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

There are several methods to damp or limit 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 save cost or have small size.

A cost effective and small size solution is shown in Figure 9-2. The R1 and C1 are composed of a damping RC network to damp the hot plug-in oscillation. As a result the over voltage spike is limited to a safe level. D1 is used for reverse voltage protection for VBUS pin. C2 is VBUS pin decoupling capacitor and it should be placed as close as possible to VBUS pin. C2 value should be less than C1 value so R1 can dominate the equivalent ESR value to get enough damping effect. R2 is used to limit inrush current of D1 to prevent D1 getting damage when adapter hot plug-in. R2 and C2 should have 10-µs time constant to limit the dv/dt on VBUS pin to reduce inrush current when adapter hot plug in. R1 has high inrush current. R1 package must be sized enough to handle inrush current power loss according to resistor manufacturer’s data sheet. The filter components' value always need to be verified with real application and minor adjustments may need to fit in the real application circuit.

Figure 9-2 Input Filter