L7986TA STMicroelectronics, L7986TA Datasheet - Page 28

• Manufacturer Part Number: L7986TA
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986TA

• Low Dropout Operation: 100% duty cycle

Application informations
6.5
28/43
Thermal considerations
The thermal design is important to prevent the thermal shutdown of device if junction
temperature goes above 150°C. The three different sources of losses within the device are:
Equation 32
Where D is the duty cycle of the application and the maximum R
220 mΩ. Note that the duty cycle is theoretically given by the ratio between V
but actually it is quite higher to compensate the losses of the regulator. So the conduction
losses increases compared with the ideal case.
Equation 33
Where T
and the current flowing into it during turn ON and turn OFF phases, as shown in
T
switching time is 40 ns.
Equation 34
where I
The junction temperature T
Equation 35
Where T
Rth
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
SW
JA
a)
b)
c)
is the equivalent switching time. For this device the typical value for the equivalent
is the equivalent thermal resistance junction to ambient of the device; it can be
Q
A
RISE
is the quiescent current (I
conduction losses due to the not negligible R
equal to:
switching losses due to power MOSFET turn ON and OFF; these can be
calculated as:
Quiescent current losses, calculated as:
is the ambient temperature and P
and T
P
SW
FALL
=
V
IN
are the overlap times of the voltage across the power switch (V
⋅
I
J
OUT
can be calculated as:
Doc ID 022098 Rev 1
⋅
(
------------------------------------------ - Fsw
P
T
ON
T
Q
RISE
J
=2.4 mA).
=
=
P
+
2
T
R
Q
T
A
DSON
FALL
+
=
TOT
Rth
V
IN
)
⋅
is the sum of the power losses just seen.
⋅
JA
(
⋅
I
OUT
I
Q
⋅
P
TOT
)
=
2
DSon
⋅
V
D
IN
of the power switch; these are
⋅
I
OUT
DSon
⋅
T
SW
over temperature is
⋅
F
SW
OUT
Figure
and V
L7986TA
DS
IN
15.
,
)