Data Sheet

LTM4640

analog.com

Rev. 0

26 of 38

simplicity—but also, not ignoring practical realities—an approach has been taken by using FEA software modeling

and laboratory testing in a controlled environment chamber to reasonably define and correlate the thermal

resistance values supplied in this data sheet: (1) Initially, FEA software accurately builds the mechanical geometry

of the LTM4640 and the specified PCB with all of the correct material coefficients and accurate power loss source

definitions; (2) this model simulates a software-defined JEDEC environment consistent with JSED5112 to predict

power loss heat flow and temperature readings at different interfaces that enable the calculation of the JEDEC-

defined thermal resistance values; (3) the model and FEA software evaluates the LTM4640 with heat sink and

airflow; (4) having solved for and analyzed these thermal resistance values and simulated various operating

conditions in the software model, a thorough laboratory evaluation replicates the simulated conditions with

thermocouples within a controlled environment chamber while operating the device at the same power loss as the

one which was simulated. An outcome of this process and due diligence yields the set of derating curves shown in

Figure 29 through Figure 34. After these laboratory tests have been performed and correlated to the LTM4640, then

the θJB and θBA are summed together to provide a value that should closely equal the θJA value because

approximately 100% of power loss flows from the junction through the board into ambient with no airflow or top

mounted heat sink.

The 1V, 1.5V, and 3.3V power loss curves in Figure 26 through Figure 28 can be used in coordination with the load

current derating curves in Figure 29 through Figure 34 for calculating an approximate θJA thermal resistance for the

LTM4640 with various airflow conditions. The power loss curves are taken at room temperature and are increased

with a multiplicative factor according to the ambient temperature. This approximate factor is 1.2 for 120°C, at

junction temperature. The maximum load current is achievable while increasing ambient temperature if the

junction temperature is less than 120°C, which is a 5°C guard band from a maximum junction temperature of 125°C.

When the ambient temperature reaches a point where the junction temperature is 120°C, then the load current is

lowered to maintain the junction at 120°C, while increasing ambient temperature up to 120°C. The derating curves

are plotted with the output current starting at 20A and the ambient temperature at 30°C. The output voltages are

1V, 1.5V, and 3.3V. These are chosen to include the lower and higher output voltage ranges to correlate the thermal

resistance. Thermal models are derived from several temperature measurements in a controlled temperature

chamber and thermal modeling analysis. The junction temperatures are monitored while ambient temperature is

increased with and without airflow. The power loss increase with ambient temperature change is factored into the

derating curves. The junctions are maintained at 120°C maximum while lowering output current or power with

increasing ambient temperature.

The decreased output current decreases the internal µModule loss as the ambient temperature is increased. The

monitored junction temperature of 120°C minus the ambient operating temperature specifies how much µModule

temperature rise can be allowed. For example, in Figure 30, the load current is derated to ~10A at ~95°C with no

airflow or heat sink, and the power loss for the 12V to 1V at 10A output is about 1.8W. The 1.8W loss is calculated

with the ~1.5W room temperature loss from the 12V to 1V power loss curve at 10A, and the 1.2 multiplying factor at

120°C junction temperature. If the 95°C ambient temperature is subtracted from the 120°C junction temperature,

then the difference of 25°C divided by 1.8W equals a 13.9°C/W θJA thermal resistance. Table 9 specifies a 14°C/W

value, which is very close. Table 10 and Table 11 provide equivalent thermal resistances for 1.5V and 3.3V outputs

with and without airflow and heat sinking. The derived thermal resistances in Table 9, Table 10, and Table 11 for the

various conditions can be multiplied by the calculated power loss as a function of ambient temperature to derive

temperature rise above ambient, thus maximum junction temperature. Room temperature power loss can be

derived from the efficiency curves in the Typical Performance Characteristics section and adjusted with the previous

ambient temperature multiplicative factors. The printed circuit board is a 1.6mm thick 6-layer board with two-

ounce copper for all six layers. The PCB dimensions are 90mm × 90mm.