ST AN1608 APPLICATION NOTE

AN1608

®

CHECK THE ROBUSTNESS OF CLT3-4BT6

CONTENT

DESCRIPTION OF THE CLT3-4BT6 PRODUCT

■

CLT3-4BC6 DEMONSTRATION BOARD

■

EMC REQUIREMENTS

■

ROBUSTNESS AND IMMUNITY OF THE

■

CLT3-4BT6 DEVICE
CONCLUSION

■

REFERENCES

■

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

1/7

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.

■

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

■

- 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

■

- 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

3/7

APPLICATION NOTE

3. EMC REQUIREMENTS

3.1. Description of the procedure to evaluate
the robustness of the CLT3-4BT6

The reference to evaluate the robustness of the
CLT3-4BT6 product is the IEC61131-2 interna­tional standard. This international standard gives
all requirements and conditions of tests that must
be performed on the programmable logic control­lers PLC and their associated peripherals.

Thispaper focuses on the most stressfully tests for
the CLT3-4BT6 product. The immunity of the
CLT3-4BT6 is tested according to the standards.

The IEC61131-2 standard specifies the
Electromagnetic Compatibility (EMC) requirements
and the nature of the tests to perform in order to
determine if the system meets these requirements

paragraph 7: "EMC requirements" and paragraph 8:

(

"EMC type tests and verifications" of the Ed2 of the
standard

). The levels of each test depend on the zone
where the system will be installed. The most typical
industrial environmental levels correspond to zone B:
local power distribution zone and dedicated power
distribution zone (

of the IEC61131-2-Ed2 standard

see table 28: "EMC immunity zones"

). The following

paragraphs recall the test levels for this zone.

3.4. Surge test (according to the IEC61000-4-5)

Since the voltage surge consists in a single but en­ergetic pulse, the CLT3-4BT6 device embeds an
over-voltage protection on each point. The ab­sorbed energy complies at least with the require­ments of the IEC61131-2 standard. The high
energy surge test must be applied on all input pins
of the system. For all analog inputs, the coupling
method is a 47Ω serial resistance and a 0.5µF ca­pacitor.For dc power line, thecoupling is 2Ω, 18µF
with differential mode, and 12Ω, 9µF with common
mode.

The required voltage surge levels are:

analog or dc I/O: 0.5kV (line to line and line to

■

earth coupling modes),
dc power line: 0.5kV (line to line),

■

dc power line: 1kV (line to earth).

■

The PLC system shall continue to operate as in­tended. Temporary degradation of the perfor­mance is acceptable during the test, but the
system must recover by itself after the test (

rion according to the IEC61131-2 standard

B crite-

).

3.5. Conducted disturbance tests (according to
the IEC61000-4-6)

3.2. ESD tests (according to the IEC61000-4-2)

The electrostatic discharge test shall be applied to
operator accessible devices. This means that these
tests have to be performed on each connector pin.

The required levels are:

■

air discharge: +/-8kV

■

contact discharge: +/-4kV

The PLC system shall continue to operate as in­tended. Temporary degradation of the perfor­mance is acceptable during the test, but the
system must recover by itself after the test (

rion according to the IEC61131-2 standard

B crite-

).

3.3.Burst tests (according to the IEC61000-4-4)

Thefast transient burst tests must be applied on all
the input pins of the system. A capacitive
clamp-coupling device (50-200pF) must be used
as described in the IEC61000-4-4 standard.

The required burst voltage levels are:

■

analog or dc I/O: +/-1kV,

■

dc power line: +/-2kV.

The PLC system shall continue to operate as in­tended. Temporary degradation of the perfor­mance is acceptable during the test, but the
system must recover by itself after the test (

rion according to the IEC61131-2 standard

B crite-

).

The conducted radio frequency interference test
mustbe applied onall input pinsof the system.The
frequency range is 150kHz to 80MHz, with a 80%
amplitude modulation by a 1kHz sinusoidal wave.

A CDN (Coupling Device Network) or a current
coupling clamp (as described in the IEC61000-4-6
standard) has to be used to apply the stress to the
system.

The required level is:

■

3Vrms, whatever the tested system input is.

The PLC system shall continue to operate as in­tended. No loss of function or performance is ac­ceptable (

IEC61131-2 standard

"A" criterion according to the

).

