ST AN1608 APPLICATION NOTE

AN1608

®

CHECK THE ROBUSTNESS OF CLT3-4BT6

CONTENT

DESCRIPTION OF THE CLT3-4BT6 PRODUCT

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CLT3-4BC6 DEMONSTRATION BOARD

■

EMC REQUIREMENTS

■

ROBUSTNESS AND IMMUNITY OF THE

■

CLT3-4BT6 DEVICE
CONCLUSION

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REFERENCES

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1. DESCRIPTION OF CLT3-4BT6 PRODUCT

1.1. Functional description

TheCLT3-4BT6(CurrentLimitedTermination)isa
quadruple input digital termination device
designed for 24V DC automation applications. It
achieves the front-end circuitry of a digital input
module (I/O) in industrial automation.

Available in a four channels configuration, it offers
a high-density termination by minimizing the exter­nal component count. It is housed in a TSSOP20
surface mount package to reduce the printed
board size.

Made of a parallel input voltage protection, aserial
input-output current limiting circuit and an
opto-coupler driver, each channel circuit
terminates the connection between the logic input
and the associated high side sensor or switch.

The CLT3-4BT6 device is used between the sen­sors and the opto-coupler of an input module. The
current limiting circuit, connected between the in­put and the output pins, is compensated all over
the temperature range. Furthermore, each chan­nel runs independently of the other. Thanks to its
low tolerance, the current limitation allows the
drastic reduction of the dissipation compared to a
resistive input: the overall module requires less
cooling capability and becomes smaller.

The output block of each termination channel
controls the operation of an opto-coupler that is
internally enabledbyaLightEmittingDiode.When
the input current is less than 1.5mA, an integrated

APPLICATION NOTE

CLT3-4BT6 DEMOBOARD:

output circuit derivates the input current to
maintain the opto-coupler off.

When the CLT input voltage V
(that corresponds to a module input voltage higher
than 11V with a 1.2kΩ serial resistor), a minimum
output current of 1.5mA secures the opto-coupler
in the on state.

The CLT3-4BT6 protects the input module against
transient electromagnetic interferences such as
those described in the IEC61131-2 standard.

The opto-coupler plays a role in the CLT3-4BT6
operation. The drop voltage of its input diode
introduces a voltage offset in series with the
CLT3-4BT6 channel: for a good CLT3-4BT6
operation, this drop voltage should remain below
2V (see

CLT3-4BT6 datasheet

1.2. Application requirements

A reverse blocking diode is connected between
the module ground connection and the common
pin COM of the CLT3-4BT6 device to protect the
module against spurious reverse supply
connection. This diode also protects the
CLT3-4BT6 device against negative surge
voltages.

An external output capacitor is placed either at the
input or the output of the CLT3-4BT6 to filter the
transient disturbances injected in the inputs of the
module and secure the immunity of the module
itself.

All immunity requirements are described by the
IEC61131-2 international standard.

is higher than 5V

IN

for more details).

October 2002 - Ed: 1

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APPLICATION NOTE

2. CLT3-4BT6 DEMONSTRATION BOARD

2.1. Description of the CLT3-4BT6 demoboard

This CLT3-4BT6 demonstration board allows the
evaluation of the function of the CLT3-4BT6
product. It can be easily inserted in a real
application, between sensors and digital bus
controller. The output LEDs give the state of each
line of the CLT3-4BT6, according to the state of
each input.

ThePCBofthedemonstration board hasthe struc­ture described by the

figure 1

. It has been de-

signed and optimized with the components

table 1

presented in the

. This configuration gives a

good example of a robust interface design.

Table 1: bill of material.

Reference Nb Part description Package

J1, J2 2 Connector 6 pins 2.54 pitch

Rin1, Rin2,

4 1.2 kΩ, 0.25 W 1206

Rin3, Rin4

RC 1 4.7 kΩ, 0.33 W 1206

C1, C2, C3,C44 22 nF, 60 V 1206

R_LED 1 4 resitors array

1206

330Ω, 0.13W

LED1, LED2,

4 LED 10 mA 1206

LED3, LED4

U1 1 CLT3-4BT6 TSSOP20
U2 1 TLP281-4 SO-16

The copper surface under the CLT device im­proves the thermal dissipation capability of the
TSSOP20 package. The copper area on the PCB
demoboard is around 1.2cm², and makes it possi­ble to decrease the Rth of the package to a value
lower than 100°C/W.

