ST AN2859 APPLICATION NOTE

AN2859

Application note

Multiplexed diagnostics of AC switches using two STCC08s

Introduction

The aim of this application note is to present opportunities to reduce the number of input pins used on a microcontroller unit (MCU) to diagnose failures of several AC switches with the STCC08. This document deals with the multiplexed diagnostics of two STCC08 and gives technical recommendations on the implementation of this solution.

STCC08 overview

The STCC08 has been designed to improve home appliance safety. This new device can drive an AC switch (Triac, ACST and ACS) with a gate current I back to the microcontroller unit a signal image of the voltage across the controlled AC switch (this signal defines the AC switch state). The STCC08 has three functional blocks (see

Figure 1).

■ A "gate driver" block used to drive an AC switch and to interface directly the STCC08 with

the MCU (CMOS compatible)

■ A "power switch signal shaping" block used to measure the AC switch voltage in both AC

line cycles

■ An "AVF driver" block used to give an image of the AC switch voltage to the MCU (digital

information)

up to 10 mA and to send

GT

Figure 1. STCC08 block diagram

G

V

CC

AC

Power switch

signal shaping

Gate driver

+

-

STCC08

AVF driver

IN

R

IG

AVF

GND

IN

AVF

N/C

AC

1

2

3

4

SO -8

8

7

6

5

For more information about the STCC08, please refer to the ST Application note AN2716.

December 2009 Doc ID 15255 Rev 1 1/29

GND

R

IG

G

V

CC

www.st.com

Contents AN2859

1 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 (STCC08 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

Neutral

R

Shunt

Line

Load2

ACS2

R

AC

V

AC

G

CC

STCC08

2

IN2

R

AVF

and STCC082) is

1

IG

V

CC

R1

R2

V

AVF2

MCU

V

CC

Neutral

R

Shunt

Line

Load1

ACS1

R

AC

V

AC

G

CC

STCC08

1

To distinguish the state of each AC switch (ACS1 and ACS2) a divider bridge is used. Resistors R STCC08 (V

, R2, R3 and R4 are designed to convert the V

1

AVF 1

and V

) into an analog signal (V

AVF 2

STATE

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

signal (see Section 1.2).

STATE

Note: The STCC08 AVF output is an open collector output. Resistors R

STCC08 AVF output and limit the collector current to 5 mA. For further information, and in particular, resistor values for R

AC

, R

, and RIG, refer to the ST Application note AN2716.

shunt

IN1

R

IG

V

CC

R3

AVF

R4

GND

V

digital signal given by each

AVF

AVF1

V

STATE

). Knowing the control state of

and R3 bias the

1

Doc ID 15255 Rev 1 3/29

Multiplexed diagnostics AN2859

1.2 Failure mode detection of two AC switches

Figure 3 to Figure 12 give the V

V

, V1, V2 and V3 are levels reached by the parameter V

0

signal level according to the state of each AC switch.

STATE

and depends on R1, R2, R3,

STATE

and R4 resitor values.Ta ble 1 shows that we only need four different levels to define the state of each AC switch.

Figure 3. Case 1: V

VCC/COM

I

Load_2

V

AC

Line

VCC/COM

V

AC

Line

RShunt

Load2

RShunt

Load1

I

Load_1

ACS2

RAC

ACS1

RAC

= V3 (except at each zero crossing of the AC line)

STATE

VCC

VS TATE

V3

V2

V1

V0

I

Load_2

I

Load_1

STCC08

G

AC

STCC08

G

AC

AVF

IN2

AVF

IN1

R1

2

R2

V

AVF2

VCC

R3

1

R4

V

AVF1

STAT E

V

AC

V

AC

ACS1 and ACS2 are not in conducting state

Figure 4. Case 2: V

VCC/COM

I

Load_2

V

AC

Line

Load2

RShunt

VCC/COM

V

AC

Line

Load1

RShunt

I

Load_1

ACS2

RAC

ACS1

RAC

STATE

= V

STCC08

G

AC

STCC08

G

AC

0

AVF

IN2

AVF

IN1

VCC

V

AC

R1

2

R2

V

AVF2

I

Load_2

I

Load_1

V

AC

VCC

VSTATE

R3

1

R4

V

AVF1

STAT E

V3

V2

V1

V0

ACS1 and ACS2 are in conducting state

4/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Figure 5. Case 3: V

V /COM

CC

I

Load_2

V

AC

Line

VCC/COM

V

AC

Line

RShunt

Load2

RShunt

Load1

I

Load_1

ACS2

RAC

ACS1

RAC

= V1 (except at each zero crossing of the AC line)

STATE

V

CC

R1

STCC08

G

AC

STCC08

G

AC

2

AVF

IN2

1

AVF

IN1

R2

V

AVF2

V

CC

R3

V3

R4

V2

V

AVF1

STATE

V1

V0

ACS1 is in conducting state and ACS2 is not in conducting state

Figure 6. Case 4: V

= V2 (except at each zero crossing of the AC line)

