GND contacts) or 6.5KΩ

(VBAT contacts) and goes LOW, when the input

resistance decreases below 4K

Ω (GND contacts)

or 4.8K

Ω (VBAT contacts); these values are typical

values for the switching thresholds. The limit val-

ues of RI = 1.8K

Ω (GND contacts) and RI =

1.8K

Ω (VBAT contacts) for LOW and RI = 20KΩ

(GND contacts) and 29K

Ω (VBAT contacts) for

HIGH implies that a contact with RCON =100

Ω (at

IIN = 10mA) will be recognized as ON = LOW and

a contact with RCOFF = 19K

Ω (GND contacts) or

28K

Ω (VBAT contact) will be recognized as OFF =

HIGH.

These limits are valid within the supply voltage

range 6V < VS < 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 RIN 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 CIN must be charged

through the resistor RIN. In this case the total de-

lay 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 buffers are less than 5pF.

L9705

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