SLVSB10F July 2012 – November 2020 TPS54020
PRODUCTION DATA
The amount of bus capacitance is the third criteria. This requirement is optional. However, extra output bus capacitance should be considered in systems where the electrical environment is unpredictable, or not fully defined, or can be subject to severe events such as hot plug events or even electrostatic discharge (ESD) events.
During a hot plug event, when a discharged load capacitor is plugged into the output of the regulator, the instantaneous current demand required to charge this load capacitance will be far too rapid to be supplied by the control loop. Often the peak charging current can be multiple times higher than the current limit of the regulator. Additional output capacitance will help maintain the bus voltage within acceptable limits. For hot plug events, the amount of required bus capacitance can be calculated if the load capacitance is known, based on the concept of conservation of charge.
An ESD event, or even non-direct lightning surges at the primary circuit level can cause glitches at this converter system level. A glitch of sufficient amplitude to falsely trip OVP or UVLO can cause several clock cycles of disturbance. In such cases, it is beneficial to design in more bus capacitance than is required by the simpler load transient and ripple requirements. The amount of extra bus capacitance can be calculated based on maintaining the output voltage within acceptable limits during the disturbance. This capacitance can be as much as required to fully support the load for the duration of the interrupted converter operation.