STATE

VCC

V

AVF2

V

AVF1

V

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

STCC08

AVF

G

AC

2

IN2

1

IN1

VSTATE

V

STAT E

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

ACS1 is not in conducting state and ACS2 is in conducting state

Doc ID 15255 Rev 1 5/29

Multiplexed diagnostics AN2859

Figure 7. Case 5: V

toggles between V1 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V /COM

CC

I

Load_2

STCC08

2

R1

R2

AVF

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_1

ACS2

RAC

ACS1

RAC

G

AC

STCC08

AVF

G

AC

IN2

IN1

V

AVF2

V

CC

V

R3

1

R4

V3

V2

V

AVF1

STATE

V1

V0

ACS1 is failed in diode mode and ACS2 is not in conducting state

Figure 8. Case 6: V

toggles between V2 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V

AVF2

AVF1

V

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

IN2

STCC08

AVF

G

AC

IN1

2

1

STATE

V

STATE

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

ACS1 is not in conducting state and ACS2 is failed in diode mode

6/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Figure 9. Case 7: V

toggles between V1 and V2 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V

AVF2

V

AVF1

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

STCC08

AVF

G

AC

2

IN2

1

IN1

ACS1 and ACS2 are failed in diode mode not on the same AC line polarities

V

STAT E

I

Load_2

I

Load_1

V

AC

Figure 10. Case 8: V

toggles between V0 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

VCC

V /COMCC

ILoad_2

STCC08

R1

2

R2

ILoad_2

AVF

V

AC

Line

V /COMCC

V

AC

Line

Load2

RShunt

ILoad_1

Load1

RShunt

ACS2

RAC

ACS1

RAC

G

AC

STCC08

AVF

G

AC

IN2

IN1

VAVF2

ILoad_1

VCC

VSTAT E

R3

1

R4

V3

V2

VAVF1

VSTAT E

V1

V0

ACS1 and ACS2 are failed in diode mode on the same AC line polarities

V

AC

V

AC

Doc ID 15255 Rev 1 7/29

Multiplexed diagnostics AN2859

Figure 11. Case 9: V

V /COM

CC

I

Load_2

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

ACS2

RAC

Load_1

ACS1

RAC

toggles between V2 and V0 at each AC line cycle

STATE

V

CC

R1

STCC08

2

R2

AVF

G

V

AVF2

V

AC

IN2

CC

R3

STCC08

1

R4

AVF

G

V

AVF1

V

STAT E

AC

IN1

ACS1 is failed in diode mode and ACS2 is failed in short circuit

Figure 12. Case 10: V

V /COM

CC

I

Load_2

V

AC

Line

Load2

RShunt

V /COM

CC

I

V

AC

Line

Load1

RShunt

Load_1

ACS2

RAC

ACS1

RAC

toggles between V1 and V0 at each AC line cycle

STATE

V

CC

R1

STCC08

2

R2

AVF

G

V

AVF2

V

AC

IN2

CC

R3

STCC08

1

R4

AVF

G

V

AVF1

V

STATE

AC

IN1

V3

V2

V1

V0

V3

V2

V1

V0

V

STAT E

STATE

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

ACS1 is failed in shot circuit and ACS2 is failed in diode mode

8/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Table 1. Variation of the V

Knowing the control state of each STCC08 (IN1 and IN2) and according to Ta bl e 1, the MCU is able to detect the AC switch state by analyzing V states of each ACS according to the V 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 V

The STCC08 AVF output signal is an image of the AC switch voltage. This signal toggles between V is higher or not than I electrical variation between ICs may result in the state of the AVF signal of each STCC08 (either V V I V 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 V (V

signal according to the AC switch states

STATE

ACS1 state ACS2 state V

ON ON V

ON OFF V

OFF ON V

OFF OFF V

signal. Appendix A defines the

signal level (V0, V1, V2 and V3) and the control

STATE

signal reading synchronization

AVF

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

CC

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

CC

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

STATE

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

ACT2

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

AVF

= V0).

STATE

(see AN2716). In case of multiplexed diagnostics the slight I

ACT

STATE

is read between t0 and t

STATE

status

= V

0

1

2

3

ACT1

ACT

and

1

Doc ID 15255 Rev 1 9/29

Multiplexed diagnostics AN2859

Figure 13. V

I

ACT2

I

ACT1

-I

ACT1

-I

ACT2

V

CC

V

CC

VSTATE

V

3

V

1

ϕ

t = 0

0

signal variation due to the I

STATE

VLine

I=AC1 IAC2

AVF1 (STCC08_1)

AVF2 (STCC08_2)

Δ

t

1

2

Detection

parameter dispersion

ACT

t

3

t =

5

t

4

t (s)

1

2 x f

The V is advised to read the V

should be read between times t2 and t3. To simplify the AC switches detection, it

STATE

signal around the AC line peak voltage to avoid any

AVF

inappropriate interpretation of the AC switches state. Note that when the IN1 and/or IN2 signals are removed, a parasitic detection of the AC switch state exists up to the next AC load current zero crossing (see AN2716). Anyway to ensure a reliable detection of the AC switch state when the IN1 and/or IN2 control is removed, the AVF reading should be read 10 ms after the IN1 and/or IN2 control has been removed and at the next peak mains voltage.