3.6. Reverse analog input polarity tests

The test procedure is described by the
IEC61131-2 standard (

Ed2 of the standard

paragraph 5.4.4.5 of the

). A signal of reverse polarity
(negative voltage) for unipolar analog inputs is ap­plied for 10s. The result of this test shall be as
stated by the manufacturer.

Each input of the CLT3-4BT6 device may be bi­ased to a reverse polarity. This case corresponds
to a connection mistake, or a reverse biasing that
is generated by the demagnetization of a moni­tored inductive solenoid.

4/7

APPLICATION NOTE

The involved input withstands the high reverse
current up to 20mA; its opto-coupler is off and pro­tected by the conducting input diode. The other in­puts remain operational, and some extra
dissipation can happen in their clamping
protections.

Considering the supply operation, a reverse block­ing diode can be connected between the module
ground and the common pin COM to protect the
CLT3-4BT6 device against any spurious reverse
supplyconnection. Then, the whole module supply
voltage rating is extended to +/-30V.

The thermal management of this accidental situa­tion, is described in the §5.

4. ROBUSTNESS AND IMMUNITY OF THE
CLT3-4BT6 DEVICE

The

table 2

shows the minimal requirements of the
IEC61131-2 Ed2 international standard. Further­more, it gives an overview of the high immunity
that a CLT3-4BT6 device will ensure to the whole
interface module.

Table 2: Immunity of the CLT3-4BT6 device.

Minimum requirements of

international standards

Tests conditions Levels Tests conditions Levels Behavior of the CLT

ESD test
IEC61000-4-2

Burst test
IEC61000-4-4

Surge test
IEC61000-4-5

Conducted
disturbance test
IEC61000-4-6

Reverse input
polarity test

Air discharge ± 8kV RC= 4.7kΩ

Contact discharge ± 4kV RC= 4.7kΩ

Analog input ± 1kV RIN= 1.2kΩ C = 22nF ±4kV No failure,

DC power line ± 2kV RC= 4.7kΩ

Analog input 42Ω, 5µF

DC

power line

150kHz

to 80MHz

-Vcc applied to one input during 10s -30VDCapplied to one input,

differential and
common mode

2Ω, 18µF

differential mode

12Ω, 9µF

common mode

22nF capacitors

at the output

± 0.5kV Analog input RIN= 1.2kΩ ± 1kV

± 0.5kV DC power line RC= 4.7kΩ ± 1kV

± 1kV ± 1kV

3V

RMS

AM ± 80%

The ambient temperature to take into account is ob­viouslythe air temperatureclose to the component.

The main equation corresponds to the following:

TTTPRth

=−=⋅

∆

ja j a d ja

−−

With:

Tj: junction temperature,

■

Ta: ambient temperature,

■

Pd: power to dissipate,

■

Rth

■

: junction to ambient thermal resistance.

j-a

This paragraph presents the method to evaluate
the dissipated power, and also the evolution of the
Rth value versus the copper surface on the PCB.

The maximal ambient temperature fixes the maxi­mal allowed ∆T

. To estimate the maximal power

j-a

dissipation, it is possible to refer to the parameter of
the datasheet that gives the maximal specification
of the limitation of current. The purpose is to make
the sum of all "thermal supplies" inside the die.
Thereare 3 main sources origins of thepower dissi­pation: the 2 current limiters embedded in each line
of the CLT device (see

figure 3

low current consumption of the V

Robustness of the CLT3-4BT6 demoboard

RIN= 1.2kΩ

RIN= 1.2kΩ

150kHz to

80MHz

+30VDCon the others

RIN= 1.2kΩ

C = 22nF

at the INPUT

± 8kV

± 6kV

10

VRMS

AM ± 80%

on page 6), and the

pin (IC).

C

No failure,
no disturbance

no disturbance

No failure,
temporary disturbance

No failure,
no disturbance

No failure, no cross talk

5. THERMAL MANAGEMENT

TheCLT3-4BT6 device limits the current that flows
across each line. This causes an increase of the
junction temperature. The maximal allowed junc­tion temperature of the CLT3-4BT6 is 150°C.

The TSSOP20 package has a thermal resistance
specified in the datasheet. This parameter allows
the determination of the maximal ambient temper­ature during the operation of the device.

Then, the maximal power dissipation can be esti­mated as follow:

PV IV IVI

=⋅ ⋅ ⋅ + ⋅ ⋅ + ⋅4075 025(.)(.)

