Engineers within the commercial and industrial arenas often face the tough challenge of interfacing low-voltage microcontrollers and digital signal processors (DSPs) to high-voltage sensor switches and other digital, high-voltage circuitry. In most cases, these interfaces must gain feedback in the form of binary (one/zero, or high/low) status information.
A new generation of devices called digital input serializers (DISs) tackles this thorny issue head on. They can efficiently sense digital input voltages from as low as 6 V dc up to around 300 V dc while interfacing to low-power microcontrollers. This article discusses DIS operation and how to configure the device for low-, medium-, and high-voltage input signals.
To better understand the functional principle of a DIS, it’s best to look at the device in the context of an entire interface design (Fig. 1). A high-voltage bus typically supplies a bank of sensor switches (S0-S7); each switch’s on/off status is detected by the eight field inputs of the device (IP0-IP7). Internal signal processing converts the input signals into low-volt levels and applies them to the inputs of a parallel-in, serial-out shift register.
Upon a load pulse from the microcontroller to the /LD input, the internal input data are latched into the shift register. The microcontroller reads the shift-register content by applying a clock signal to the CLK input. Then the data is serially shifted out of the DIS and, via a digital isolator, into one of the controller registers.
High-voltage interfaces require digital isolators to galvanically isolate strongly varying ground potentials of remote sensor switches from the local ground of the controller electronics. The range of sensor switches applicable to high-voltage interfaces includes proximity switches, relay contacts, limit switches, push-buttons, and many more.
For high-input voltages, the implementation of input resistors RIN0 to RIN7 is necessary to raise the input switching threshold to higher levels. Systems with lower input voltages usually get along without input resistors.
Supply voltages of up to 34 V can be applied directly to the supply terminal and the eight input terminals without the need for protection resistors (Fig. 1, again). In the case of the supply voltage, an internal linear voltage regulator provides a smooth 5-V output to the device’s internal circuitry as well as external isolators or microcontrollers. Another auxiliary function, the on-chip temperature sensor, indicates an alarm condition to the controller should the junction temperature reach 150°C.
Direct application of up to 34 V at the device inputs is tolerable thanks to adjustable input current-limiting. Generally, high-voltage interfaces with purely resistive inputs consume significantly more power with rising input voltage due to rising input current. In comparison, the inputs of a DIS dramatically reduce power consumption by limiting the input current to a constant level, which can be adjusted with an external precision resistor.
In addition, each channel checks its input signal for strength and stamina. The current- and voltage-detect functions possess internal signal thresholds to ensure that leakage current or residual voltage won’t trigger a channel.
In the case of an “on” condition (switch closed), a current comparator detects whether the input current is higher than a pre-defined leakage threshold, and a voltage comparator checks whether the input voltage is higher than an internally fixed reference voltage. If both comparator outputs are logic high, a programmable debounce filter checks whether the new change in input status is caused by a noise transient or by a true input signal.
For an on condition, the filter output is high and the current-limiter output connects to a signal-return output (REx). Each RE-output allows a light-emitting diode (LED) to connect to ground for the visible indication of a sensor switch’s status. Thus, if a switch is closed, the LED is turned on. For an “off” condition (switch open), the filter output is low and the current-limiter output switches to ground, keeping the LED turned off.
When configuring a DIS for a certain application, only two parameters are of interest: the input current limit (IIN-LIM), and the on threshold (VIN-ON). Both parameters are adjusted via the external resistors RLIM and RIN0 to RIN7. While RLIM defines the current limit for all eight input channels, the on thresholds can be individually set for each channel by using different values for RIN.
