LTM4658EVPBF datasheet(16/26 Pages) AD

16 / 26 page

LTM4658

Rev. 0

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The 2.5VIN, 3.3VIN and 5VIN power loss curves in Figure 6,

Figure 7 and Figure 8 can be used in coordination with

the load current derating curves in Figure 9 to Figure 14

for calculating an approximate θJA thermal resistance

for the LTM4658 with various heat sinking and airflow

conditions. The power loss curves are taken at room

temperature, and are increased with multiplicative factors

according to the junction temperature. This approximate

factor is ~1.2 assuming the junction temperature is reach-

ing 120°C. Maximum load current is achievable while

increasing ambient temperature as long as the junction

temperature is less than 120°C, which is 5°C guardband

from 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 10A and the ambient

temperature at 30°C. The output voltages are 1.0V, 1.5V

and2.5V.Thesearechosentoincludethelowerandhigher

outputvoltagerangesforcorrelatingthethermalresistance.

Thermal models are derived from several temperature

measurementsinacontrolledtemperaturechamberalong

withthermalmodelinganalysis.Thejunctiontemperatures

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 will

APPLICATIONS INFORMATION

decrease the internal module loss as ambient tempera-

ture is increased. The monitored junction temperature of

120°C minus the ambient operating temperature specifies

how much module temperature rise can be allowed. For

example,todeterminethemaximumambienttemperature

allowable when VIN = 5V, VOUT = 1V and 10A load current

without a heat sink and airflow, simply find out the power

loss from Figure 7 which equals to 1.54W in this case,

then multiply by the 1.2 coefficient for 120°C junction tem-

perature, If the 81.6°C ambient temperature is subtracted

from the 120°C junction temperature, then the difference

of 43°C divided by 1.85W equals a 21°C/W for θJA the

system equivalent thermal resistance. Table 5 specifies

20.8°C/W value which is very close. Table 6 and Table 7

provide equivalent thermal resistances for 1.5V and 2.5V

outputs with and without airflow. The Pin Configuration

section shows the simulation data for the worst case

scenario. Table 5 through Table 7 provide equivalent

thermal resistances for 1.0V, 1.5V and 2.5V outputs with

and without airflow. The derived thermal resistances in

Table 5 through Table 7 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

andadjustedwiththeaboveambienttemperaturemultipli-

cative factors. The printed circuit board is a 1.6mm thick

four-layer board with two ounce copper for the two outer

layers and one ounce copper for the two inner layers. The

PCB dimensions are 95mm × 76mm.

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