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May 28

Finding the right bulk decoupling capacitor value

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Almost all electronic system designs feature one or more bulk decoupling capacitors. These are typically placed at the point of entry of the power cables. They are mostly of Aluminum Electrolyte, Tantalum or in case of low leakage systems multi-layer ceramic capacitors. The purpose of these capacitors is to provide large current spikes of lower frequency content while the local decoupling capacitors placed closer to the ICs provide short surges of current typically at the high frequencies. More often super capacitors are being used in systems where cost may not be a large concern. For typical IOT systems using coin cells or Alkaline batteries which are expected to last for couple of years, the selection of the right decoupling capacitor will make a large difference to the overall battery life. Too Low a value If the value of the decoupling capacitor is too low, then the high current spikes are drawn directly from the battery. In case of batteries with limited surge current capabilities like CR2032, the battery capacity gets degraded substantially. This will degrade the overall battery life. Too High a value Another choice would be to simply over rate the capacitor value and chose an arbitrarily high value. This would add space on the board and increase the overall cost of the system. However this is not guaranteed to increase the battery life. Although the large capacitor will reduce the peak current drawn from the battery, it will also have a high leakage current. This leakage current will get added with the overall system current and would actually end up degrading the battery life. Thus the reduction in peak current would be offset by an increase in the overall leakage current. So choosing a very high value is also not optimal. The Optimum Value How do we determine the optimum value in this case? The answer is to profile the current drawn by the IOT device and simulate the system to get the right value of the capacitor. Using the exact current model from the device which can be obtained by the ZS-2102-A, one may be able to simulate the power distribution system along with the bulk decoupling capacitor model in order to keep the peak current from the battery below recommended levels. Using the leakage model for the capacitor one can calculated the overall average current for the whole profile. This would yield the best battery life for the product. Finally after adding the right capacitor to the system, one can re-measure the power profile and battery life using the ZS-2102-A, just to confirm the overall exercise. We would soon add an experimental data for this exercise. The battery profiling feature on the GUI is being implemented now and is expected to be available by the middle of June. Using that battery profile with pulsed loads, the degradation in capacity due to the current peaks can be easily computed. This would yield a more accurate battery life estimation than the one based on the average capacity and would be able to predict the optimum capacitor value.

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