Note: It is recommended that the AVF signal be read during several AC line cycles around the AC

line peak voltage.

10/29 Doc ID 15255 Rev 1

AN2859 V

V

level definition

STATE

2 V

According to the state of each AC switch, V0, V1, V2 and V3 levels are defined by equations

1, 2, 3 and 4 (see also Appendix B). In this document V

the STCCO8 values of AVF at low level are respectively 0 V and 1 V.

Equation 1

ACS1 and ACS2 are on

Equation 2

ACS1 is on and ACS2 is off

Equation 3

ACS2 is on and ACS1 is off

STATE

=

0

V

=

1

V

=

2

level definition

and STCCO82 AVF output at the low level. The minimum and maximum

1

()

+

·RV·RV

4AVF2_L2AVF1_L

RR

42

()

RR·V·RV

++

RRR

++

421

()

RR·V·RV

++

RRR

++

432

21AVF1_L4CC

43AV F2_ L2CC

AVF 1_L

and V

are respectively

AVF 2_L

Equation 4

ACS1 and ACS2 are off

VV =

CC3

The tolerance of the resistors (R dispersion and the DC power supply characteristics induce a dispersion on V V

levels (see Ta bl e 2 ).

3

Table 2. Variation of the V

ACS1 state ACS2 state V

ON ON V

ON OFF V

OFF ON V

OFF OFF V

, R2, R3 and R4), the STCC08 output AVF signal electrical

1

signal according to the AC switch states

STATE

< V

0_Min

< V

1_Min

< V

2_Min

3_Max

> V

STATE

, V1, V2 and

0

status

< V

0_Max

< V

1_Max

< V

2_Max

> V

3_Min

Knowing the previous equations 1, 2, 3 and 4, the resistors standard value and the tolerance of the resistors, V

x_Max

and V

(x = 0, 1, 2, or 3) values are defined respectively by

x_Min

equations 5, 6, 7, 8, 9, 10, 11 and 12.

Doc ID 15255 Rev 1 11/29

V

STATE

level definition AN2859

V the application. X resistor tolerance X V

and V

CC_Min

AVF_L_Min

CC_Max

are respectively the minimum and maximum power supply voltage of

and X

R_Max

and X

values are fixed by the STCC08 AVF output electrical dispersion at low level

with:

V

AVF_L_Max

= V

AVF1_L_Max

and

V

AVF_L_Min

= V

AVF1_L_Min

= V

Equation 5

ASC1 is on and ACS2 is on.

⎛

+

⎜ ⎝

=

AVF_L_Max0_Max

·VV =

42

⎛

+

⎜ ⎝

Equation 6

ASC1 is on and ACS2 is on.

⎛ ⎜ ⎝

=

AVF_L_Min0_Min

·VV

⎛ ⎜ ⎝

⎞

+

⎟

42

⎠

⎞

+

⎟

42

⎠

R_Min

= V

AVF2_L_Max

AVF2_L_Min

⎞

X·RR

⎟

R_Max

⎠

⎞

X·RR

⎟

42

R_Min

⎠

X·RR

R_Min

X·RR

R_Max

are the tolerances of the resistors. For example, with 5%

are respectively 1.05 and 0.95. V

AVF_L_Max

and

= 1 V

= 0 V

X

R_Max

·V X

R_Min

=

AVF_L_Max

V0

Equation 7

ASC1 is on and ACS2 is off.

⎞

X·RR·VX·R·V

++

⎟

21

R_Max

⎠

V

1_Max

=

CC_Max

⎛

4

⎛ ⎜ ⎝

AVF_L_MaxR_Max

++

⎜ ⎝

⎞

X·RRR

⎟

421

R_Min

⎠

Equation 8

ASC1 is on and ACS2 is off.

⎞

X·RR·VX·R·V

++

⎟

21

R_Min

⎠

V

1_Min

⎛

CC_Min

4

AVF_L_MinR_Min

=

⎛ ⎜ ⎝

++

⎜ ⎝

⎞

X·RRR

⎟

421

R_Max

⎠

Equation 9

ASC1 is off and ACS2 is on.

⎛

V

2_Max

CC_Max

=

2

⎛ ⎜ ⎝

AVF_L_MaxR_Max

++

⎜ ⎝

⎞

X·RRR

⎟

432

R_Min

⎠

⎞

X·RR·V.XR·V

++

⎟

43

R_Max

⎠

12/29 Doc ID 15255 Rev 1

AN2859 V

Equation 10

ASC1 is off and ACS2 is on.

