AN2859 datasheet(3/29 Pages) STMICROELECTRONICS

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AN2859

Multiplexed diagnostics

1.1

Principle

The multiplexed diagnostic allows the detection of the state of several AC switches

independently using only one MCU input. In this case, an analog/digital converter input

(ADC) of the MCU should be used and must be configured with no pull-up resistor. In this

document, only the multiplexed diagnostic of two STCC08 (STCC081 and STCC082) is

described (see Figure 2). Note that two output pins of an MCU should be used to control

each STCC08 (IN1 and IN2).

Figure 2. Multiplexed diagnostic schematic of two STCC08

To distinguish the state of each AC switch (ACS1 and ACS2) a divider bridge is used.

Resistors R1, R2, R3 and R4 are designed to convert the VAVF digital signal given by each

STCC08 (VAVF1 and VAVF2) into an analog signal (VSTATE). Knowing the control state of

each STCC08 (IN1 and IN2), the MCU is able to identify the state of each AC switch by

analyzing the VSTATE signal (see Section 1.2).

Note:

The STCC08 AVF output is an open collector output. Resistors R1 and R3 bias the

STCC08 AVF output and limit the collector current to 5 mA. For further information, and in

particular, resistor values for RAC, Rshunt, and RIG, refer to the ST Application note AN2716.

AVF

IN2

MCU

AVF

Load2

Line

Neutral

ACS2

STCC08

AVF

VCC

GND

RIG

IN1

AVF

GND

Load1

Line

Neutral

ACS1

VCC

VSTATE

VAVF2

VAVF1

STCC08

RAC

RShunt

VCC

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