AN2716 datasheet(8/28 Pages) STMICROELECTRONICS

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STCC08 description

AN2716

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Doc ID 14460 Rev 2

1.4.2

AC switch state detection principle

The AC switch state detection is achieved thanks to the STCC08 “power switch signal

shaping” block. This block measures the current (IAC) through the STCC08 AC input, which

is the image of the AC switch voltage (VT) during both AC line cycle (VLine). As illustrated in

Figure 7, three cases can be considered.

●

Case 1: If the AC switch is off (VT = VLine), a low IAC current, in phase shift with the AC

line (see Section 1.5.1), flows through resistor RAC for the both AC line polarities. In this

case, the STCC08 AVF output signal (VAVF) remains at high level state (+VCC).

●

Case 2: If the AC switch is on (VT ≈ 0), no IAC current flows through the resistor RAC. In

this case, the AVF output signal (VAVF) remains at low level state (GND).

●

Case 3: If the AC switch is off and the AC line voltage falls to zero voltage, no IAC

current flows through the resistor RAC. In this case, the AVF output signal (VAVF) goes

to zero level (GND).

Knowing the STCC08 IN input state, the MCU is then able to determine the AC switch state

by analyzing the AVF signal (see Section 1.4.3).

Figure 7.

AC switch failure detection principle

The IAC current follows the waveform of the voltage across the AC switch. The AVF signal

toggles between VCC and zero level (GND) at a certain IAC current value noted as IACT (see

Figure 7). The IACT electrical parameter dispersion is shown in Table 2.

The IAC peak current value (IAC-peak) must be lower than 2.2 mA (maximum IAC current

allowed through STCC08 AC input). Equation 2 defines the minimum resistor RAC value

(RAC_Min) to limit this IAC current in the worst case (if the AC load fails in short circuit). For

example, with VLine_rms_max = 230 V + 10%, the RAC resistor value must be higher than

163 k

Ω.

Table 2.

IACT electrical parameter dispersion

Maximum (IACT_max)

Minimum (IACT_min)

IACT

236 µA

82 µA

VAVF

IAC

IACT

-IACT

AC switch off

AC switch on

t (s)

ILoad

VCC

Power switch

signal shaping

VCC

AVF

Load

Line

Neutral

ACS

STCC08

IAC

ILOAD

VAVF

RAVF

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