⎞

++

X·RR·VX·R·V

⎟

43

R_Min

⎠

V

2_Min

⎛

CC_Min

2

AVF_L_MinR_Min

=

⎛ ⎜ ⎝

++

⎜ ⎝

⎞

X·RRR

⎟

432

R_Max

⎠

Equation 11

ASC1 is off and ACS2 is off.

VV =

3_Max

CC_Max

Equation 12

ASC1 is off and ACS2 is off.

VV =

3_Min

CC_Min

level definition

STATE

Doc ID 15255 Rev 1 13/29

Resistance settings AN2859

3 Resistance settings

Equation 13 shows how to select values for R1 and R3 resistances. I

AVF _Max

is the maximum

current sunk by the STCC08 AVF pin and should be lower than 5 mA.

Equation 13

V·2

RRR ≥==

31

Knowing the R AVF output electrical dispersion and the DC power supply characteristic, R

CC_Max

I

AVF _Ma x

and R3 resistor standard values, the tolerance of the resistors, the STCC08

1

and R4

2

resistances value should be chosen by using equations 14 , 15 , and 16 (see also

Appendix C).

Equation 14

>⇒

⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

VV

0_Max1_Min

⎡ ⎢

⎛ ⎜ ⎝

>⇒

R

4

⎞

+

⎟

2

AVF_L_Max

⎢

⎠

⎢ ⎣

CC_Min

−

⎛ ⎜

·V·RR

⎜ ⎜ ⎝

AVF_L_Max

X

R_Max

R_Min

⎛ ⎜

·VV

⎜ ⎜ ⎝

X

2

⎞ ⎟

⎟ ⎟ ⎠

R_Max

R_Min

−

V

⎞ ⎟

⎟ ⎟ ⎠

AVF_L_Min

2

⎤ ⎥ ⎥ ⎥

⎦

Equation 15

>⇒

⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪

R

⎪ ⎪ ⎪ ⎪ ⎪ ⎩

VV

1_Max2_Min

2

⎡

⎡ ⎢

2

⎢ ⎢

⎣

<⇒

4

CC_Min

−

VV·R

AVF_L_Max

⎛

X

⎜ ⎜

⎜

X

⎝

CC_Max

R_Max

R_Min

⎤

⎞

⎢

⎥

⎟

−

AVF_L_Max

⎢

⎥

⎟ ⎟

⎢

⎥

⎠

⎣

⎦

2

X

R_Max

R_Min

⎞ ⎟

−

V

⎟ ⎟ ⎠

⎛ ⎜

·V

⎜ ⎜ ⎝

AVF_L_Min

⎛ ⎜

·V·R

⎜ ⎜ ⎝

X

R_Max

R_Min

2

⎞ ⎟

V

−

⎟ ⎟ ⎠

AVF_L_Min

⎤ ⎥ ⎥ ⎥

⎦

Equation 16

>⇒

⎧ ⎪ ⎪ ⎪ ⎪ ⎪

⎪ ⎨

⎪ ⎪ ⎪ ⎪ ⎪ ⎪

⎩

VV

2_Max3_Min

⎡ ⎢

2

⎢ ⎣

>⇒

R

4

CC_Min

−

X

R_Min

·V X

R_Max

CC_MinCC_Max

−

V

AVF_L_Max

·VV·R

X

R_Min

R_Max

⎤ ⎥ ⎥ ⎦

−

R

14/29 Doc ID 15255 Rev 1

AN2859 Detection windows digital value setting

4 Detection windows digital value setting

To detect the state of both AC switches, an MCU analog/digital converter input (ADC) should be used. The conversion result (N and of the MCU voltage reference (V as ideal (see Equation 17).

Equation 17

N =

ADC

V

STATE

V

Ref

N

2·

According to the state of the AC switches, the V value of V

(see equations 1, 2 and 3). This has an impact on the conversion result if the

CC

voltage reference of the ADC transfer function depends directly on V detection levels to implement in the MCU firmware should be determined by taking into account the DC power supply variation with V

23 and 24).

Equation 18

ASC1 is on and ACS2 is on.

⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨

V

AVF _L

N

=⇒

0

V

CC

⎛ ⎜ ⎝

·

⎛ ⎜ ⎝

⎪ ⎪ ⎪

N

0_Max

⎪ ⎪ ⎩

V

=⇒

AVF_L_Max

V

CC_Min

⎞

RR

+

⎟

42

⎠

N

2·

⎞

RR

+

⎟

42

⎠

⎛

+

⎜ ⎝

·

⎛

+

⎜ ⎝

42

) of the V

ADC

). Note that the ADC transfer function is considered

Ref

REF

⎞

X·RR

⎟

R_Max

⎠ ⎞

X·RR

⎟

R_Min

⎠

N

=

2·

signal depends on the ADC size (N)

STATE

signal is not directly dependent on the

STATE

. In this case, the

CC

= VCC (see equations 18, 19, 20, 21, 22,

V

AVF_L_Max

V

CC_Min

X

R_Max

· X

R_Min

N

2·

Equation 19

ASC1 is on and ACS2 is on.

