AN2846 datasheet(8/35 Pages) STMICROELECTRONICS

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Application guidelines

AN2846

8/35

Doc ID 15130 Rev 1

The proposed RIN value of 2.2 kΩ has been calculated to meet the IEC 61131-2 threshold

requirements. Users can set their own particular application threshold voltages by applying,

in the formula given above, the VIN they want to achieve and find the corresponding RIN

value. Take note, the higher the RIN, the better will be the immunity against voltage surges.

A particular useful application is when an input type 2 is needed. Figure 7 shows the

solution of connecting RIN7 and input RIN8 = 1.5 kΩ in parallel and tuning the ILIM with

RREF = 9.1 kΩ to get 3.5 mA (see Figure 4) in each input branch. Of course corresponding

bits (B14, B15) will be set together at the right state according to the level applied at the

common input. Unused LED outputs must be grounded to maintain the flow of the

current in its corresponding chanel.

Figure 7.

IN7 and IN8 parallel wired for type 2

The different threshold voltages and the ILIMIT = 7.0 mA are shown in Figure 8 below.

Figure 8.

Threshold voltages - type 2 configuration using two inputs in parallel

REF= 9.1kΩ

RIN= 1.5k Ω

Type 3 inputs

Type 2 inputs

IN1

IN2

IN3

IN4

IN5

IN6

IN7

IN8

LED2

LED1

LED5

LED6

LED7

LED8

LED4

REF

LED3

(2V/div)

(5mA/div)

LED

I_OFF

= 8.4 V

I_ON

= 9.9 V

LIMIT

= 7.0mA

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