a5974ad STMicroelectronics, a5974ad Datasheet - Page 29

• Manufacturer Part Number: a5974ad
• Description: Up To 2 A Step-down Switching Regulator For Automotive Applications
• Manufacturer: STMicroelectronics

A5974AD
8.3.2
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
Figure 15. Power losses estimation (V
The red trace represents the maximum power which can be taken away, as calculated
above, whilst the other traces are the total internal losses for different output voltage.
The embedded conduction losses are proportional to the duty cycle required for the
conversion. Assuming the input voltage constant, the switching losses are proportional to
the output current while the quiescent losses can be considered as constant.
As a consequence, in
system can manage a continuous output current up to 1.8 A. The device could deliver a
AMB
TH
is a static parameter of the package; it sets the maximum power loss which can be
=80 °C. The calculations are performed considering the R
represents the estimation of power losses for different output voltages at V
Figure
P
MAX DC
THJ-A
15, the maximum power loss is for V
Doc ID 018762 Rev 1
=
A
T
= 80 °C, 140 °C is the maximum operating temperature
J
---------- -
R
(t)
ΔT
TH
=
60
=
+
T
------------------------------------- -
IN
J MAX
1.38 42
= 5 V, f
TH
R
⋅
TH
–
= 40 °C/W, therefore, the maximum power
T
AMB
≅
SW
128°C
= 500 kHz)
=
60
----- -
40
=
1.5W
OUT
Application information
DS(on)
=3.3 V, where the
of the power
IN
=5 V
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