0N

=

0_Min

Equation 20

ASC1 is on and ACS2 is off.

⎧ ⎪ ⎪ ⎪ ⎪ ⎪

⎪ ⎨

⎪ ⎪ ⎪ ⎪ ⎪

⎪

⎩

N

=⇒

1

1_Max

⎡

4

CC

⎢ ⎢ ⎢ ⎢

⎣

⎛ ⎜

CC

⎝

⎡

AVF _L_ Max

⎢ ⎢

+=⇒

RN

4

⎢ ⎢

⎣

AVF _L

V

CC_Min

⎛ ⎜ ⎝

++

RRR·V

⎛ ⎜ ⎝

⎤

⎞

++

RR·VV·R

⎟

21

⎥

⎠

N

⎥

2·

⎞

⎥

⎟

421

⎥

⎠

⎦

⎤

⎞

+

RR·V

⎟

21

⎥

⎠

⎥

·

⎥ ⎥

⎦

N

X·2

R_Max

⎛ ⎜ ⎝

⎞

++

X·RRR

⎟

421

R_Min

⎠

Doc ID 15255 Rev 1 15/29

Detection windows digital value setting AN2859

Equation 21

ASC1 is on and ACS2 is off.

⎧ ⎪ ⎪ ⎪ ⎪ ⎪

⎪ ⎨

⎪ ⎪ ⎪ ⎪ ⎪

⎪

⎩

N

=⇒

1

1_Min

⎡

4

CC

⎢ ⎢ ⎢ ⎢

⎣

⎡ ⎢ ⎢ ⎢ ⎢

⎣

⎛ ⎜

CC

⎝

AVF_L_Min

+=⇒

RN

4

AVF _L

V

CC_Max

⎛ ⎜ ⎝

++

RRR·V

⎛ ⎜ ⎝

⎤

⎞

++

RR·VV·R

⎟

21

⎥

⎠

N

⎥

2·

⎞

⎥

⎟

421

⎥

⎠

⎦

⎤

⎞

+

RR·V

⎟

21

⎥

⎠

⎥

·

⎥ ⎥

⎦

N

X·2

R_Min

⎛ ⎜ ⎝

⎞

++

X·RRR

⎟

421

R_Max

⎠

Equation 22

ASC1 is off and ACS2 is on.

⎧ ⎪ ⎪ ⎪ ⎪ ⎪

⎪ ⎨

⎪ ⎪ ⎪ ⎪ ⎪

⎪

⎩

N

=⇒

2

2_Max

⎡

2

CC

⎢ ⎢ ⎢ ⎢

⎣

⎛ ⎜

CC

⎝

⎡

AVF_L_Max

⎢ ⎢

+=⇒

RN

2

⎢ ⎢

⎣

AVF _L

V

CC_Min

⎛ ⎜ ⎝

++

RRR·V

⎛ ⎜ ⎝

⎤

⎞

++

RR·VV·R

⎟

43

⎥

⎠

N

⎥

2·

⎞

⎥

⎟

432

⎥

⎠

⎦

⎤

⎞

+

RR·V

⎟

43

⎥

⎠

⎥ ⎥ ⎥

⎦

N

X·2

++

R_Max

X·RRR

432

R_Min

·

()

Equation 23

ASC1 is off and ACS2 is on.

⎧ ⎪ ⎪ ⎪ ⎪ ⎪

⎪ ⎨

⎪ ⎪ ⎪ ⎪ ⎪

⎪

⎩

N

=⇒

2

1_Min

⎡

2

CC

⎢ ⎢ ⎢ ⎢

⎣

⎡ ⎢ ⎢ ⎢ ⎢

⎣

⎛ ⎜

CC

⎝

AVF_L_Min

+=⇒

RN

2

AVF _L

V

CC_Max

⎛ ⎜ ⎝

RRR·V

++

⎛ ⎜ ⎝

⎤

⎞

RR·VV·R

++

⎟

43

⎥

⎠

N

⎥

2·

⎞

⎥

⎟

432

⎥

⎠

⎦

⎤

⎞

+

RR·V

⎟

43

⎥

⎠

⎥

·

⎥ ⎥

⎦

N

X·2

R_Min

⎛ ⎜ ⎝

⎞

++

X·RRR

⎟

432

R_Max

⎠

Equation 24

ASC1 is off and ACS2 is off.

NNN >=

2_Max3_Min3_Max

16/29 Doc ID 15255 Rev 1

AN2859 Application example

5 Application example

Table 3. Defined values of the application

Symbol Value Unit

I

AVF _Max

V

CC_Min

V

CC_Max

V

AVF _L_ Min

V

AVF _L_ Ma x

5mA

4.5 V

5.5 V

0V

1 V

N (MCU ADC resolution) 10 bits

The first step is to calculate R1 and R3 resistor values using Equation 13. The second step is to choose the R

Table 4. R1, R2, R3 and R4 resistor values

and R4 resistor values to fulfil equations14 and 15 (see alsoTa ble 4 ).

