When there combination low load and maximum supply input voltage

Maximum Regulator Dissipation
In fact maximum series-pass dissipation does not necessarily occur at full load current, because as the current rises the voltage across the series-pass element falls. The maximum dissipation will occur at less than full output if the voltage dropped across the DC supply’s equivalent series resistance is greater than half the difference between the no-load input voltage and the output voltage. Figure 15.29 shows this graphically.

Minimum Load Requirement
A further problem, particularly with switch-mode supplies, is that the stability of the regulator cannot always be assured down to zero load. For this reason some rails have to be run with a minimum load, such as a bleed resistor, to remain within specifi cation, and this represents an unnecessary additional power drain. Many circuits, of course, always take a minimum current and so the minimum load is not then a problem.

where, I L is the DC load current,
V ripple is the acceptable ripple voltage