In Industrial 4.0 applications, smart sensors are becoming increasingly prevalent in factory environments. In newly constructed automation systems in some factories, the use of sensors has significantly increased. However, the widespread use of sensors has also brought about an important change: the need to handle a large amount of IOs in these legacy controllers, which requires the development of high-density IO modules with size and heat constraints. In this article, I will focus on the design considerations for digital IOs modules, as well as the features and functions of the solutions offered by Maxim Integrated (now part of ADI).
Transition from discrete to integrated solutions in PLCs
Traditionally, digital IOs in programmable logic controllers (PLCs) has been composed of discrete components, such as resistors/capacitors or individual FET drivers. The demand to minimize the size of controllers while being able to handle 2 to 4 times the number of channels has driven the shift from discrete solutions to integrated solutions.
The disadvantages of the discrete approach are quite apparent, especially when each module handles 8 or more channels. High heat/power dissipation and the need for a large number of discrete components make this approach impractical, considering factors such as size, mean time between failures (MTBF), and rugged system specifications.
When building systems with high-density digital input (DI) and digital output (DO) modules, both size and heat dissipation must be considered. For digital input, support for different input types, including types 1/2/3, is required, and in some cases, both 24V and 48V inputs must be supported. Robust operation characteristics are crucial in all cases, and sometimes open circuit detection is also essential. For digital output, the system uses different FET configurations to drive the load, with the precision of driving current being an important consideration. Moreover, diagnostic functions are often critical in many cases.
Considerations for designing high-density digital input/output modules
Traditional discrete designs use resistor divider networks to convert 24V/48V signals into signals usable by a microcontroller, and a discrete RC filter can also be used at the front end. If isolation is needed, an external optocoupler is sometimes employed.
A typical discrete approach to building digital input circuits is often used for a certain number of digital inputs, typically 4 to 8 per board. Beyond this number, such designs quickly become impractical. This discrete solution introduces various problems, including high power consumption and associated board hot spots, the need for an optocoupler per channel, and too many components leading to lower FIT rate or even requiring larger form factor devices. More importantly, the discrete design approach means that the input current increases linearly with the input voltage. Assuming a 2.2KΩ input resistor and 24V VIN, when the input is high, such as at 24V, the input current is 11mA, corresponding to a power consumption of 264mW. A typical 8-channel module would consume more than 2W, and a 32-channel module more than 8W.
From a thermal perspective alone, this discrete design cannot support multiple channels on a single board. One of the greatest advantages of an integrated digital input design is the significant reduction in power consumption, which reduces heat dissipation. Most integrated digital input devices allow configurable input current limits to drastically reduce power consumption. When the current limit is set to 2.6mA, power consumption is significantly reduced, with each channel consuming approximately 60mW, and the power rating for an 8-channel digital input module can now be set to below 0.5W.
Another reason to avoid discrete logic design is that DI modules sometimes need to support different types of inputs. The IEC published standard 24V digital input specifications are classified into Types 1, 2, and 3. Types 1 and 3 are usually combined since their current and threshold limits are very similar. Type 2 has a 6mA current limit, which is much higher. When using the discrete method, a redesign might be necessary, as most discrete values need to be updated.
However, integrated digital input products generally support all three types. In essence, Types 1 and 3 are generally supported by integrated digital input devices. However, to meet the Type 2 input minimum current requirement of 6mA, two channels need to be paralleled for one field input, and only the current-limiting resistor needs to be adjusted, requiring minimal board changes.
A typical discrete digital output design has a FET with a driving circuit driven by the microcontroller, and different methods can be used to configure the FET to drive the microcontroller.
A high-side load switch is defined as one that must be controlled by an external enable signal and connects or disconnects power to a given load. Compared to a low-side load switch, the high-side switch sources current to the load, while the low-side switch connects or disconnects the ground connection to the load, and therefore sinks current from the load. While both use a single FET, the problem with a low-side switch is that the load could shorted to the ground. The high-side switch protects the load from ground shorts, but the low-side switch is cheaper to implement. Sometimes, output drivers are also configured as push-pull switches, requiring two MOSFETs.
