AN1608
APPLICATION NOTE
®
CLT3-4BT6 DEMOBOARD:
CHECK THE ROBUSTNESS OF CLT3-4BT6
CONTENT
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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
The CLT3-4BT6 (Current Limited Termination) is a
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, a serial
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 enabled by a Light Emitting Diode. When
the input current is less than 1.5mA, an integrated
October 2002 - Ed: 1
output circuit derivates the input current to
maintain the opto-coupler off.
When the CLT input voltage VIN is higher than 5V
(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 for more details).
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.
1/7
APPLICATION NOTE
2. CLT3-4BT6 DEMONSTRATION BOARD
2.1. Description of the CLT3-4BT6 demoboard
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.
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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.
The PCB of the demonstration board has the structure described by the figure 1. It has been designed and optimized with the components
presented in the table 1. This configuration gives a
good example of a robust interface design.
Table 1: bill of material.
Reference Nb Part description
Package
J1, J2
2
Rin1, Rin2,
Rin3, Rin4
4
1.2 kΩ, 0.25 W
1206
RC
1
4.7 kΩ, 0.33 W
1206
C1, C2, C3,
C4
4
22 nF, 60 V
1206
R_LED
1
4 resitors array
330Ω, 0.13W
1206
LED 10 mA
1206
LED1, LED2, 4
LED3, LED4
Connector 6 pins 2.54 pitch
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 directly to any type of sensor, especially those specified by the EN60947-5-2 standard.
The output connector gives access to the 4 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
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2/7
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 (between the 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 the CLT3-4BT6 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).
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APPLICATION NOTE
Fig. 1: Electrical diagram of the CLT3-4BT6 demonstration board for reinforced EMC.
Vcc
Vcc2
RC
4.7k
2
Rin1
1.2k
3
1
Vcc
2
4
Rin2
1.2k
3
5
4
6
5
7
Rin3
1.2k
8
6
J1
9
Rin4
1.2k
10
D1
C1
22nF
COM
COM
In1
Out1
ESD12
COM12
IN2
Out2
Vc
COM
ESDc
COM
In3
Out3
ESD34
COM34
In4
Out4
COM
COM
CLT3-4
C2
22nF
C3
22nF
20
1
A1
C1
K1
E1
A2
C2
K2
E2
16
19
2
18
17
3
15
Output
14
1
16
4
15
13
2
Vcc2
3
14
5
A3
C3
12
4
13
6
12
K3
E3
A4
C4
K4
E4
11
5
6
11
7
8
10
J2
9
Opto-coupler
4
3
2
1
Input
U2
U1
1
R_LED
LED1
LED2
LED3
LED4
5
6
7
8
C4
22nF
Fig. 2: Assembly top view of the demonstration board.
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APPLICATION NOTE
3. EMC REQUIREMENTS
3.4. Surge test (according to the IEC61000-4-5)
3.1. Description of the procedure to evaluate
the robustness of the CLT3-4BT6
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, the coupling 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 (B criterion according to the IEC61131-2 standard).
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.
This paper 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 (see table 28: "EMC immunity zones"
of the IEC61131-2-Ed2 standard). The following
paragraphs recall the test levels for this zone.
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 (B criterion according to the IEC61131-2 standard).
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3.5. Conducted disturbance tests (according to
the IEC61000-4-6)
The conducted radio frequency interference test
must be applied on all input pins of 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 ("A" criterion according to the
IEC61131-2 standard).
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3.3. Burst tests (according to the IEC61000-4-4)
The fast 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 (B criterion according to the IEC61131-2 standard).
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3.6. Reverse analog input polarity tests
The test procedure is described by the
IEC61131-2 standard (paragraph 5.4.4.5 of the
Ed2 of the standard). 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.
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
supply connection. 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.
The ambient temperature to take into account is obviously the air temperature close to the component.
The main equation corresponds to the following:
∆Tj − a =Tj −Ta = Pd ⋅ Rthj − a
With:
Tj: junction temperature,
Ta: ambient temperature,
Pd: power to dissipate,
Rthj-a: junction to ambient thermal resistance.
