ST AN1608 APPLICATION NOTE

	AN1608
®	APPLICATION NOTE

CLT3-4BT6 DEMOBOARD: 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

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

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.

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.

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	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,	4	22 nF, 60 V	1206
C4
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 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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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 (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).

APPLICATION NOTE

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

			Vcc			Vcc2
			RC
			4.7k		U2
			U1
			1	20	1	16
			COM	COM	A1	C1
Input	Rin1		2	19
	1.2k		In1	Out1	2	15
1			3	18	K1	E1
Vcc			ESD12	COM12						Output
2	Rin2		4	17	3	14
	1.2k		IN2	Out2	A2	C2			1
3			5	16
			Vc	COM	4	13			2	Vcc2
4			6	15	K2	E2
			ESDc	COM						3
5	Rin3		7	14	5	12
	1.2k		In3	Out3	A3	C3				4
6			8	13
			ESD34	COM34	6	11			5
J1	Rin4		9	12	K3	E3
	1.2k		In4	Out4					6
			10	11	7	10
			COM	COM	A4	C4				J2
		C1
	D1
		22nF	CLT3-4		8	9
					K4	E4
			C2
			22nF
			C3		Opto-coupler			4 3 2 1
			22nF
			C4						R_LED
			22nF
								5 6 7 8
						LED4	LED3	LED2	LED1

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

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