ST1S14PHR Datasheet, PDF (26/46 Pages) STMicroelectronics – Up to 3 A step-down switching regulator

Application information
ST1S14
7.3
Thermal considerations
The dissipated power of the device is tied to three different sources:
• Conduction losses due to the not insignificant R_DSON, which are equal to:
Equation 24
P_ON = R_DSON ⋅ (I_OUT)² ⋅ D
where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between V_OUT and V_IN, but in practice it is substantially higher than this value to
compensate for the losses in the overall application. For this reason, the conduction losses
related to the R_DSON increase compared to an ideal case.
• Switching losses due to turning on and off. These are derived using the following
equation:
Equation 25
P_SW = V_IN ⋅
I_OUT ⋅
(_---T_{----R---I--S---E----+-----}T_{----F---A---L---L---})
2
⋅
F_SW= V_IN ⋅
I_OUT ⋅
T_{SW_EQ} ⋅
F_SW
where T_RISE and T_FALL represent the switching times of the power element that cause the
switching losses when driving an inductive load (see Figure 15). T_SW is the equivalent
switching time.
Figure 15. Switching losses
• Quiescent current losses.
Equation 26
P_Q = V_IN ⋅ I_Q
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