Abstract
The input voltage transient problem is related to the power-up sequence. If the wall adapter is plugged into an AC outlet and powered up first, plugging the wall adapter output into a portable device can cause input voltage transients that could damage the DC/DC converters inside the device. A recent trend in the design of portable devices has been to use ceramic capacitors to filter DC/DC converter inputs. Ceramic capacitors are often chosen because of their small size, low equivalent series resistance, and high root mean square current capability. Unfortunately, using ceramic capacitors for input filtering can cause problems. Applying a voltage step to a ceramic capacitor causes a large current surge that stores energy in the inductances of the power leads. A large voltage spike is created when the stored energy is transferred from these inductances into the ceramic capacitor. These voltage spikes can easily be twice the amplitude of the input voltage step. Unfortunately, using a transient voltage suppressor with a higher voltage rating would not provide a sufficiently low clamping voltage. Optimizing the input capacitors requires clear understanding of what is happening during transients.
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