2

Resistor settings Standard value (5% tolerance)

= R3 > 1.1 kΩ 2.2 kΩ

R

1

R

2

R

4

15 kΩ

6.8 kΩ

The third step is to calculate the window detection levels (see Tab l e 5 ) according to equations 5, 6, 7, 8, 9, 10, 11, 18, 19, 20, 21, 22, 23 and 24. The window detection digital levels will be stored in the MCU firmware to distinguish the state of each AC switch.

Table 5. Detection window values

Analog values (Volts) Equivalent digital values

Windows detection level

Max. Min. Max. Min.

V

0

V

1

V

2

V

3

1.105 0 252 0

2.514 1.154 501 262

4.214 2.545 802 579

5.5 4.5 1024 > 802

Doc ID 15255 Rev 1 17/29

Conclusion AN2859

6 Conclusion

This application note illustrates how designers can diagnose the state of two AC switches with only one single microcontroller ADC input. The way to implement this solution in the application and the external resistor choice is described in this document.

This solution is used to detect the failure modes of two AC switches and to inform the MCU so that appropriate actions to put the system into a safe state can be taken. This function improves the system safety by detecting "diode mode" in both polarities of the AC mains, "short circuit" and "open circuit" of each AC switch independently.

The main benefit of this solution is to reduce the cost of the microcontroller when a platform needs to monitor several AC switches because it requires one less pin.

18/29 Doc ID 15255 Rev 1

AN2859 AC switch state deduction

Appendix A AC switch state deduction

Table 6. AC switch states when IN1 = IN2 = 0

IN1 IN2 V

00V

00

Table 7. AC switch states when IN1 = 0 and IN2 = 1

Toggle between

IN1 IN2 V

01V

01

Toggle between

value ACS1 diagnostic ACS2 diagnostic

STATE

0

1

2

3

V

and V

3

2

and V

V

3

1

and V

V

3

0

V2 and V

V

STATE

V

V3 and V

V

V2 and V

V1 and V

1

0

and V

1

0

value ACS1 diagnostic ACS2 diagnostic

0

1

2

3

and V

3

2

and V

3

1

0

and V

2

1

0

Shorted circuit Shorted circuit

Shorted circuit OFF

OFF Shorted circuit

OFF OFF

OFF Diode mode

Diode mode OFF

Diode mode Diode mode

Diode mode Shorted circuit

Shorted circuit Diode mode

Shorted circuit ON

Shorted circuit OPEN circuit

OFF ON

OFF OPEN circuit

OFF NA

Diode mode OPEN circuit

Diode mode NA

Diode mode ON

Shorted circuit NA

Doc ID 15255 Rev 1 19/29

AC switch state deduction AN2859

Table 8. AC switch states when IN1 = 1 and IN2 = 0

IN1 IN2 V

10V

10

Table 9. AC switch states when IN1 = IN2 = 1

Toggle between

IN1 IN2 V

11V

11

Toggle between

value ACS1 diagnostic ACS2 diagnostic

STATE

0

1

2

3

and V

V

3

2

V3 and V

V

V2 and V

V1 and V

STATE

V

V1 and V

1

and V

3

0

1

0

value ACS1 diagnostic ACS2 diagnostic

0

1

2

3

and V

3

2

and V

3

1

and V

3

0

and V

2

1

and V

2

0

ON Shorted circuited

ON OFF

Open circuit Shorted circuited

Open circuit OFF

Open circuit Diode mode

NA OFF

NA Diode mode

NA Shorted circuit

ON Diode mode

ON ON

ON OPEN circuit

Open circuit ON

Open circuit OPEN circuit

Open circuit NA

NA OPEN circuit

NA NA

NA ON

ON NA

20/29 Doc ID 15255 Rev 1

AN2859 V

signal voltage definition

STATE

Appendix B V

The V linear circuits defined on Figures14, 15, 16 and 17 (according to the state of each AC switch). The voltage resulting (V results are added algebraically. The input current of the MCU A/D conversion block (I neglected.

