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LTM4644/LTM4644-1

4644fe

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rise above ambient, thus maximum junction temperature.

Room temperature power loss can be derived from the ef-

ficiency curves in the Typical Performance Characteristics

section and adjusted with the above junction temperature

multiplicative factor. 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.

The 16A represents all four channels in parallel at 4A each.

The four parallel channels have their currents reduced at

the same rate to develop an equivalent θJA circuit evalu-

ation with thermal couples or IR camera used to validate

the thermal resistance values.

Maximum Operating Ambient Temperature

Figures 30 and 31 display the Maximum Power Loss

Allowance Curves vs ambient temperature with various

heat sinking and airflow conditions. This data was derived

from the thermal impedance generated by various ther-

mal derating examinations with the junction temperature

measured at 120°C. This maximum power loss limitation

serves as a guideline when designing multiple output

rails with different voltages and currents by calculating

the total power loss.

For example, to determine the maximum ambient tem-

perature when VOUT1 = 2.5V at 0.6A, VOUT2 = 3.3V at 3A,

VOUT3 = 1.8V at 1A, VOUT4 = 1.2V at 3A, without a heat sink

and 400LFM airflow, simply add up the total power loss

for each channel read from Figure 9 to Figure 15 which in

this example equals 2.5W, then multiply by the 1.35 coef-

ficient for 120°C junction temperature and compare the

total power loss number, 3.4W with Figure 30. Figure 30

indicates with a 3.4W total power loss, the maximum am-

bient temperature for this particular application is around

86°C. For reference, the actual thermal derating test in the

chamber resulted in a maximum ambient temperature

of 86.3°C, very close to the calculated value. Also from

Figure 30, it is easy to determine with a 3.4W total power

loss, the maximum ambient temperature is around 77°C

with no airflow and 81°C with 200LFM airflow.

Safety Considerations

The LTM4644 modules do not provide galvanic isolation

from VIN to VOUT. There is no internal fuse. If required,

a slow blow fuse with a rating twice the maximum input

current needs to be provided to protect each unit from

catastrophic failure. The device does support thermal

shutdown and overcurrent protection.

APPLICATIONS INFORMATION

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