9.2.2.2 Input Capacitor Selection

In a dc/dc boost converter, since the input current is continuous, only minimum input capacitance is required. The TPS81256 device integrates a low ESR decoupling capacitor to prevent large voltage transients that can cause misbehavior of the device or interference in other circuits in the system.

For most applications, the input capacitor that is integrated into the TPS81256 should be sufficient. If the application exhibits a noisy or erratic switching frequency, experiment with additional input capacitance to find a remedy. Multilayer ceramic capacitors are an excellent choice for input decoupling of the step-up converter as they have extremely low ESR and are available in small footprints. Additional input capacitors should be located as close as possible to the device.

The TPS81256 uses a tiny ceramic input capacitor. When a ceramic capacitor is combined with trace or cable inductance, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or can even damage the part. In this circumstance, additional "bulk" capacitance, such as electrolytic or tantalum, should be placed between the input of the converter and the power source lead to reduce ringing that can occur between the inductance of the power source leads and C_I.