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LTM4646

4646f

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over 125°C delta equates to ~0.35%/°C loss increase. A

125°C maximum junction minus 25°C room temperature

equates to a 100°C increase. This 100°C increase multi-

plied by 0.35%/°C equals a 35% power loss increase at

the 125°C junction, thus the 1.35 multiplier.

The derating curves are plotted with VOUT1 and VOUT2 in

parallel single output operation starting at 20A of load

with low ambient temperature. The output voltages are

1V, 2.5V and 5V. These are chosen to include the lower

and higher output voltage ranges for correlating the ther-

mal resistance. Thermal models are derived from several

temperature measurements in a controlled temperature

chamber along with 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 while increasing ambient temperature.

The decreased output current will decrease the internal

module loss as ambient temperature is increased.

The monitored junction temperature of 120°C minus the

ambient operating temperature specifies how much tem-

perature rise can be allowed. As an example in Figure 13,

the load current is derated to ~14.5A at ~95°C with no

air or heat sink and the power loss for the 12V to 1.0V

at 14.5A output is a ~3.04W loss. The ~3.04W loss is

calculated with the ~2.25W room temperature loss from

the 12V to 1.0V power loss curve at 14.5A, and the 1.35

multiplying factor at 125°C ambient. If the 95°C ambient

temperature is subtracted from the 125°C junction tem-

perature, then the difference of 30°C divided by 3.04W

equals a 9.9°C/W JA thermal resistance. Table 2 speci-

fies a 10.1°C/W value which is pretty close. The airflow

graphs are more accurate due to the fact that the ambient

temperature environment is controlled better with airflow.

As an example in Figure 13, the load current is derated to

~16.5A at ~95°C with 200LFM of airflow and the power

loss for the 12V to 1.0V at 16.5A output is a 3.7W loss.

The 3.7W loss is calculated with the ~2.75W room tem-

perature loss from the 12V to 1.0V power loss curve at

16.5A, and the 1.35 multiplying factor at 125°C ambient.

If the 95°C ambient temperature is subtracted from the

125°C junction temperature, then the difference of 30°C

divided by 3.7W equals a 8.1°C/W JA thermal resistance.

Table 2 specifies a 8.1°C/W value which is pretty close.

Tables 2 through 4 provide equivalent thermal resistances

for 1V, 2.5V and 5V outputs with and without airflow.

The derived thermal resistances in Tables 2 through 4

for the various conditions can be multiplied by the calcu-

lated 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 power loss curves and adjusted with

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