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LTM4633

4633f

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applicaTions inForMaTion

defined chamber. This

θJB + θBA value should accurately

equal the

θJAvaluebecauseapproximately100%ofpower

loss flows from the junction through the board into ambi-

ent with no air-flow or top mounted heat sink.

LTM4633 Thermal Considerations and Output Current

Derating

The power loss curves at 5V input, 8V input, and 12V

input are in Figures 7 to 9. These power loss curves can

be used in coordination with the load current derating

curves in Figures 10 to 15 for calculating an approximate

θJA thermal resistance for the LTM4633 with various heat

sinking and airflow conditions. The power loss curves

are taken at room temperature, and are increased with a

multiplicative factor of 1.4 at 125°C junction. This factor

comes from the fact that the power loss of the regulator

increases about 50% from 25°C to 150°C, thus a 50%

spread over 125°C delta equates to ~0.4%/°C power loss

increase. A 125°C maximum junction minus 25°C room

temperature equates to a 100°C increase. This 100°C

increase multiplied by 0.4%/°C equals a 40% power loss

increase at the 125°C junction, thus the 1.4 multiplier.

The derating curves are plotted with the output current

starting at 30A and the ambient temperature at 40°C.

The 30A come from each of the three channels operating

at 10A each. This simplifies the loading for this thermal

testing. The output voltages are 1.0V and 1.8V when all

three channels are loaded together in parallel. Channel 1

and Channel 2 are designed to operate with outputs up

to 1.8V. Two additional derating curves are shown with

Channel 1 and Channel 2 operating at 1.8V at 10A each

for a total of 20A while Channel 3 is at 5V with 10A load

current derated over ambient temperature. This is done

to look at some of the different output power conditions

to correlate thermal resistance numbers that can be used

for derating the LTM4633 power module with different

output power requirements. The power loss curve values

at a particular output voltage and output current for each

output are taken and multiplied by 1.4 for increased power

loss at 125°C junction. Thermal models are derived from

several temperature measurements in a controlled tem-

perature 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 125°C maximum while lowering output

current or power with increasing ambient temperature.

The decreased output current will decrease the internal

module loss as ambient temperature is increased. The

monitored junction temperature of 125°C minus the ambi-

entoperatingtemperaturespecifieshowmuchtemperature

rise can be allowed. For example, in Figure 11, the 1V

load current is derated to ~20A at ~85°C with no air and

with heat sink. In Figure 9, the 12V to 1.0V power loss

at 6.66A per channel is 1.4W. The total power loss would

be 3 times 1.4W or 4.2W. The 4.2W is then multiplied by

the 1.4 multiplier for 125°C junction. This 5.88W value is

used with the total temperature rise of 125°C minus the

85°C ambient to calculate

θJA thermal resistance. If the

85°C ambient temperature is subtracted from the 125°C

junction temperature, then the difference of 40°C divided

by 5.88W equals a 6.8°C/W

θJA thermal resistance. Table

2 specifies a 6°C/W value which is very close. Tables 2 to

4 provide equivalent thermal resistances for 1.0V, 1.8V,

and combination 1.8V and 5V outputs with and without air

flow and heat sinking. The derived thermal resistances in

Tables 2 and 4 for the various conditions can be multiplied

by the calculated power loss as a function of the 125°C

maximum junction temperature to determine if the tem-

perature rise plus ambient is below the 125°C maximum

junction temperature. Thermal or infrared imaging should

be performed to validate the calculated results. Room

temperature power loss can be derived from the power

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