Voltage variations on a system supply input

Voltage input is the major excitation for any system. If there is an ideal load on a voltage input, then we do not experience any issues with the system functionality. In reality nothing is ideal. The loads 

keep changing, supply keeps varying, components use din the system experience ageing. As we discuss on this point, let us look at the major variations that can be seen on the supply input which can cause system malfunction.

1. Brown-out - This is the condition where the supply has dipped for short duration below a certain threshold. Most of the microcontrollers have a brown out feature inbuilt inside them and when voltage dips below the threshold defined in the brown out register or as per hardware connections inside the microcontroller, the microcontroller resets. Brown out is also termed voltage droop. The actual definition is like % drop of supply from it's desired value. The voltage dip occurs due to sudden load variations especially the inductive loads.

2. Voltage Fluctuation - Voltage fluctuation may not be as severe as voltage dip/droop. This is % variation of the supply input. Most of the datasheets define it as supply input tolerance. For example, 

microcontroller LPC2148 defined it's maximum and minimum limits as 3.6V and 3.0V respectively for a typical operating voltage of 3.3V. The effect of voltage fluctuation is not so severe as voltage dip. However, voltage fluctuation for a longer periods can degrade the performance of the load it is connected to. This is where the decoupling helps to restrict the fluctuations. The main reason for fluctuations is the inefficient power supply rather than load conditions.

3. Voltage Interruption - This is the most severe of all the above cases. When we say voltage interruption here, the voltage drops to zero for a certain period. Even though this period is for only few milliseconds, if not handled properly, the loads power-down and power-up again causing malfunction of the system. One of the primary reasons for this voltage interruption is the sudden load variations. The only remedy for this is to have holding capacitor at the supply input to system so that the capacitor charge can provide the desired voltage for the short period of interruption. Designing a voltage hold up circuit is always a challenge as the temperature, load conditions and variations, maximum and minimum voltage conditions are all to be considered.

4. Harmonic Distortion - The major reason for the harmonic distortion is the load variations causing irregular current draw from the supply. Harmonic distortion causes radiation and causes failures in 

EMI/EMC. When we talk about the term distortion here, it is the variation of the actual waveform due to harmonic contents variation. The non-linear loads cause voltage and current fluctuations.