LTM4657 datasheet(21/28 Pages) AD

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LTM4657

Rev. A

For more information www.analog.com

Figure 21. Thermal Image of LTM4657 Running from 12V Input

and 1V Output at 8A Load at 25°C Ambient Without Airflow and

Heat Sink

APPLICATIONS INFORMATION

The 1.0V, 1.5V, 3.3V and 5V power loss curves in

Figure 10 to 13 can be used in coordination with the load

current derating curves in Figure 14 to 20 for calculating

an approximate θJA thermal resistance for the LTM4657

with various airflow conditions. The power loss curves

are taken at room temperature, and are increased with a

multiplicative factor according to the ambient tempera-

ture. This approximate factor is: 1.2 for 120°C at junction

temperature. Maximum load current is achievable while

increasing ambient temperature as long as the junction

temperature is less than 120°C, which is a 5°C guard band

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 8A and the ambient

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

3.3V and 5V. These are chosen to include the lower and

higher output voltage ranges for correlating the thermal

resistance. Thermal models are derived from several

temperature measurements in a controlled temperature

chamber along with thermal modeling analysis. The junc-

tion temperatures are monitored while ambient tempera-

ture 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 decrease the internal module loss as

ambient temperature is increased. The monitored junction

temperature of 120°C minus the ambient operating tem-

perature specifies how much module temperature rise can

be allowed. As an example, in Figure 17 the load current

is derated to ~5.5A at ~100°C with no air flow or heat sink

and the power loss for the 12V to 1.5V at 5.5A output is

about 1.2W. The 1.2W loss is calculated with the ~1W

room temperature loss from the 12V to 1.5V power loss

curve at 5.5A, and the 1.2 multiplying factor at 120°C

junction temperature. If the 100°C ambient temperature

is subtracted from the 120°C junction temperature,

then the difference of 20°C divided by 1.2W equals a

16.8°C/W θJA thermal resistance. Table 4 specifies a

17°C/W value which is very close. Table 3, Table 4, Table 5

and Table 6 provide equivalent thermal resistances for

1.0V, 1.5V, 3.3V and 5V outputs with and without air-

flow and heat sinking. The derived thermal resistances

in Table 3, Table 4, Table 5 and Table 6 for the various

conditions can be multiplied by the calculated power loss

as a function of ambient temperature to derive tempera-

ture rise above ambient, thus maximum junction tem-

perature. Room temperature power loss can be derived

from the efficiency curves in the Typical Performance

Characteristics section and adjusted with the above ambi-

ent temperature multiplicative factors. The printed circuit

board is a 1.6mm thick 4-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. A Typical thermal image based on this PCB is

shown in Figure 21.

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