JAJSF42B April 2018 – June 2019 DLPC3478
The underlying thermal limitation for the DLPC3478 is that the maximum operating junction temperature (TJ) not be exceeded (this is defined in the ). This temperature is dependent on operating ambient temperature, airflow, PCB design (including the component layout density and the amount of copper used), power dissipation of the DLPC3478, and power dissipation of surrounding components. The DLPC3478’s package is designed primarily to extract heat through the power and ground planes of the PCB. Thus, copper content and airflow over the PCB are important factors.
The recommended maximum operating ambient temperature (TA) is provided primarily as a design target and is based on maximum DLPC3478 power dissipation and RθJA at 0 m/s of forced airflow, where RθJA is the thermal resistance of the package as measured using a JEDEC standard high-k 2s2p PCB with two, 1-oz. power planes. This JEDEC test PCB is not necessarily representative of the DLPC3478 PCB; the reported thermal resistance may not be accurate in the actual product application. Although the actual thermal resistance may be different, it is the best information available during the design phase to estimate thermal performance. However, after the PCB is designed and the product is built, TI highly recommended that thermal performance be measured and validated.
To do this, measure the top center case temperature under the worse case product scenario (max power dissipation, max voltage, max ambient temperature) and validated not to exceed the maximum recommended case temperature (TC). This specification is based on the measured φJT for the DLPC3478 package and provides a relatively accurate correlation to junction temperature. Take care when measuring this case temperature to prevent accidental cooling of the package surface. TI recommends a small (approximately 40 gauge) thermocouple. The bead and thermocouple wire typically contact the top of the package and are covered with a minimal amount of thermally conductive epoxy. Route the wires closely along the package and the board surface to avoid cooling the bead through the wires.