2.2. Operating instructions of the CLT3-4BT6
demo-board

This paragraph provides some basic advices to
implement properly the demonstration board in or­der to evaluate the CLT3-4BT6 product.

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Inputs / Outputs

The input connector gives access to the 4 input
signals of the module (In1, 2, 3 and 4), and the
power supply access of the CLT3-4BT6 (Vcc and
Com). It is then easy to connect this module di­rectlyto any type ofsensor, especially those speci­fied by the EN60947-5-2 standard.

Theoutput connector givesaccess to the4 outputs
of the module (output of the opto-coupler, Out1, 2,
3 and 4). Furthermore, this connector is also used
to connect the output power supply (Vcc2 and

Com2). It is then easy to connect the output of the
module directly on a digital bus controller.

Power supply

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- Vcc power supply

The Vcc power supply shall be a 24Vdc voltage
supply. Actually, the CLT3-4BT6 device needs a
10 to 30Vdc voltage to work.

- Vcc2 power supply

The output power supply can be a 5Vdc voltage
supply. This voltage supplies the 4 collectors of
each opto-coupler output.

NOTE: Do not short the opto-coupler output to
ground. This may result in the destruction of the
output transistor of this opto-coupler.

Discrete components

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- Discrete capacitors:

Some 22nF capacitors are used in order to im­prove the noise immunity of the whole module and
to filter the high frequency electrical noise in the off
state.

They can be placed at the output of the CLT (be­tweenthe CLT device and the opto-coupler) as de­scribed in the

datasheet

. This configuration meets
the minimum level of immunity described in the
IEC61131-2 standard. The configuration corre­sponding to capacitors placed at the input of the
CLT device (between input resistors and the CLT
device), as in the demonstration board topology,
allows an increase of this immunity. However, the
normal operation of the module remains within
specification whatever the position of the capaci­tors is.

- Input resistors:

The input resistors are used in order to limit the
current that could appear in case of voltage surge
clamped by the CLT3-4BT6. These resistors shall
then withstand the high overvoltage that may be
applied to the module during surge tests.

- LEDs resistors:

The value of these resistors shall be set according
to the input power supply value used, and the nor­mal current of displaying LEDs. A resistor array
can be used to control these low power LEDs (re­ducing of the PCB size).

- Diode D1:

The diode used between the COM of the input
power supply, and the COM of theCLT3-4BT6 de­vice can be a general purpose component as a
1000V 1A rectifier.

■

Opto-coupler

The opto-coupler should be chosen according to
its input diode drop voltage and should fit the wide
range output voltage of the CLT3 output (0.2V).

2/7

APPLICATION NOTE

Fig. 1: Electrical diagram of the CLT3-4BT6 demonstration board for reinforced EMC.

Vcc2

16

C1

15

E1

14

C2

13

E2

12

C3

11

E3

10

C4

9

E4

Vcc

Vcc

RC

4.7k
U1

1

COM

C4
22nF

2

In1

3

ESD12

4

IN2

5

Vc

6

ESDc

7

In3

8

ESD34

9

In4

10

COM

CLT3-4

Input

1

2
3
4

5

6

J1

Rin1

1.2k

Rin2

1.2k

Rin3

1.2k

Rin4

1.2k

D1

C1
22nF

C2
22nF

C3
22nF

COM
Out1

COM12

Out2
COM
COM
Out3

COM34

Out4
COM

20
19
18
17
16
15
14
13
12
11

U2

1

2

3

4

5

6

7

8

A1

K1

A2

K2

A3

K3

A4

K4

Opto-coupler

45

678

Output

1
2

Vcc2
3
4

5
6

J2

123

R_LED

Fig. 2: Assembly top view of the demonstration board.

LED4

LED1

LED2

LED3

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