a5975d STMicroelectronics, a5975d Datasheet - Page 29

• Manufacturer Part Number: a5975d
• Description: Up To 3 A Step-down Switching Regulator For Automotive Applications
• Manufacturer: STMicroelectronics

A5975D
8.3.2
The overall losses are:
Equation 26
The junction temperature of device is:
Equation 27
Equation 28
Thermal impedance Z
The thermal impedance of the system, considered as the device in the HSO8 package
soldered on the application board, takes on an important rule when the maximum output
power is limited by the static thermal performance and not by the electrical performance of
the device. Therefore, the embedded power elements could manage a higher current but the
system is already taking away the maximum power generated by the internal losses.
In case the output power increases, the thermal shutdown is triggered because the junction
temperature triggers the designed thermal shutdown threshold.
The R
generated from the system given the operation conditions.
If we suppose, as an example, T
before triggering the thermal shutdown and R
loss achievable with the thermal performance of the system is:
Figure 15
and T
element equal to 0.4 A.
AMB
TH
is a static parameter of the package; it sets the maximum power loss which can be
=40°C. The calculations are performed considering the R
represents the estimation of power losses for different output voltages at V
=
P
TOT
0.4 3
⋅
=
2
P
R
⋅
MAX DC
0.3
DS on
(
THJ-A
+
Doc ID 018760 Rev 1
12 3 70 10
)
⋅
=
(
A
⋅
T
I
T
---------- -
R
OUT
= 40 °C, 140 °C is the maximum operating temperature
J
ΔT
(t)
J
TH
⋅
=
=
)
70
2
=
T
⋅
⋅
A
D
T
------------------------------------- -
+
+
J MAX
+
–
0.93 42
9
R
V
⋅
thJA
R
TH
IN
250 10
TH
⋅
–
⋅
= 40 °C/W, therefore, the maximum power
T
I
⋅
OUT
⋅
P
AMB
≅
TOT
116°C
3
⋅
+
T
=
SW
12 2.5 10
100
--------- -
40
⋅
⋅
F
SW
=
⋅
2.5W
+
Application information
V
DS(on)
–
3
IN
≅
⋅
1.11W
I
Q
of the power
=
IN
=5 V
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