Meaning
Electronic modules must receive a stable supply voltage to ensure that their digital components function correctly. A supply voltage sag is a temporary drop in the power rail voltage that occurs when a subsystem draws a sudden surge of current. This drop can push the voltage below the minimum operating threshold of the integrated circuits.
It is common during high-power events such as radio transmissions.
System Failure Mode
When the voltage on the primary rail drops too low, the digital circuits can behave unpredictably. A severe supply voltage sag can cause the main processor to execute a brown-out reset, which halts the device and disrupts ongoing operations. This failure can lead to data loss or corrupt the system firmware if a write operation is interrupted, necessitating a complete system reboot.
It is especially common in battery-powered products where the internal resistance of the battery increases as it discharges.
Mitigation Strategy
Preventing these drops requires storing energy close to the pins of the high-power components. Designers place decoupling capacitors and bulk storage capacitors on the board to supply the initial current surge when the subsystem activates. This localized energy reserve minimizes the supply voltage sag and keeps the power rail stable during peak load events.
Laboratory Analysis
Verifying the power integrity of the board involves monitoring the voltage rails with a high-bandwidth oscilloscope. Technicians trigger the high-power modes of the device and measure the depth of the supply voltage sag under worst-case load conditions. This measurement ensures that the power delivery network has sufficient margin to keep the voltage within safe limits.