Figure 14. Equivalent circuit ACS1 and ACS2 are on

STCC08

voltage is defined according to the theorem of superposition applied on the

STATE

AVF

2

AVF

1

STATE

V

CC

R1

R2

V

AVF2_L

V

CC

R3

R4

V

AVF1_L

signal voltage definition

) from each source is calculated separately, and the

STATE

R2

R1

V

STAT E

V

CC

AVF2_L

V

AVF1_L

R4

I

A/D

≈ 0

) is

A/D

MCU

A / D

conversion

V

STAT E

R3

V

CC

Equation 25

R

4

V

STATE

V

==

AVF 1_L

0

V·

AVF 2_L

· R

R

+

2

4

Equation 26

⎛ ⎜ ⎝

=

AVF_L_Max0_Max

·VV

⎛ ⎜ ⎝

⎞

X·RR

+

⎟

42

R_Max

⎠ ⎞

X·RR

+

⎟

42

R_Min

⎠

Equation 27

⎛ ⎜ ⎝

=

AVF_L_Min0_Min

·VV

⎛ ⎜ ⎝

⎞

X·RR

+

⎟

42

R_Min

⎠

⎞

X·RR

+

⎟

42

R_Max

⎠

0

=

Doc ID 15255 Rev 1 21/29

V

STATE

signal voltage definition AN2859

Figure 15. Equivalent circuit ACS1 is on and ACS2 is off

V

STCC08

CC

R1

V

AVF2_H

V

R3

V

AVF1_L

CC

R2

R4

V

STAT E

I

≈ 0

A/D

R2

R1

V

CC

AVF1_L

R4

R3

V

R1

V

CC

AVF

2

AVF

1

STAT E

MCU

A / D

conversion

Equation 28

R

4

VV

==

1STATE

V·

+

CC

RRR

++

124

RR

+

21

V·

AVF1_L

RRR

++

421

Equation 29

⎞

X·RR·VX·R·V

++

⎟

21

R_Max

⎠

V

1_Max

=

CC_Max

⎛

4

⎛ ⎜ ⎝

AVF_L_MaxR_Max

++

⎜ ⎝

⎞

X·RRR

⎟

421

R_Min

⎠

Equation 30

⎛

V

1_Min

CC_Min

=

4

⎛ ⎜ ⎝

AVF _L_ MinR_Min

++

⎜ ⎝

⎞

X·RRR

⎟

421

R_Max

⎠

⎞

X·RR·V.XR·V

++

⎟

21

R_Min

⎠

22/29 Doc ID 15255 Rev 1

AN2859 V

Figure 16. Equivalent circuit ACS2 is on and ACS1 is off

V

CC

R1

STCC08

AVF

2

AVF

1

V

AVF2_L

V

R3

V

AVF1_H

CC

R2

R4

V

STAT E

R1

VCC

V

AVF2_L

R2

Equation 31

R

2

VV

==

2STATE

V·

+

CC

RRR

++

342

RR

+

43

V·

AVF2_L

RRR

++

243

signal voltage definition

STATE

I

≈ 0

A/D

R4

V

R3

V

CC

STAT E

MCU

A / D

conversion

Equation 32

⎞

X·RR·VX·R·V

++

⎟

43

R_Max

⎠

V

2_Max

=

CC_Max

⎛

2

⎛ ⎜ ⎝

AVF_L_MaxR_Max

++

⎜ ⎝

⎞

X·RRR

⎟

432

R_Min

⎠

Equation 33

⎛

V

2_Min

CC_Min

=

2

⎛ ⎜ ⎝

AVF_L_MinR_Min

++

⎜ ⎝

⎞

X·RRR

⎟

432

R_Max

⎠

⎞

X·RR·VX·R·V

++

⎟

43

R_Min

⎠

Doc ID 15255 Rev 1 23/29

V

STATE

signal voltage definition AN2859

Figure 17. Equivalent circuit ACS1 and ACS2 are off

V

STCC08

CC

R1

≈ 0

CC

R4

R3

IA/D

V

AVF

2

AVF

1

V

AVF2_H

V

R3

V

AVF1_H

CC

R2

R4

V

STAT E

R2

R1

V

CC

V

STAT E

MCU

A / D

conversion

Equation 34

RR

+

·VVV =

==

CC3STATE

43

RRRR

+++

4321

·V

+

CC

RR

+

21

V

CC

RRRR

+++

4321

Equation 35

VV =

3_Max

CC_Max

Equation 36

VV =

3_Min

CC_Min

24/29 Doc ID 15255 Rev 1

AN2859 Resistor settings

⎠

⎝

Appendix C Resistor settings

Figure 15, and equations 37, 38 and39 define conditions to identify the state of each AC

switch.

Equation 37

VV >

0_Max1_Min

Equation 38

VV >

1_Max2_Min

Equation 39

VV <

3_Min2_Max

C.1 First case: V

Equations 26 and 30 define respectively V account R the STCC08 AVF output electrical dispersion and the DC power supply characteristic, the condition on R

Equation 40

Equation 41

CC_Min

Equation 42

>

R

4

resistor’s standardized values (see Equation 13), the resistor’s tolerance (XR),

1

and R4 resistors is defined in Equation 42.

2

VV >

0_Max1_Min

4

⎛ ⎜

⎡

⎛ ⎜ ⎝

⎞

⎢

+

⎟

21

⎢

⎠

⎢

⎣

CC_Min

1_Min

AVF_L_MinR_Min

++

AVF_L_Max

−

AVF_L_Max

⎞ ⎟

421

> V

⎛ ⎜ ⎝

X·RRR

R_Max

⎛

X

⎜

R_Max

·V·RR

⎜ ⎜

X

R_Min

⎝

0_Max

++

⎞ ⎟

⎟ ⎟ ⎠

⎛

X

⎜

R_Max

·VV

⎜ ⎜

X

R_Min

⎝

⎞ ⎟

21

⎠

−

V

0_Max

X·RR·VX·R·V

R_Min

AVF_L_Min

⎞ ⎟

⎟ ⎟ ⎠

and V

V

>

AVF_L_Max

⎤ ⎥ ⎥ ⎥

⎦

(see Appendix A). To take into

1_Min

Doc ID 15255 Rev 1 25/29

Resistor settings AN2859

C.2 Second case: V

Equations 29 and 33 define respectively V account R output electrical dispersion and the DC power supply characteristic, the condition on R R

resistors is defined by Equation 46.