[]

CLT ININ INCC

The worst case scenario occurs when IINand I
are maximal. These maximal values are given in

datasheet

the

:

ImA

=37.

IN

IA

=µ800

C

5/7

C

APPLICATION NOTE

Fig. 3: Equivalent circuit of 1 line when the

CLT3-4BT6 limits the current.

Vcc

Sensor

V

SENSOR

R

xI

IN IN

I

IN

V

IN

0.25 x I

V

0.75 x I

IN

CLT

1.5mA

IN

i

1

­+

V

OUT

To calculate the drop voltage across the CLT de­vice,the circuit of

figure3

givesthe following equa-

tions:

VV V RI

=− −⋅

IN CC SENSOR IN IN

VVV

=−

CLT IN OUT

The worst case scenario corresponds to the maxi­mal supply voltage value (V
drop voltage across the sensor (V
across the diode of the opto-coupler (V

), and to the minimal

CC

SENSOR

).

OUT

) and

For example:

■ V

= 30Vmax

CC

■ V

■ V

SENSOR
OUT

= 0Vmin

= 0.7Vmin

⇒ P

TOTAL

= 395mW

In case of a maximal allowed ambient temperature
equal to 100°C, the thermal resistance must be
lower than 127°C/W. Since the maximal Rth value
of the TSSOP20 package (0cm²) is 120°C/W,The
CLT3-4runs correctly at this ambienttemperature.

In case of a maximal allowed ambient temperature
equal to 100°C, the thermal resistance must be
lower than 95°C/W.

The

figure 4

is taken from the datasheet of the
CLT3-4BT6 device. It gives the junction to ambient
thermal resistance as a function of the copper sur­face used as a heatsink (FR4 epoxy PCB, 35µm
for the thickness of the copper). The 95°C/W can
be reached with the use of 1.6cm² of copper sur­face.

Fig. 4: Typical junction to ambient thermal resis­tance versus PCB layout surface for the
TSSOP20.

(°C/W)

R

th(j-a)

160
140
120
100

80
60
40
20

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

S(cm²)

As an example, the copper surface on the demo
board is equal to 1.2cm² allowing operation ambi­ent temperature up to 100°C.

6. CONCLUSION

But,there is anothercase to take into account, with
higher power dissipation: spurious reverse supply
connection (

see §3.6

take as a reference the

). In that case, it is possible to

figure 6

of the

datasheet

that gives typical currents in this case. For exam­ple, referring to current and clamping voltage de­scribed by the datasheet, we can consider:

PVIVIVI

=⋅ ⋅ + ⋅ + ⋅34()

TOTAL CL IN CC CC D

With I
the reverse connection supply: I

corresponding to the current in the line with

4

4=I1+I2+I3+IC.

This current flows across the diode, and causes

⇒ P

.

D

TOTAL

= 528mW

the drop voltage V

6/7

The demonstration board allows the evaluation of
the robustness of the CLT3-4BT6 product against
electromagnetic disturbances.

Thanks to the use of the CLT3-4BT6, the immunity
of a PLC system exceeds the requirement stan­dard levels, providing an extra safety to the appli­cation.

Furthermore,the demo board shows the functional
characteristics of this new product, and gives an
example of application.

The layout has been designed as versatile as pos­sible, and can be used in most industrial or auto­motive applications.

7. REFERENCES

Datasheet of the CLT3-4BT6 product.

■

EN60947-5-2: "Low-voltage switchgear and

■

controlgear - Part 5-2: Control circuit devices
and switching elements - Proximity switches".

IEC61131-2: "Programmable controller; Part2:

■

Equipment Requirements and tests".
IEC61000-4-2: "Electrostatic discharge

■

immunity test".
IEC61000-4-4: "Electrical fast transient/burst

■

immunity test".
IEC61000-4-5: "Surge immunity test".

■

IEC61000-4-6: "Immunity to conducted

■

disturbances, induced by radio-frequency
fields".

APPLICATION NOTE

Informationfurnishedisbelieved to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of
useofsuch information nor for any infringement of patents or other rights of thirdpartieswhich may result from its use. No license is granted by
implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to
change without notice. This publication supersedes and replaces all information previously supplied.
STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written ap­proval of STMicroelectronics.

The ST logo is a registered trademark of STMicroelectronics

© 2002 STMicroelectronics - Printed in Italy - All rights reserved.

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