Datasheet

 
 
SLVS399A − JANUARY 2002 − REVISED MAY 2006
20
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APPLICATION INFORMATION
power dissipation and junction temperature
The major source of power dissipation for the TPS2140/41/50/51 comes from the internal voltage regulator and
the N-channel MOSFET. 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
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(LDO)
+ P
D(switch)
Finally, calculate the junction temperature:
T
J
= P
D
× R
θJA
+ 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 data sheet.
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.
thermal protection
Thermal protection prevents damage to the IC when heavy-overload or short-circuit faults are present for
extended periods of time. The overcurrent faults force the TPS2140/41/50/51 into constant-current mode at
first, which causes the voltage across the high-side switch to increase; under short-circuit conditions, the voltage
across the switch is equal to the input voltage. The increased dissipation causes the junction temperature to
rise to high levels.
If either the power distribution switch or the LDO is in overcurrent, a thermal sensor trips at approximately 135°C,
turning off both circuits. Normal operation resumes when the die temperature drops approximately 10°C. If
neither the power distribution switch nor the LDO is in overcurrent, a second thermal sensor trips at
approximately 160°C. Normal operation resumes when the die temperature drops approximately 10°C.
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