Datasheet

DocID024346 Rev 1 27/42
LED2001 Application information
7.3 Thermal considerations
The dissipated power of the device is tied to three different sources:
• Conduction losses due to the R
DSON
, which are equal to:
Equation 33
where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between V
OUT
(n
LED
∗ V
LED
+ 100 mV) 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 turn-ON and turn-OFF. These are derived using the following
equation:
Equation 34
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 17). T
SW
is the equivalent
switching time.
Figure 17. Switching losses
• Quiescent current losses.
Equation 35
Example (see Section Example 1):
P
ON
R
RDSON_HS
I
OUT
()⋅
2
D ⋅=
P
OFF
R
RDSON_LS
I
OUT
()⋅
2
1D–()⋅=
P
SW
V
IN
I
OUT
T
RISE
T
FALL
+()
2
----------------------------------------- F
SW
V
IN
= I
OUT
T
SW_EQ
F
SW
⋅⋅ ⋅ ⋅⋅ ⋅=
AM12908v1
P
Q
V
IN
I
Q
⋅=