AN2716 datasheet(11/28 Pages) STMICROELECTRONICS

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AN2716

STCC08 description

Doc ID 14460 Rev 2

11/28

1.5

AVF signal reading synchronization

1.5.1

Detection window width definition

When the AC switch is off (not controlled or failed in open circuit), an IAC current flows

through the STCC08 AC input. The value of this current depends on RAC and Rshunt

resistors, the AC line voltage, and the AC load impedance. The AVF signal toggles between

VCC and zero level (GND) for an IAC current value noted IACT (see Figure 9). This has an

impact on the AC switch state detection. If the STCC08 is not controlled (IN = 0) and the

AVF signal is read when the IAC current is lower than IACT, the AC switch can be interpreted

as failed in short circuit (see Table 3).

Figure 9.

STCC08 AVF version when the AC switch is off

Note:

φAC is the phase shift between the AC line and the IAC current when the AC switch is off.

This phase shift is very low and can be neglected for most AC loads found in home

appliances (see Appendix A).

Table 3.

STCC08 status truth table

MCU control (IN)

AVF output state (1)

The AVF output must be loaded with an external resistor (RAVF) or connected directly to the MCU, in pull

up input configuration (see Section 1.4.1).

AC switch state

0+VCC (Except at each zero crossing of the AC line)

Off (no failure)

Toggle from +VCC to 0

Diode mode

Short-circuit

On (no failure)

1+VCC (Except at each zero crossing of the AC line)

Open-circuit

VLine

IAC

AVF

IACT

-IACT

t (s)

0 =

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