Datasheet

THERMAL INFORMATION
THERMAL PROTECTION POWER DISSIPATION
P =(V V ) I- ´
D IN OUT OUT
(4)
PACKAGE MOUNTING
TPS781 Series
www.ti.com ........................................................................................................................................................... SBVS102B – MARCH 2008 – REVISED MAY 2008
Thermal protection disables the output when the The ability to remove heat from the die is different for
junction temperature rises to approximately +160 ° C, each package type, presenting different
allowing the device to cool. Once the junction considerations in the PCB layout. The PCB area
temperature cools to approximately +140 ° C, the around the device that is free of other components
output circuitry is enabled. Depending on power moves the heat from the device to the ambient air.
dissipation, thermal resistance, and ambient Performance data for JEDEC low- and high-K boards
temperature, the thermal protection circuit may cycle are given in the Dissipation Ratings table. Using
on and off again. This cycling limits the dissipation of heavier copper increases the effectiveness in
the regulator, protecting it from damage as a result of removing heat from the device. The addition of plated
overheating. through-holes to heat-dissipating layers also
improves the heatsink effectiveness. Power
Any tendency to activate the thermal protection circuit
dissipation depends on input voltage and load
indicates excessive power dissipation or an
conditions. Power dissipation (P
D
) is equal to the
inadequate heatsink. For reliable operation, junction
product of the output current times the voltage drop
temperature should be limited to +125 ° C maximum.
across the output pass element (V
IN
to V
OUT
), as
To estimate the margin of safety in a complete design
shown in Equation 4 :
(including heatsink), increase the ambient
temperature until the thermal protection is triggered;
use worst-case loads and signal conditions. For good
reliability, thermal protection should trigger at least
+35 ° C above the maximum expected ambient
Solder pad footprint recommendations for the
condition of your particular application. This
TPS781 series are available from the Texas
configuration produces a worst-case junction
Instruments web site at www.ti.com through the
temperature of +125 ° C at the highest expected
TPS781 series product folder s.
ambient temperature and worst-case load.
The internal protection circuitry of the TPS781 series
has been designed to protect against overload
conditions. However, it is not intended to replace
proper heatsinking. Continuously running the TPS781
series into thermal shutdown degrades device
reliability.
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