2021.05.26 Thermal Design
The previous article explained, in connection with thermal resistance data, the relevant JEDEC standards, measurement environments, and the like. In this article, an example of actual thermal resistance data is presented.
Example of Actual Thermal Resistance Data
In many cases, the data sheet of an IC provides information relating to thermal resistance for the IC. However, there are some differences in the conditions and the types of thermal resistance provided depending on the type of IC--whether it is for example an op-amp used for signal processing with low power consumption, or a voltage regulator used to supply power in which thermal design is more important. Moreover, there are also differences depending on the IC manufacturer.
Below, thermal resistance information provided on the data sheet for an LDO linear regulator with a 500 mA output is presented as one example.
Two package types are prepared for this IC, and so the thermal resistances are indicated for each (TO263-5 and TO252-J5). Both are 5-pin surface-mount packages for power system use and are provided with fins.
We proceed to examination of the information provided. As indicated in Note 1, this thermal resistance data (in the red rectangle) conforms to the JESD51-2A (still-air) standard explained in the previous article.
The following two types of thermal resistance are provided.
・Junction-to-ambient thermal resistance: θJA（℃/W）
・Junction-to-top thermal characterization parameter: ΨJT（℃/W）
Moreover, thermal resistances indicate values for two PCB conditions--when the device is mounted on a single-layer PCB, and when it is mounted on a 4-layer PCB. The single-layer PCB is a PCB conforming to JESD51-3, as indicated in Note 3; the 4-layer PCB conforms to JESD51-5 and JESD51-7 (Note 4). The conditions for these PCBs are indicated in the table.
Relation between Thermal Resistance and PCB
In the example presented above, JESD51-standard PCB conditions were explicitly indicated as the conditions for thermal resistances. This means that thermal resistances are not determined solely by the IC package, but are greatly affected by the conditions of the PCB on which it is mounted. In recent years, surface-mount packages have come to be used extremely often, and when considering IC thermal resistances, the heat dissipation (reduction in thermal resistance) due to the PCB on which the IC is mounted must necessarily be included. It is not realistic to perform thermal calculations considering only the thermal resistance of the package.
This graph indicates the relations between the thermal resistances (θJA, ΨJT) and the area of the copper foil for heat dissipation. The package used in measurements was an 8-pin SOP type package with heat dissipation fins on the rear surface; data is given for copper foil areas ranging from 15.7 mm2 to 1200 mm2. Other elements include the number of PCB layers, PCB materials, and copper foil thickness, but the graph should be viewed considering that these remain constant, and only the relation between copper foil area and thermal resistance is displayed.
In this example, the relation between the copper foil area and θJA, which is the thermal resistance from the IC junction (chip) through the PCB to the ambient (air), is prominent. In actual practice, the copper foil area required for heat dissipation such that Tjmax is not exceeded under the conditions of use (an appropriate θJA) must be secured.
Put conversely, when the conditions of the thermal resistance provided are not explicitly stated, the conditions must always be confirmed. As the numerical values of the above example illustrate, thermal resistances can vary greatly depending on the conditions.
・Thermal resistance data is generally presented on IC data sheets and the like, but there may be differences in the information given depending on the type of IC and the manufacturer.
・Thermal resistances differ considerably depending on PCB conditions. Measurement conditions must always be confirmed.