SLYY242 November   2024

 

  1.   1
  2.   Overview
  3.   At a glance
  4.   Introduction
  5.   Safety considerations and potential failures in power-supply designs
  6.   Introduction to functional safety and standards in industrial systems
  7.   Voltage monitoring using voltage supervisor ICs
  8.   How voltage supervision affects functional safety ratings
  9.   Safe torque-off design example
  10.   Conclusion

Voltage monitoring using voltage supervisor ICs

There are many methods for monitoring the voltage. Additionally, which voltage you are choosing to monitor can also vary. In any industrial application, you may need to monitor voltages as high as 48V or as low as 0.8V for overvoltage or undervoltage conditions. Thankfully, there are effective methods to monitor important voltage rails in your system that can enable a few aspects of any functionally safe design. With accurate voltage supervision, you’ll know when to completely shut off a system, reset an MCU, or make another system-level choice to achieve the safe state. Without constant monitoring of the safety related voltage rails, the system cannot take action in the event of potentially dangerous situation.

There are ways to design a voltage monitoring circuit using discrete components, but in a functional safety-focused system, it becomes much easier to determine the diagnostic coverage if the voltage monitoring functionality is integrated into one sub-system circuit. That is why voltage supervisor ICs are especially helpful for functional safety – they include different combinations of threshold accuracy, quiescent current, reset time delay, latching capability, voltage hysteresis, output type and BIST.

Table 4 lists some voltage supervisor parameters and features.

Table 4 Important voltage supervisor parameters.
Parameter or feature Description
Threshold accuracy The accuracy percentage around the nominal threshold voltage.
Maximum input voltage The maximum voltage that the device can monitor.
Quiescent current The amount of current the device consumes while idle.
Reset time delay The amount of time it takes for the device to release from a fault condition once there is no longer a fault.
Voltage hysteresis The difference between the threshold and the deasserting threshold. This parameter helps prevent false deassertion if the monitored voltage is oscillating.
Output topology The output pin of the voltage supervisor (open drain or push pull) with either an active-low or active-high format.
Latch Once a fault occurs, the pin indicating the fault remains asserted until the supervisor IC receives a signal to clear the logic.
BIST Internal device diagnostics to check for internal faults.

Voltage supervisor ICs monitor a voltage; once that voltage enters an undervoltage or overvoltage state, the voltage supervisor can notify an MCU, flip a power switch or drive a gate. A voltage supervisor can detect that a power supply has changed and quickly disconnect the power supply safely and effectively. Supervisors that monitor both undervoltage and overvoltage are also called window supervisors. Which type of voltage monitoring you are doing also affects the functional safety rating.

Table 5 lists these ratings.

Table 5 How voltage monitoring affects DC.
Voltage monitoring type Potential diagnostic coverage or safe failure fraction
Overvoltage 60%
Window (Overvoltage and Undervoltage) 90% to 99%

When designing your safety circuit, is important to consider the level of diagnostic coverage. Additionally, using a voltage supervisor IC can decrease the number of necessary circuit components, allowing for a simpler design.