Datasheet

LTM4628
22
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
The 1.0V and 3.3V power loss curves in Figures 11 and 12
can be used in coordination with the load current derating
curves in Figures 13 to 24 for calculating an approximate
θ
JA
thermal resistance for the LTM4628 with various heat
sinking and airflow conditions. The power loss curves are
taken at room temperature, and are increased with mul
-
tiplicative factors according to the ambient temperature.
The approximate factors are: 1.35 for 115°C and 1.4 for
120°C. The derating curves are plotted with V
OUT1
and
V
OUT2
in parallel single output operation starting at 16A
and the ambient temperature at 40°C. The output volt-
ages a
re 1.
0V, and 3.3V. 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 a
long with thermal modeling analysis.
The junction temperatures are monitored while ambi-
ent t
emperature
is increased with and without airflow.
The power lo
ss increase with ambient temperature change
is factored into the derating curves. The junctions are
maintained at 115°C to 120°C maximum while lowering
output current or power with increasing ambient tem
-
perature. The d
ecreased output current will decrease the
internal module l
oss as ambient temperature is increased.
The monitored j
unction temperature of 120°C minus
the ambient operating temperature specifies how much
temperature rise can be allowed. As an example, in
Figure 14 the load current is derated to ~12A at ~80°C with
no air or heat sink and the power loss for the 12V to 1.0V
at 12A output is about 3.65W. The 3.65W loss is calculated
with the ~2.7W room temperature loss from the 12V to
1.0V power loss curve at 12A, and the 1.35 multiplying
factor at 120°C junction. If the 80°C ambient tempera
-
ture is
sub
tracted from the 120°C junction temperature,
then the difference of 40°C divided by 3.65W equals a
10.9°C/W
θ
JA
thermal resistance. Table 2 specifies a
9.5°C/W to 10°C/W value which is very close. Table 2 and
Table 3 provide equivalent thermal resistances for 1.0V and
3.3V outputs with and without airflow and heat sinking.
The derived t
hermal resistances in Tables 2 and 3 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. The no-airflow θ
JA
values have some variation
from 9.5°C/W to 11°C
/W depending on the 115°C to 120°C
holding junction temperature. All other airflow thermal
resistance values are more accurate. Room temperature
power loss can be derived from the efficiency curves in
the Typical Performance Characteristics section and ad
-
justed w
ith t
he above ambient temperature multiplicative
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. The BGA heat sinks are
listed in Table 3.