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 1.2kserial 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 k, 0.25 W 1206
Rin3, Rin4
RC 1 4.7 k, 0.33 W 1206
C1, C2, C3,C44 22 nF, 60 V 1206
R_LED 1 4 resitors array
1206
330, 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).
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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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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 47serial resistance and a 0.5µF ca­pacitor.For dc power line, thecoupling is 2, 18µF with differential mode, and 12, 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.
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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= 4.7k
Contact discharge ± 4kV RC= 4.7k
Analog input ± 1kV RIN= 1.2k C = 22nF ±4kV No failure,
DC power line ± 2kV RC= 4.7k
Analog input 42, 5µF
DC
power line
150kHz
to 80MHz
-Vcc applied to one input during 10s -30VDCapplied to one input,
differential and common mode
2, 18µF
differential mode
12, 9µF
common mode
22nF capacitors
at the output
± 0.5kV Analog input RIN= 1.2k ± 1kV
± 0.5kV DC power line RC= 4.7k ± 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= 1.2k
RIN= 1.2k
150kHz to
80MHz
+30VDCon the others
RIN= 1.2k
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
800
C
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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
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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.
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