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We are designing a circuit using LM317KTT (DDPAK Package) LDO and ADC ADS124S06. Following are our design requirements:
Power Dissipation through LDO = (15V – 5V)*60 mA = 600mW
Please let us know following:
1. Calculations for temperature rise of the LDO.
2. Recommended heat sink area and design guidelines such that there would be least heating.
This LDO is planned to be used to power up a product having high precision ADC, so heating shall be minimized to have any impact on measurement.
Hi Surya,
You already did the first step for calculating the junction temperature of the linear regulator by calculating the power dissipation (Pd) in the regulator at 600 mW. The first pass equation for calculating the junction temperature is the following:
Tj = Pd x Rtheta_ja + Ta
Ta is your application ambient temperature. Rtheta_ja can be found in the Thermal Information table of the datasheet. This table is populated by simulation on the JEDEC High-k board.
Tj = Pd x Rtheta_ja + Ta
Tj = 600 mW x 38 C/W + Ta
Tj = 22.8 C + Ta
Your junction temperature will be ~22.8 C higher than the ambient temperature of your application with a layout similar to the JEDEC High-K board. Since the maximum recommended junction temperature for LM317 is 125 C, this means your ambient temperature can be up to 102.2 C.
While the JEDEC board gives a standard that is useful for comparing linear regulators and packages for thermal performance, layout will impact the thermal performance. The primary heatsink for the LM317 is the metal connected to OUTPUT. As such, maximizing the copper attached to OUTPUT will yield the best thermal performance in your application.
Very Respectfully,
Ryan
