The relationship between thermal
resistance and power dissipation can be expressed as:
Equation 2.
Where:
- TJ = Junction
temperature (°C)
- TA = Ambient
temperature (°C)
- θJA =
Junction-to-ambient thermal resistance (°C/W)
- PD = Power
dissipation (W)
Calculate the appropriate power dissipation to determine required heat-sink
area. At the same time, consider the relationship between power dissipation and
thermal resistance to minimize shutdown conditions and allow for proper long-term
operation (junction temperature of 125 °C). After the heat-sink area has been
selected, verify proper thermal protection by testing worst-case load conditions.
For applications with limited board size, refer to
Figure 7-3 for the approximate thermal resistance relative to heat-sink area. Increasing
heat-sink area beyond 2 in
2 provides little improvement in thermal
resistance. To achieve the 32 °C/W stated in the
Electrical Characteristics,
a copper plane size of 9 in
2 was used. The HTSSOP-20 PowerPAD integrated
circuit package is a good choice for continuous power levels from 2 W to 4 W,
depending on ambient temperature and heat-sink area. Higher power levels can be
achieved in applications with a low on-off duty cycle, such as remote meter
reading.