Datasheet

Application information L7986TA
28/43 Doc ID 022098 Rev 3
where D is the duty cycle of the application and the maximum R
DSon
overtemperature is 220
mΩ. Note that the duty cycle is theoretically given by the ratio between V
OUT
and V
IN
, but
actually it is quite higher in order to compensate the losses of the regulator. So the
conduction losses increase compared with the ideal case.
b) switching losses due to power MOSFET turn-on and turn-off; these can be
calculated as:
Equation 36
where T
RISE
and T
FALL
are the overlap times of the voltage across the power switch (V
DS
)
and the current flowing into it during turn-on and turn-off phases, as shown in Figure 15.
T
SW
is the equivalent switching time. For this device the typical value for the equivalent
switching time is 40 ns.
c) quiescent current losses, calculated as:
Equation 37
where I
Q
is the quiescent current (I
Q
=2.4 mA).
The junction temperature T
J
can be calculated as:
Equation 38
where T
A
is the ambient temperature and P
TOT
is the sum of the power losses just seen.
Rth
JA
is the equivalent thermal resistance junction to ambient of the device; it can be
calculated as the parallel of many paths of heat conduction from the junction to the ambient.
For this device the path through the exposed pad is the one conducting the largest amount
of heat. The Rth
JA
measured on the demonstration board described in the following
paragraph is about 40 °C/W for the HSOP package.
P
SW
V
IN
I
OUT
T
RISE
T
FALL
+()
2
----------------------------------------------
Fsw⋅⋅ ⋅ V
IN
I
OUT
T
SW
F
SW
⋅⋅⋅==
P
Q
V
IN
I
Q
⋅=
T
J
T
A
Rth
JA
P
TOT
⋅+=