Description

AN3031

6/28

Doc ID 16144 Rev 1

2.1

SCLT3-8BT8

The SCLT3-8BT8 (current limited termination) is an eight-channel digital input termination

device designed for 24 VDC automation applications. Typical applications for this device

include PLCs (programmable logic controllers), PACs (programmable automation

controllers), distributed I/O systems, etc. The device works as an interface for mechanical

switches, relay contacts and two-wire or three-wire digital sensors (also known as proximity

switches).

Available in eight-channel configuration, it offers a high-density termination by minimizing

the external component count. It is housed in a HTSSOP-38 surface mount package to

reduce the printed circuit board size.

The chip consists of a parallel input voltage protection circuit, a current limiting circuit which

regulates the channel current and feeds the indicator LEDs, a digital filter to prevent glitches,

a voltage regulator and SPI communication peripheral.

The SCLT3-8BT8 device is connected between the sensors and the application

microcontroller, FPGA or ASIC. The current limiting circuit is compensated over the entire

temperature range. Thanks to its low tolerance, the current limitation allows drastic reduction

of dissipation compared to a resistive input (discrete solution) - the overall module requires

less cooling capability and is smaller.

Thanks to integrated SPI communication, the solution reduces the number of lines which

typically need to be galvanically isolated, making the application more cost-effective. In a

16-bit input interface, conventional solutions require 16 isolators to decouple the control part

from an industrial environment. With the SCLT3-8BT8, the two-chip solution (16-bit

interface) connected in a daisy chain configuration requires only 3 isolators. In addition to

the input data, it also provides device status information including thermal alarm,

undervoltage indication, and others. Parity check bits for higher reliability are also included

in the frame extension.

The SCLT3-8BT8 device principal block diagram and recommended application structure is

provided in Figure 2 and Figure 3.