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 ∆Tj-a. To estimate the maximal power
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.
There are 3 main sources origins of the power dissipation: the 2 current limiters embedded in each line
of the CLT device (see figure 3 on page 6), and the
low current consumption of the VC pin (IC).
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Table 2: Immunity of the CLT3-4BT6 device.
Minimum requirements of
international standards
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
Robustness of the CLT3-4BT6 demoboard
Tests conditions
Levels
Tests conditions
Levels
Air discharge
± 8kV
± 8kV
Contact discharge
± 4kV
RC = 4.7kΩ
RIN = 1.2kΩ
RC = 4.7kΩ
RIN = 1.2kΩ
Analog input
± 1kV
RIN = 1.2kΩ
DC power line
± 2kV
RC = 4.7kΩ
± 6kV
C = 22nF
± 4kV
Analog input
42Ω, 5µF
differential and
common mode
± 0.5kV
Analog input
RIN = 1.2kΩ
± 1kV
DC
power line
2Ω, 18µF
differential mode
± 0.5kV
DC power line
RC = 4.7kΩ
± 1kV
12Ω, 9µF
common mode
± 1kV
22nF capacitors
at the output
3VRMS
AM ± 80%
150kHz
to 80MHz
-Vcc applied to one input during 10s
5. THERMAL MANAGEMENT
The CLT3-4BT6 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.
Behavior of the CLT
No failure,
no disturbance
No failure,
no disturbance
No failure,
temporary disturbance
± 1kV
150kHz to
80MHz
RIN = 1.2kΩ
C = 22nF
at the INPUT
10VRMS
AM ± 80%
-30VDC applied to one input,
+30VDC on the others
No failure,
no disturbance
No failure, no cross talk
Then, the maximal power dissipation can be estimated as follow:
P = 4 ⋅[(V CLT ⋅ 0.75 ⋅ I IN ) + (V IN ⋅ 0.25 ⋅ I IN )] + V C ⋅ I C
The worst case scenario occurs when IIN and IC
are maximal. These maximal values are given in
the datasheet:
I IN = 3.7mA
I C = 800µA
5/7
APPLICATION NOTE
Fig. 3: Equivalent circuit of 1 line when the
CLT3-4BT6 limits the current.
In case of a maximal allowed ambient temperature
equal to 100°C, the thermal resistance must be
lower than 95°C/W.
Vcc
Sensor
VCLT
VSENSOR
IIN
0.75 x IIN
RIN x IIN
i1
VIN
+
VOUT
1.5mA
0.25 x IIN
To calculate the drop voltage across the CLT device, the circuit of figure 3 gives the following equations:
V IN =V CC −V SENSOR − RIN ⋅ I IN
V CLT =V IN −V OUT
The worst case scenario corresponds to the maximal supply voltage value (VCC), and to the minimal
drop voltage across the sensor (VSENSOR) and
across the diode of the opto-coupler (VOUT).
For example:
VCC = 30Vmax
VSENSOR = 0Vmin
VOUT = 0.7Vmin
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.
Rth(j-a) (°C/W)
160
140
120
100
80
60
40
20
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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)
⇒ PTOTAL = 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
CLT3-4 runs correctly at this ambient temperature.
But, there is another case to take into account, with
higher power dissipation: spurious reverse supply
connection (see §3.6). In that case, it is possible to
take as a reference the 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:
PTOTAL = 3 ⋅ (V CL ⋅ I IN ) + V CC ⋅ I CC + V D ⋅ I 4
With I4 corresponding to the current in the line with
the reverse connection supply: I4 = I1 + I2 + I3 + IC.
This current flows across the diode, and causes
the drop voltage VD.
⇒ PTOTAL = 528mW
6/7
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
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.
APPLICATION NOTE
7. REFERENCES
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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:
immunity test".
"Electrostatic
discharge
IEC61000-4-4: "Electrical fast transient/burst
immunity test".
IEC61000-4-5: "Surge immunity test".
IEC61000-4-6:
"Immunity
disturbances, induced by
fields".
to
conducted
radio-frequency
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