During adapter hot plug-in, the parasitic inductance and the input capacitor from the adapter cable form a second-order system. The voltage spike at the AVCC pin may be beyond the IC maximum voltage rating and damage the IC. The input filter must be carefully designed and tested to prevent an overvoltage event on the AVCC pin.

There are several methods to damping or limiting the overvoltage spike during adapter hot plug-in. An electrolytic capacitor with high ESR as an input capacitor can damp the overvoltage spike well below the IC maximum pin voltage rating. A high-current capability TVS Zener diode can also limit the overvoltage level to an IC safe level. However, these two solutions may not be lowest cost or smallest size.

A cost-effective and small-size solution is shown in Figure 10-2. R1 and C1 are composed of a damping RC network to damp the hot plug-in oscillation. As a result, the overvoltage spike is limited to a safe level. D1 is used for reverse voltage protection for the AVCC pin. C2 is the AVCC pin decoupling capacitor and it should be placed as close as possible to the AVCC pin. R2 and C2 form a damping RC network to further protect the IC from high dv/dt and high voltage spike. The C2 value should be less than the C1 value so R1 can dominant the equivalent ESR value to get enough damping effect for hot plug-in. R1 and R2 must be sized enough to handle in-rush current power loss according to the resistor manufacturer’s data sheet. The filter component values always need to be verified with a real application and minor adjustments may be needed to fit in the real application circuit.

If the input is 5 V (USB host or USB adapter), then D1 can be saved. R2 has to be 5 Ω or higher to limit the current if the input is reversely inserted.

Figure 10-2 Input Filter