AN2846 datasheet(23/35 Pages) STMICROELECTRONICS

23 / 35 page

AN2846

Application circuit

Doc ID 15130 Rev 1

23/35

3.2

Component definitions

The reference resistor RREF tolerance gives the accuracy of the input limiters. 1% accuracy

is suggested.

The typical type 3 SCLT3-8 application uses RREF = 15 kΩ and RIN = 2.2 kΩ. Type 2 can be

also achieved as shown in Table 3.

The input MELF resistors are used to sustain high voltages occurring during surge tests.

Using type 2, the power dissipated by the SCLT3-8 reaches 1W with VI = VCC = 24 V. A

copper heat sink area of 1 cm2 will set TJ at 150 °C with an ambient temperature of 65 °C.

The RC value has to be chosen with attention. The voltage drop across this resistor is the

product of SCLT3-8 supply current and any load current supplied by VDD: regulator output

current and sourced MISO current. The resulting voltage VC must be in any case above the

8 V activation threshold to avoid a spurious power loss detection. 1 k

Ω meets this

requirement and allows 2 kV of voltage surge.

The 22 nF input capacitors are used to improve the noise immunity of the whole module.

Their function is to filter the high frequency electrical noise, and to secure the off state of the

module.

Adding a 33 nF capacitor on VC pin ensures high immunity against electrical noise such as

that described in the IEC 61131-2 standard.

ROSC = 51 kΩ and pin DVR grounded set the input digital filter to eliminate pulse widths

below 20 µs.

A 33 nF capacitor connected on VDD output ensures a good output of the voltage regulator.

The LEDs must be chosen according to their input diode drop voltage. LED outputs can

drive LEDs with forward voltage up to 2.7 V.

Low pass RC filters have been inserted into digital inputs /CS and SCK to improve the

immunity against fast transient bursts. R = 220

Ω and C = 470 pF give a good compromise

between immunity result and SCLT-3 speed which can run up to 1 MHz.

Table 3.

Type 2 and 3 configurations

Type 3

Type 2

Unit

RREF

3.9

RIN

2.2

0.75

Typical ILIMIT

2.3

6.5

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