Interfacing high voltage applications with low power controllers

Many commercial and industrial applications face the challenge of interfacing low-voltage microcontrollers and digital signal processors (DSPs) to high-voltage sensor switches and other digital, high-voltage circuits. In most cases, feedback is required through these interfaces in the form of binary (1/0, or high/low) status information.

A new generation of interface devices, called digital input serializers (DIS), can sense digital input voltages as low as 6 Vdc and as high as 300 Vdc in the most energy-efficient manner while interfacing with low-power microcontrollers.

This article describes the operation of a digital input serializer and its configuration for low, medium, and high voltage input signals.

How it Works To better understand how the DIS works, the device is examined in the context of a complete interface design (see Figure 1). In general, a high-voltage bus powers a set of sensor switches, S0 – S7, whose on/off states are detected by the device’s eight field inputs, IP0 – IP7. Internal signal processing converts the input signal to a low-volt level and applies it to the input of a parallel-input, serial-output shift register. The internal input data is latched into the shift register due to the microcontroller’s load pulse applied to the /LD input. The microcontroller applies a clock signal to the CLK input, which serially shifts the data from the DIS and into a controller register through a digital isolator, completing the reading of the shift register contents.

High voltage interfaces require the use of digital isolators to electrically isolate the wildly varying ground potentials of remote sensor switches from the local ground of the controller electronics.

Figure 1 Typical structure of a digital input serializer

Some sensor switches suitable for high voltage interfaces include proximity switches, relay contacts, limit switches, push button switches, etc. For high input voltages, the implementation of input resistors RIN0 to RIN7 is necessary to raise the input switching threshold to a higher level, while input resistors are generally not required for low input voltage systems.

Figure 1 shows that a supply voltage of up to 34V can be applied directly to the power supply terminals and eight inputs without the need for protection resistors. With this supply voltage, an internal linear regulator can provide a stable 5V output to power the device's internal circuitry and an external isolator or microcontroller. Another auxiliary feature is an on-chip temperature sensor that alerts the controller when the junction temperature reaches 150oC.

Adjustable input current limit makes it possible to use high voltages up to 34V directly at the device input. For a purely resistive input high voltage interface, the power consumption rises dramatically as the input voltage rises with the increase in input current. In contrast, the DIS input greatly reduces power consumption by limiting the input current to a constant level that can be adjusted using an external precision resistor.

In addition, each channel performs strength and endurance checks on its input signals. This current and voltage detection function has some internal signal thresholds to ensure that the channel is not triggered by leakage current or residual voltage.

In the on-state (switch closed) case, the current comparator detects whether the input current is above a predefined leakage current threshold, while the voltage comparator detects whether the input voltage is above an internally set reference voltage. If both comparator outputs are logic high, the programmable debounce filter checks whether the new change in input state is caused by a noise transient or a true input signal.

In the on state, the filter output is high and the current limiter output is connected to the signal return output (Rex). Each RE-output has a light emitting diode (LED) connected to the ground plane, allowing a visual indication of the sensor switch state. Therefore, if a switch is closed, the LED is on. In the off state (switch open), the filter output is low and the current limiter output is grounded, so the LED is not on.

Input Configuration When configuring the digital input serializer for an application, there are only two important parameters, the input current limit IIN-LIM and the turn-on threshold VIN-ON. Both parameters are adjusted by external resistors RLIM and RIN0 to RIN7. Although RLIM defines the current limit for all eight input channels, the turn-on threshold for each channel can be set individually by using different RIN values.

