LTM4671 datasheet(24/36 Pages) AD

24 / 36 page

LTM4671

Rev. B

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The 1V to 5V power loss curves in Figure 10 to Figure

16 can be used in coordination with the load current

derating curves in Figure 17 to Figure 26 for calculating

an approximate θJA thermal resistance for the LTM4671

with various heat sinking and airflow conditions. The

power loss curves are taken at room temperature and

are increased with a multiplicative factor according to

the junction temperature. This approximate factor is 1.3

considering internal junction temperature hitting 120°C at

the point of derating starts. The derating curves are taken

withthreedifferentoutputpowercombinations,lowpower

(VOUT0 = VOUT3 = 1V, VOUT1 = VOUT2 = 1.5V), medium

power (VOUT0 = VOUT3 = 1.8V, VOUT1 = VOUT2 = 3.3V) and

high power (VOUT0 = VOUT3 = 3.3V, VOUT1 = VOUT2 = 5V).

Output current starting at 100% of the full load current

(IOUT0 = IOUT3 = 12A, IOUT1 = IOUT2 = 5A) and the ambient

temperature starting at 30°C. 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 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 120°C maximum while lowering output cur-

rent or power with increasing ambient temperature. The

decreasedoutputcurrentwilldecreasetheinternalmodule

loss as ambient temperature is increased. The monitored

junctiontemperatureof120°Cminustheambientoperating

temperature specifies how much module temperature rise

can be allowed. The printed circuit board for this test is a

1.6mm thick six layers board with two ounce copper for

the two outer layers and one ounce copper for the four

inner layers. The PCB dimensions are 121mm × 112mm.

Figure 27 and Figure 28 display the maximum power loss

allowance curves vs ambient temperature with various

heat sinking and airflow conditions. This data was derived

from the thermal derating curves in Figure 17 to Figure 26

with the junction temperature measured at 120°C. This

maximumpowerlosslimitationservesasaguidelinewhen

designing multiple output rails with different voltages and

currents by calculating the total power loss. For example,

to determine the maximum ambient temperature when

VIN = 12V, VOUT0 = 1V at 10A, VOUT1 = 1.8V at 3A, VOUT2

= 3.3V at 2A, VOUT3 = 1.5V at 10A, without a heat sink

and any airflow, simply add up the total power loss for

each channel read from Figure 10 to Figure 16 which in

this example equals 4.8W (1.6W + 0.7W + 0.6W + 1.9W),

then multiply by the 1.3 coefficient for 120°C junction

temperature and compare the total power loss number,

6.3W with Figure 27. Figure 27 indicates with a 6.3W total

power loss, the maximum ambient temperature for this

application is around 66°C. Also from Figure 27, it is easy

to determine with a 6.3W total power loss, the maximum

ambient temperature is around 73°C with 200LFM airflow

and 77°C with 400LFM airflow.

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