An input capacitor, C_IN, is required to provide a local bypass for the input voltage source. A low ESR input capacitor is best for input voltage filtering and minimizing interference with other circuits. For most applications, a 10-µF ceramic capacitor is recommended from V_IN to ground (system ground, not –V_OUT). The C_IN capacitor value can be increased without any limit for better input voltage filtering.

For the inverting buck boost configuration of the TPS62120, it is not recommended to install a capacitor from V_IN to –V_OUT. Such a capacitor, if installed, provides an AC path from V_IN to –V_OUT. When V_IN is applied to the circuit, this dV/dt across a capacitor from V_IN to –V_OUT creates a current that must return to ground (the return of the input supply) to complete its loop. This current might flow through the internal low-side MOSFET's body diode and the inductor to return to ground. Flowing through the body diode pulls the SW pin and VOS pin more than 0.3 V below IC ground, violating their absolute maximum rating. Such a condition might damage the TPS62120 and is not recommended. Therefore, a capacitor from V_IN to –V_OUT is not needed or recommended. If such a capacitor (CBP) is present, then a Schottky diode should be installed on the output, per Figure 4-1. Startup testing should be conducted to ensure that the VOS pin is not driven more than 0.3 V below IC ground when V_IN is applied.

Figure 4-1 If Installing CBP, Installing Schottky D1 is Required