LTM4641
APPLICATIONS INFORMATION—THERMAL CONSIDERATIONS AND
OUTPUT CURRENT DERATING
formance of the LTM4641 on DC1543 hardware; a 4-layer
FR4 PCB measuring 96mm × 87mm × 1.6mm using outer
and inner copper weights of 2oz and 1oz, respectively. The
power loss curves are taken at room temperature, and are
increased with multiplicative factors with ambient tem-
perature. These approximate factors are listed in Table 3.
(Compute the factor by interpolation, for intermediate
temperatures.) The derating curves are plotted with the
outputcurrentstartingat10Aandtheambienttemperature
at 40°C. The output voltages are 6V, 3.3V and 1.5V. These
are chosen to include the lower and higher output voltage
rangesforcorrelatingthethermalresistance.Thermalmod-
els are derived from several temperature measurements
in a controlled temperature chamber along with thermal
modeling analysis. The junction temperatures are
monitoredwhileambienttemperatureisincreasedwithand
without air flow, and with and without a heat sink attached
with thermally conductive adhesive tape. The BGA heat
sinks evaluated in Table 7 (and attached to the LTM4641
with thermally conductive adhesive tape listed in Table 8)
yield very comparable performance in laminar airflow
despite being visibly different in construction and form
factor. 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
currentorpowerwhileincreasingambienttemperature.The
decreasedoutputcurrentwilldecreasetheinternalmodule
loss as ambient temperature is increased. The monitored
junction temperature of 120°C minus the ambient operat-
ing temperature specifies how much module temperature
rise can be allowed. As an example in Figure 38, the load
current is derated to ~8A at ~81°C ambient with no air or
8A
condition is ~3.1W. The 3.74W loss is calculated
OUT
with the ~3.1W room temperature loss from the 36V to
IN
1.5V
power loss curve at 8A (Figure 20), and the 1.205
OUT
multiplying factor at 81°C ambient (interpolating from
Table 3). If the 81°C ambient temperature is subtracted
from the 120°C junction temperature, then the difference
of 39°C divided by 3.74W yields a thermal resistance, θ ,
JA
of 10.4°C/W—in good agreement with Table 6. Tables 4,
5 and 6 provide equivalent thermal resistances for 6V,
3.3V and 1.5V outputs with and without air flow and heat
sinking. The derived thermal resistances in Tables 4, 5
and 6 for the various conditions can be multiplied by the
calculatedpowerlossasafunctionofambienttemperature
to derive temperature rise above ambient, thus maximum
junction temperature. Room temperature power loss can
be derived from the efficiency curves in the Typical Per-
formance Characteristics section and adjusted with the
above ambient temperature multiplicative factors.
Table 3. Power Loss ꢁultiplicative Factors vs Aꢃbient
Teꢃperature
POWER LOSS ꢁULTIPLICATIVE
AꢁBIENT TEꢁPERATURE
FACTOR
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
Up to 40°C
50°C
60°C
70°C
80°C
90°C
100°C
110°C
120°C
heat sink and the power loss for this 36V to 1.5V
at
IN
OUT
4641f
40
Linear [ Linear ]
