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 LED. 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 1.2kW 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.