The current limiter implements a comparator function internally, and its threshold current ITH is exactly the same as the maximum input current IIN-LIM. ITH is derived from the reference current IREF using a current mirror with a reflection coefficient of n = 72. Since IIN-LIM is the same as ITH, the maximum input current can be expressed as:

Equation 1

IREF is in turn calculated from the ratio of the internal 1.25V bandgap reference to the external resistor RLIM:

Equation 2

Plugging Equation 2 into Equation 1 yields IIN-LIM as a function of RLIM:

Equation 3

Solving Equation 3 yields RLIM, the resistor value required to set the desired current limit:

Equation 4

The field input turn-on threshold voltage VIN-ON is related to the current limit, input resistor, and the turn-on threshold voltage VIP-ON of the device input. VIP-ON is equal to the fixed 5.2V reference voltage of the internal voltage detection comparator. Therefore, VIP-ON can be expressed as:

Equation 5

Plugging in the value of VIP-ON and then substituting it into the IIN-LIM calculation in Equation 3 yields:

Equation 6

Then solve for RIN to find the input resistor value required to set the ideal turn-on threshold for a specified current limit:

Equation 7

Therefore, only two equations are needed to fully configure the DIS for various applications, namely, Equation 3 for setting the current limit and Equation 7 for achieving the desired turn-on threshold voltage. Based on these two equations, Table 1 lists various resistor combinations for different input threshold voltages and current limits.

Table 1 Various input configurations

The asterisks in Table 1 indicate very high input voltages that exceed the maximum device voltage of 34 V. In this case, a 30-V Zener diode connected between IPx and ground prevents device input destruction. By setting the switching threshold in the middle of the input voltage range, VIN-ON = VIN-max/2, the maximum Zener current will be equal to the input current limit, IZ-max = IIN-LIM, and the total input current will be twice the current limit.

To save energy, the current limit should be set to 0.5mA. Obviously, at this low input current, it does not make sense to connect the indicator LEDs to the Rex outputs, since they will not light up. Instead, we should place them on the controller side where the CMOS outputs can easily implement the LED driving function.

Serial Interface Figure 1 shows that for bus supplies up to 24V nominal, or 34V maximum, the digital input serializer can regulate the bus voltage down to 5V to provide sufficient power for the digital isolator or microcontroller. However, regulating the bus supply voltage before the DIS under high voltage conditions will greatly reduce the overall power efficiency. In non-isolated applications, using a micro charge pump and providing backup power to the DIS from the controller power supply is more energy efficient. However, in isolated applications, an isolated DC-DC converter is required to provide controller power across the isolation barrier.

The reason for implementing electrical isolation is that digital input serializers are typically used to detect output voltages from remotely mounted sensors and signal sources, such as the output of an AC rectifier, whose ground potential is significantly different from the local controller ground. Connecting the various ground potentials to each other can cause large ground loop currents to flow. Using digital isolators can prevent this from happening.

As mentioned earlier, control of the DIS digital interface is easy to implement. The system controller simply sends a short, low-activity load pulse to the /LD input of the DIS through one of its general-purpose outputs to latch the current field input state into the DIS shift register. Afterwards, it applies a clock signal to the CLK line to shift out the register contents in a serial manner.

The shift register structure of the DIS allows multiple devices to be daisy-chained by simply connecting the serial output SOP of the preceding device to the serial input SIP of the succeeding device, as shown in Figure 2. This approach allows the design of compact digital input modules with high channel counts while using only one serial interface.

When reading the contents of multiple DIS devices at once, a short read cycle time is a basic requirement, and the maximum speed of the standard microcontroller SPI interface can reach 10 MHz or 20 Mbps. However, the serial interface of the DIS can support data rates up to 300 Mbps, which even exceeds the data rate of some high-speed isolators. Therefore, if you want to shorten the read cycle time to an absolute minimum, a very high clock frequency is required, and the propagation delay of the isolator must also be eliminated.

For this reason, microcontrollers are often replaced by field programmable gate arrays (FPGAs), which not only have high clock frequencies, but also allow for the implementation of a receive clock input (shown as the blue line in Figure 2). The same clock signal sent by the FPGA then begins shifting the register contents out of the DIS after being delayed by the isolator, while being fed back through another isolator channel along with the SOP signal, thereby maintaining the phase relationship between the receive clock and data.

Figure 2 Isolated 32-channel digital input module

Digital input serializers are the most versatile solution for interfacing low-power controllers with high DC voltages. The SN65HVS88x series of digital input serializers supports the interface design between low-voltage controllers and high-voltage applications, with a variety of features such as undervoltage detection, current limiting, de-jitter filtering, thermal protection, parity generation, and single 5V power supply.