Internally, the current limiter performs a comparator function whose threshold current (ITH) is identical to the maximum input current (IIN-LIM). ITH is derived from a reference current (IREF) via current mirrors with a mirror ratio of n = 72. Because IIN-LIM is identical to ITH, the maximum input current can be expressed by:
IREF, in turn, is determined by the ratio of an internal 1.25-V bandgap reference to the external resistor RLIM:
Inserting Equation 2 into Equation 1 provides IIN-LIM as a function of RLIM:
Solving Equation 3 for RLIM supplies the resistor value necessary to set the desired current limit:
The on-threshold voltage at the field input (VIN-ON) is a function of the current limit, the input resistor, and the on-threshold voltage at a device input (VIP-ON). VIP-ON equates to the fixed, 5.2-V reference voltage of the internal voltage-detect comparator. Thus, VIP-ON\\] can be expressed through:
Inserting the numerical value for VIP-ON and substituting IIN-LIM with Equation 3 yields:
Subsequently, solving for RIN provides the input resistor value required to set the desired on threshold at a specified current limit:
As a result, only two equations are required to completely configure the DIS for a variety of applications: Equation 3 to set the current limit and Equation 7 to achieve the desired on-threshold voltage. Based on these two equations, there is a variety of potential resistor combinations for various input threshold voltages and current limits (see the table).
The asterisks in the table indicate that very high input voltages can exceed the maximum device voltage of 34 V. In this case, a 30-V Zener diode connected between IPx and ground prevents a device input from destruction. When placing the switching threshold in the middle of the input voltage range (VIN-ON = VIN-max/2), the maximum Zener current will equal the input current limit (IZ-max = IIN-LIM) and the total input current will be twice the current limit.
To preserve power, the current limit is set to 0.5 mA. Clearly, when an input current is this low, it makes no sense to connect indicator LEDs to the REx output since they won’t turn on. Instead, they should be placed at the controller side, where CMOS outputs can easily perform the LED drive function.
For bus supplies up to 24 V nominal, or 34 V maximum, digital input serializers can regulate the bus voltage down to 5 V to provide sufficient supply for digital isolators or microcontrollers (Fig. 1, again). At high voltages, though, down-regulating the bus supply prior to the DIS drastically reduces overall power efficiency.
In the case of a non-isolated application, it’s more efficient to use a tiny charge pump and back-supply the DIS from the controller supply. For isolated applications, however, an isolated dc-dc converter is required to deliver the controller supply across the isolation barrier.
Galvanic isolation is required because digital input serializers are commonly used to sense the output voltages of remote located sensors and signal sources. These include the outputs of ac rectifiers, where the ground potentials might differ significantly from the local controller ground. Various ground potentials connected to one another will cause extensive ground loop currents to flow. Adding digital isolators prevents this from happening.
As previously mentioned, controlling a DIS’s digital interface is easy. The system controller simply sends a short, low-active load pulse via one of its general-purpose outputs to the /LD input of the DIS, which will latch the present field input status into the DIS shift register. Afterwards, it applies a clock signal to the CLK line to serially shift out the register content.
Thanks to the shift register structure of a DIS, multiple devices can be daisy-chained by simply connecting the serial output (SOP) of a leading device to the serial input (SIP) of a following device (Fig. 2). Thus, it becomes possible to design compact digital input modules with high channel counts using only one serial interface.
When reading the contents of multiple DIS devices, short read cycle times are vital. In such a case, the serial peripheral interface (SPI) of standard microcontrollers typically maxes out at just 10 MHz or 20 Mbits/s.
The serial interface of a DIS, however, can support data rates of up to 300 Mbits/s, which even exceeds the data rates of high-speed isolators. Therefore, to reduce the duration of a read cycle to an absolute minimum, significantly higher clock rates are required. It’s also imperative to eliminate the isolator’s propagation delay.
That’s why FPGAs usually replace microcontrollers. FPGAs provide high clock rates and enable the implementation of a receive clock input (blue trace in Figure 2). Then the clock signal that’s sent by the FPGA, which is delayed by the isolator and starts to shift the register content out of the DIS, is fed back through another isolator channel simultaneously with the SOP signal. This maintains the phase relation between receive clock and data.
Overall, a digital input serializer represents one of the most versatile solutions for interfacing a low-power controller to high dc voltages. One particular example is the SN65HVS885 digital input serializer, which provides undervoltage detection, current-limiting, debounce filtering, thermal protection, parity-generation, and a single 5-V supply.
Digital Input Serializer Promotes High-Channel Density Input Modules, Kugelstadt, Thomas, Texas Instruments,Industrial Control Design Line; June 2008.
SN65HVS880 User’s Guide (slau271), Texas Instruments; December 2008.