AN2859 datasheet(9/29 Pages) STMICROELECTRONICS

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AN2859

Multiplexed diagnostics

Knowing the control state of each STCC08 (IN1 and IN2) and according to Table 1, the

MCU is able to detect the AC switch state by analyzing VSTATE signal. Appendix A defines

the states of each ACS according to the VSTATE signal level (V0, V1, V2 and V3) and the

control state of each STCC08. In the case of failure of one of the AC switches, the MCU can

place the application in a safe configuration by switching off an appliance front-end relay.

1.3

VAVF signal reading synchronization

The STCC08 AVF output signal is an image of the AC switch voltage. This signal toggles

between VCC and zero level (GND) according to whether the STCC08 AC input current (IAC)

is higher or not than IACT (see AN2716). In case of multiplexed diagnostics the slight IACT

electrical variation between ICs may result in the state of the AVF signal of each STCC08

(either VCC or zero level) not changing at exactly the same time. This has an impact on the

VSTATE signal and on the AC switches state detection (see Figure 13). Note that IACT1 and

IACT2 define respectively the STTCO8 IAC input current for STCC081 and STCC082 to allow

VAVF signal to toggle between VCC and GND. For example, if the two STCC08 are not

controlled (IN1 = IN2 = 0) and AC1 and AC2 are not in conducting state the AC1 and AC2

can be interpreted (see Table 1) as failed in short circuit if VSTATE is read between t0 and t1

(VSTATE = V0).

Table 1. Variation of the VSTATE signal according to the AC switch states

ACS1 state

ACS2 state

VSTATE status

VSTATE = V0

OFF

VSTATE = V1

OFF

VSTATE = V2

OFF

VSTATE = V3

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