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LTM4632

4632fc

For more information www.linear.com/LTM4632

APPLICATIONS INFORMATION

power loss as that which was simulated. The outcome of

this process and due diligence yields the set of derating

curves provided in other sections of this data sheet. After

these laboratory test have been performed and correlated

to the µModule model, then the θJB and θBA are summed

together to correlate quite well with the µModule model

with no airflow or heat sinking in a properly define cham-

ber. This θJB + θBA value is shown in the Pin Configuration

section and should accurately equal the θJA value because

approximately 100% of power loss flows from the junc-

tion through the board into ambient with no airflow or top

mounted heat sink.

The 1.0V, 1.5V, and 2.5V power loss curves in Figures 9 to

11 can be used in coordination with the load current derat-

ing curves in Figures 12 to 17 for calculating an approxi-

mate θJA thermal resistance for the LTM4632 with no heat

sinking and various airflow conditions. The power loss

curves are taken at room temperature, and are increased

with multiplicative factors of 1.35 assuming junction

temperature at 120°C. The derating curves are plotted

with the output current starting at 6A by putting LTM4632

into two phase single output setup (Figure 20) and the

ambient temperature at 40°C. These output voltages are

chosen to include the lower and higher output voltage

ranges for correlating the thermal resistance. Thermal

models are derived from several temperature measure-

ments in a controlled temperature 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

Figure 8. Graphical Representation of JESD51-12 Thermal Coefficients

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 tempera-

ture is increased. The monitored junction temperature of

120°C minus the ambient operating temperature speci-

fies how much module temperature rise can be allowed.

As an example in Figure 12 the load current is derated

to ~3A at ~100°C with no air or heat sink and the power

loss for the 5V to 1V at 3A output is about 0.95W. The

0.95W loss is calculated with the ~0.7W room tempera-

ture loss from the 5V to 1V power loss curve at 3A, and

the 1.35 multiplying factor at 120°C measured junction

temperature. If the 100°C ambient temperature is sub-

tracted from the 120°C junction temperature, then the

difference of 20°C divided by 0.95W equals a 20°C/W

θJA thermal resistance. Table 2 specifies a 19°C~20°C/W

value which is very close. Table 2 to 4 provide equivalent

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

and without airflow. The derived thermal resistances in

Table 2 to 4 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 and

adjusted with the above ambient temperature multiplica-

tive 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.

4638 F08

µMODULE DEVICE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

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