Integrated DO devices can integrate multiple DO channels into a single device. Since high-side, low-side, and push-pull switches use different FET configurations, different devices can be used to implement each type of an output driver.
Built-in demagnetization function for inductive loads
One of the key advantages of integrated digital output devices is that they come with a built-in demagnetization function for inductive loads. An inductive load refers to any device that has coils of wire, which typically performs some mechanical work after being powered, such as solenoids, motors, and actuators. The magnetic field generated by the current can move the switching contacts in a relay or contactor to operate the solenoid valves, or rotate a shaft in a motor. In most industrial applications, engineers use high-end switches to control the inductive load. The challenge lies in how to discharge the inductor when the switch opens and current no longer sourced to the load. Improper discharge can lead to negative effects, including relay contacts possibly arcing, large negative voltage spikes damaging sensitive ICs, and the generation of high-frequency noise or EMI, which can further affect system performance.
In discrete solutions, the most common method for discharging the inductive load is by using a free-wheeling diode. In such circuits, when the switch is closed, the diode is reverse biased and not conduct any current. When the switch opens, the negative voltage through the inductor will forward biases the diode, allowing the stored energy to dissipate by directing the current through the diode until a steady state is reached and the current is zero.
When selecting a digital output device, several important factors need to be considered. Key specifications in the datasheet should be carefully reviewed, such as checking the maximum continuous current rating and ensuring the possibility of parallel multiple outputs when necessary to achieve a higher current drive, ensuring the output device can drive multiple high-current channels (over the temperature range), and ensuring that the on-resistance, supply current, and thermal resistance values are as low as possible. Additionally, output current drive accuracy specifications are crucial.
On the other hand, diagnostic information is critical for recovering from some out-of-range operating conditions. First, you would want to obtain diagnostic information for each output channel, including temperature, overcurrent, open circuit, and short circuit. From an overall (chip-level) perspective, important diagnostics include thermal shutdown, VDD undervoltage, and SPI diagnostics.
High-side switch/push-pull driver not limited by inductance
For many applications, particularly in the industrial sector where each I/O card has multiple output channels, the diode is usually quite large, leading to significant increases in cost and design size. Modern digital output devices use an active clamping circuit to achieve this function within the device. For example, Maxim Integrated uses a patented safe demagnetization (SafeDemag™) feature, which allows digital output devices to safely handle "unlimited inductance" 24V DC loads without being limited by inductance.
For example, Maxim Integrated's MAX14912/MAX14913 features eight 640mA smart high-side switches, which can also be configured as push-pull drivers for high-speed switching. The propagation delay from input to the high-side/low-side driver switch is 1µs (maximum). Each high-side driver has an on-resistance as low as 230mΩ (maximum) at TA = 125°C and a load current of 500mA.
The MAX14912/MAX14913 devices can be configured and controlled via pins or SPI interface. The SPI interface uses a daisy-chainable connection, effectively allowing multiple devices to be cascaded. SPI also supports command mode for the most detailed diagnostic information. The MAX14912 supports configuration via SPI in either parallel or serial setting mode, while the MAX14913 only supports configuration via SPI in serial setting mode.
The open-load detection function in high-side mode detects openwire conditions when the switch is on/off states, and the LED driver indicates fault and status conditions for each channel. An internal active clamps circuit accelerates the fast shutdown of inductive loads in high-side mode.
The MAX14912/MAX14913 comes in a 56-pin, 8mm x 8mm, QFN package and can be applied in building automation, industrial digital output, PLC systems, and other fields.
Conclusion
When designing high-density digital input or output modules, discrete solutions become impractical once the channel density exceeds a certain threshold. From the perspective of heat dissipation, reliability, and size, an integrated device options must be carefully considered. When selecting integrated DI or DO devices, it is important to pay attention to several key data points, including robust operating characteristics, diagnostics, and support for multiple input-output configurations. Maxim Integrated's MAX14912/MAX14913 meets these requirements and is suitable for the design of high-density digital input/output modules, making it an ideal choice for developing related applications.