Monitoring & metering
CHOOSING THE RIGHT TOLERANCE WINDOW
One of the common difficulties in using a window voltage supervisor is setting and selecting the appropriate tolerance window. Users tend to choose a window voltage supervisor with the same tolerance as the core voltage requirement. For example, a window voltage supervisor with a tolerance window of ±3% is used for a core voltage requirement that has a ±3% tolerance. Due to threshold accuracy, choosing the same tolerance with the operating requirement of the FPGA core voltage can trigger a reset output anywhere near the maximum overvoltage threshold OV_TH (max) and minimum undervoltage threshold UV_TH (min), leading to a potential malfunction. In Figure 4a, the monitored power supply could go beyond the core voltage tolerance and may not be detected by the supervisor on its possible actual operating threshold if threshold accuracy is not considered. This power supply going beyond the ±3% will be supplied to the core of a microprocessor, thus, a more appropriate choice of tolerance window needs to be chosen. To avoid the risks of such a condition, the OV_TH (max) and the UV_TH (min) should be set within the ±3% tolerance requirement of the core voltage. Yet, a portion of usable power supply window will be eaten
Figure 4. Window voltage supervisor tolerance setting (a) same with core voltage tolerance and (b) within the core voltage tolerance.
by accuracy, which will give a reduced power supply operating window, as shown in Figure 4b.
THE IMPACT OF THE THRESHOLD ACCURACY
Consider two window voltage supervisors with different threshold accuracies to monitor the same core voltage supply. The actual UV and OV thresholds of the one with higher accuracy deviate less than the one with lower accuracy with respect to the expected UV and OV
threshold values. From Figure 5a, note that a lower threshold accuracy will give a narrower power supply window since the reset output signal will assert when the core supply voltage is anywhere within the UV and OV monitoring range. In applications with low accuracy power supply and poor regulation, this could lead to a more sensitive system that is prone to oscillation. Meanwhile, high threshold accuracy gives an expanded and wider usable power supply window, which improves system performance by
Figure 5. Allowable power supply window and reset response (a) with low threshold accuracy and (b) with high threshold accuracy. 72 August 2025 Instrumentation Monthly
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