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POWER DISSIPATION AND JUNCTION TEMPERATURE
THERMAL PROTECTION
TPS2080, TPS2081, TPS2082 DUAL
TPS2085, TPS2086, TPS2087 QUAD
SLVS202B – SEPTEMBER 2000 – REVISED OCTOBER 2007
Figure 25. Typical Circuit for OC Pin
The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, to pass
large currents. The thermal resistance of these packages is high compared to that of power packages; it is good
design practice to check power dissipation and junction temperature. Begin by determining the r
DS(on)
of the
N-channel MOSFET relative 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 Figure 18 . Using this value, the power
dissipation per switch can be calculated by:
P
D
= r
DS(on)
× I
2
Multiply this number by the total number of switches being used, to get the total power dissipation coming from
the N-channel MOSFETs.
Finally, calculate the junction temperature:
T
J
= P
D
× R
θ JA
+ T
A
Where:
T
A
= Ambient Temperature ° C
R
θ JA
= Thermal resistance SOIC = 172 ° C/W (for 8 pin), 111 ° C/W (for 16 pin)
P
D
= Total power dissipation based on number of switches being used.
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 prevents damage to the IC when heavy-overload or short-circuit faults are present for
extended periods of time. The faults force the TPS208x into constant current mode, 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. The protection
circuit senses the junction temperature of the switch and shuts it off. Hysteresis is built into the thermal sense
circuit, and after the device has cooled approximately 20 degrees, the switch turns back on. The switch continues
to cycle in this manner until the load fault or input power is removed.
The TPS208x implements a dual thermal trip to allow fully independent operation of the power distribution
switches. In an overcurrent or short-circuit condition the junction temperature will rise. Once the die temperature
rises to approximately 140 ° C, the internal thermal sense circuitry checks which power switch is in an overcurrent
condition and turns that power switch off, thus isolating the fault without interrupting operation of the adjacent
power switch. Should the die temperature exceed the first thermal trip point of 140 ° C and reach 160 ° C, both
switches turn off. The OC open-drain output is asserted (active low) when overtemperature or overcurrent
occurs.
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