To define an appropriate soft-start time to ramp up the output voltage to the required level some aspects of the application need to be considered:
- Maximum output current
- This is the maximum load current
which the converter can supply without getting into any limitation, either
current limit or device parameter limits. Device parameter limits must never be
violated and a well-defined margin need to be considered.
- Output capacitance and the ESR of the output capacitors. The capacitance plays a
significant role as it dictates the value of the inrush current, together with
the target value of the output voltage. This current is basically only limited
by the ESR of the capacitors. (In most cases the ESR can be neglected as,
especially for ceramic capacitors, it is quite low.)
- Behavior of the load during the startup phase
- Beside the inrush current of the
output capacitor also the load on the SMPS can contribute to the current during
the start phase. This needs to be evaluated and considered individually, for
example, an ohmic load can ramp linear while a Processor or uController can only
start at a certain voltage level or even after the power supply has stabilized
and, in this case, might not need to be considered for the soft-start time.