Datasheet
Application information ST1S14
26/46 DocID17977 Rev 2
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
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
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
ON
R
DSON
I
OUT
()⋅
2
D⋅=
P
SW
V
IN
I
OUT
T
RISE
T
FALL
+()
2
---------------------------------------- -
F
SW
V
IN
= I
OUT
T
SW_EQ
F
SW
⋅⋅ ⋅ ⋅⋅ ⋅=
$09
P
Q
V
IN
I
Q
⋅=