Datasheet

DocID023432 Rev 4 27/40
LED2000 Application information
40
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
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
=