The dynamic performance of the LM2940 is dependent on the layout of the PCB. PCB layout practices that are adequate for typical LDOs may degrade the PSRR, noise, or transient performance of the LM2940. Best performance is achieved by placing CIN and COUT on the same side of the PCB as the LM2940, and as close as is practical to the package. The ground connections for CIN and COUT should be back to the LM2940 ground pin using as wide and short of a copper trace as is practical.
A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible operating conditions, the junction temperature must be within the range specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the power dissipated by the regulator, PD, must be calculated.
Figure 33 shows the voltages and currents which are present in the circuit, as well as the formula for calculating the power dissipated in the regulator:
The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX). This is calculated by using the formula:
Using the calculated values for TR(MAX) and PD, the maximum allowable value for the junction-to-ambient thermal resistance, RθJA, can now be found:
If the maximum allowable value for RθJA is found to be ≥ 23.3°C/W for the TO-220 package (with a heatsink of 21.7°C/W RθSA), ≥ 40.9°C/W for the DDPAK/TO-263 package, or ≥ 59.3°C/W for the SOT-223 package, no heatsink is needed since the package alone will dissipate enough heat to satisfy these requirements.
If the calculated value for RθJA falls below these limits, a heatsink is required.
If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, RθSA, must first be calculated:
When a value for RθSA is found using Equation 3, a heatsink must be selected that has a value that is less than or equal to this number.
RθSA is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots temperature rise vs power dissipation for the heatsink.