4

Equation 43

Equation 44

V

2_Min

Equation 45

V

1_Max

Equation 46

R

4

resistor’s standardized values, the resistor’s tolerance (XR), the STCC08 AVF

3

VV >

1_Max2_Min

CC_Min

=

CC_Max

=

2

⎛ ⎜ ⎝

4

⎛ ⎜ ⎝

⎡ ⎢

2

⎢ ⎢

⎣

<

CC_Min

−

2_Min

AVF_L_MinR_Min

++

VV·R

AVF_L_Max

> V

⎞ ⎟

432

⎠

AVF_L_MaxR_Max

++

X·RRR

R_Max

⎞

X·RRR

⎟

421

⎠

⎛

X

⎜ ⎜

⎜

X

⎝

CC_Max

1_Max

⎛

++

⎜

4

⎝

⎛ ⎜ ⎝

R_Min

R_Max

R_Min

⎛ ⎜

·V

⎜ ⎜

⎝

1_Max

⎞

·XRR·VX·R·V

⎟ ⎠

++

21

2

⎤

⎞

⎥

⎟

⎥

⎟ ⎟

⎥

⎠

⎦

X

R_Max

X

R_Min

and V

R_Min

⎞

X·RR·VX·R·V

⎟ ⎠

−

R_Max

3

2

⎞ ⎟

⎟ ⎟

⎠

2_Min

⎡ ⎢

AVF_L_Max

⎢ ⎢

⎣

V

−

AVF_L_Min

(see Appendix A). To take into

and

2

R_Max

R_Min

⎞ ⎟

⎟ ⎟

⎠

−

V

AVF_L_Min

⎛

X

⎜

·V·R

⎜ ⎜

X

⎝

⎤ ⎥ ⎥ ⎥

⎦

26/29 Doc ID 15255 Rev 1

AN2859 Resistor settings

C.3 Third case: V

Equations 32 and 36 define respectively V account R output electrical dispersion and the DC power supply characteristic, the condition on R R

resistors is defined in Equation 49.

4

Equation 47

Equation 48

CC_Max

Equation 49

R −

>

4

resistor’s standardized values, the resistors’ tolerance (XR), the STCC08 AVF

3

VV <

3_Min2_Max

2

⎛ ⎜ ⎝

⎡ ⎢

2

⎢ ⎣

X

R_Min

·V

CC_Min

X

R_Max

2_Max

AVF_L_MaxR_Max

++

−

CC_MinCC_Max

432

V

−

AVF_L_Max

< V

⎞

X·RRR

⎟ ⎠

X

·VV·R X

⎛ ⎜ ⎝

R_Min

R_Max

3_Min

⎞

++

⎟

43

⎠

⎤ ⎥ ⎥ ⎦

R

3

X·RR·VX·R·V

R_Max

2_Max

and V

V

<

CC_Min

(see Appendix A). To take into

3_Min

and

2

Doc ID 15255 Rev 1 27/29

Revision history AN2859

Revision history

Table 10. Document revision history

Date Revision Changes

08-Dec-2009 1 Initial release.

28/29 Doc ID 15255 Rev 1

AN2859

Please Read Carefully:

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Doc ID 15255 Rev 1 29/29

ST AN2859 APPLICATION NOTE

Specifications and Main Features

Frequently Asked Questions

User Manual

Figure 1. STCC08 block diagram

Contents

1 Multiplexed diagnostics

1.1 Principle

Figure 2. Multiplexed diagnostic schematic of two STCC08

1.2 Failure mode detection of two AC switches

3 Resistance settings

4 Detection windows digital value setting

5 Application example

Table 3. Defined values of the application

Table 4. R1, R2, R3 and R4 resistor values

Table 5. Detection window values

6 Conclusion

Appendix A AC switch state deduction

Table 6. AC switch states when IN1 = IN2 = 0

Table 7. AC switch states when IN1 = 0 and IN2 = 1

Table 8. AC switch states when IN1 = 1 and IN2 = 0

Table 9. AC switch states when IN1 = IN2 = 1

Figure 14. Equivalent circuit ACS1 and ACS2 are on

Figure 15. Equivalent circuit ACS1 is on and ACS2 is off

Figure 16. Equivalent circuit ACS2 is on and ACS1 is off

Figure 17. Equivalent circuit ACS1 and ACS2 are off

Appendix C Resistor settings

Revision history

Table 10. Document revision history