How does the LTC2874 provide greater current to the SIO load while maintaining IO-Link characteristics and functionality?

Introduction

IO-Link is a communication standard for point-to-point three-wire interfaces for smart sensors and actuators in industrial applications. IO-Link extends the traditional interface capabilities of these devices from a simple NC/NO switch interface (standard IO or SIO mode) to a two-way intelligent interface that can pass three different speeds (COM1 - 4.8kb/s, COM2 - 38.4) Kb/s or COM3 — 230.4kb/s) One of the code conversions to send additional information. In addition to the data pins (C/Q), the IO-LinkType A interface has a 24VDC power supply pin (L+) and a common return pin (L–).

 

When an IO-Link master powers up, it asks each connected device to determine the correct operating mode for each device: SIO, COM1, COM2, or COM3. This allows legacy legacy devices and devices that support the IO-Link standard to work seamlessly in the same system.

 

The LTC2874 has a rated CQ output current of 110mA. Higher currents of up to 440mA can be achieved by connecting multiple channels in parallel. While this exceeds the IO-Link standard specification limits, some non-standard SIO applications may require a larger current supply and/or the need to maintain the functionality of four independent channels. This article shows how to change the use of the LTC2874 hot-swap channel to provide a large current for the SIO load (called SIO+ mode) while maintaining the IO-Link features and functionality of the LTC2874.

Figure 1 : LTC2874   Four channels   IO-Link master configuration having three high current  

SIO port (SIO +) and L + 1 with hot-plugging current standard port SIO

Circuit description

As shown   As shown in ports 1–3 of 1, by connecting the hot-swap controller output of the channel to its corresponding CQ pin,   Any large supply current is provided in SIO+ mode. For high current ports, L+'s hot-swap capability is not available; however, for those applications that expect this feature, an external hot-swap controller can be added. As shown in Figure 1   4, not used for SIO output   The LTC2874 hot-swap controller can be used for normal L+ or other purposes.

In normal   In IO-Link or SIO operation, L+   The MOSFET is turned off and the CQ output is passed   TXEN, TXD and   RXD works fine. All IO-Link functions are maintained, including full speed communication and wake-up pulses at COM3 speed.

 

During SIO+ operation, the L+ MOSFET is controlled through the SPI register interface and CQ is disabled (TXEN is low or under SPI register control). The upper half of Register 0xE is responsible for controlling the L+ MOSFET. In SIO+ mode, the switching frequency is limited to approximately COM1 speed.

 

although   LTC2874 in CQ and   The L+ output will not be damaged while it is in operation, but it is not recommended to perform this mode of operation because the rising and falling trajectories of the output waveform are non-monotonic. These traces occur due to timing differences between the channels and the interaction of various current limits and source resistances.

 

The maximum output current of SIO+ mode is determined by the choice of MOSFET and sense resistor RS. The current limit is set by 50mV/RS. The typical current limit for the circuit in Figure 1 is 500mA. Taking into account tolerances and variations, a 400mA port rated output current is generated. The appropriate MOSFET must be chosen to handle voltage, current, and safe operating area (SOA) requirements. See the LTC2874 product brochure for more details.

 

MOSFET output capacitance   The 1nF maximum allowed by the IO-Link standard contributes approximately   60pF.

 

Since the circuit has two drivers in parallel, the idle drive acts as a capacitive load on the working drive. When the working drive changes state, it will generate a charging current in the idle drive. Due to the large capacitance of the MOSFET   The faster edge rate of the CQ drive, so this effect is more pronounced during IO-Link operation. To prevent the charging current pulse from ringing when the working driver is turned off, minimize the MOSFET source and   Parasitic inductance between C/Q driver outputs.

 

Figure   2 and Figure 3 show a single support   SIO+ mode port drives a resistive load, working simultaneously on SIO+ or normal   Operating waveform in IO-Link mode. The supply voltage is 24V and the resistive loads are 56Ω and 200Ω, respectively.

 

Figure 2: SIO+ operation

  Figure 3 : IO-Link operation (C/Q) for a port that supports SIO+ mode   At COM3   Speed )