Datasheet

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SLVS373 − AUGUST 2001
14
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APPLICATION INFORMATION
power dissipation and junction temperature
The main source of power dissipation for the TPS2148 and TPS2158 comes from the internal voltage regulator
and the N-channel MOSFETs. Checking the power dissipation and junction temperature is always a good
design practice and it starts with determining the r
DS
(on)
of the N-channel MOSFET according to the input voltage
and operating temperature. As an initial estimate, use the highest operating ambient temperature of interest and
read r
DS
(on)
from the graphs shown in the Typical Characteristics section of this data sheet. Using this value,
the power dissipation per switch can be calculated using:
P
D
+ r
DS(on)
I
2
Multiply this number by two to get the total power dissipation coming from the N-channel MOSFETs.
The power dissipation for the internal voltage regulator is calculated using:
P
D
+
ǒ
V
I
–V
O(min)
Ǔ
I
O
The total power dissipation for the device becomes:
P
D(total)
+ P
D(voltage regulator)
)
ǒ
2 P
D(switch)
Ǔ
Finally, calculate the junction temperature:
T
J
+ P
D
R
qJA
) T
A
Where:
T
A
= Ambient Temperature °C
R
θJA
= Thermal resistance °C/W, equal to inverting the derating factor found on the power
dissipation table in this datasheet.
Compare the calculated junction temperature with the initial estimate. If they do not agree within a few degrees,
repeat the calculation, using the calculated value as the new estimate. Two or three iterations are generally
sufficient to get a reasonable answer.
(1)
(2)
(3)
(4)