Datasheet

LTM4620A
25
4620afb
For more information www.linear.com/LTM4620A
The device does support over current protection. A tempera-
ture diode is provided for monitoring internal temperature,
and
can be used to detect the need for thermal shutdown
that can be done by controlling the RUN pin.
Power Derating
The 1V, 2.5V and 5V power loss curves in Figures 12 to 14
can be used in coordination with the load current derating
curves in Figures 15 to 24 for calculating an approximate
Θ
JA
thermal resistance for the LTM4620A with various heat
sinking and airflow conditions. The power loss curves are
taken at room temperature, and are increased with a 1.35
to 1.4 multiplicative factor at 125°C. These factors come
from the fact that the power loss of the regulator increases
about 45% from 25°C to 150°C, thus a 45% spread over
125°C delta equates to ~0.35%/°C loss increase. A 125°C
maximum junction minus 25°C room temperature equates
to a 100°C increase. This 100°C increase multiplied by
0.35%/°C equals a 35% power loss increase at the 125°C
junction, thus the 1.35 multiplier.
The derating curves are plotted with CH1 and CH2 in
parallel single output operation starting at 26A of load
with low ambient temperature. The output voltages are
1V, 2.5V and 5V. These are chosen to include the
lower
and
higher output voltage ranges for correlating the ther-
mal resistance
.
Thermal models are derived from several
temperature measurements 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 curves. The junctions are
maintained at ~120°C maximum while lowering output
current or power while increasing ambient temperature.
The decreased output current will decrease the internal
module loss as ambient temperature is increased.
The monitored junction temperature of 120°C minus
the ambient operating temperature specifies how much
module temperature rise can be allowed. As an example in
Figure 15, the load
current is derated to ~19A at ~80°C with
no air or heat sink and the power loss for the 12V to 1.0V
at 19A output is a ~5.1W loss. The 5.1W loss is calculated
with the ~3.75W room temperature loss from the 12V to
1.0V power loss curve at 19A, and the 1.35 multiplying
factor at 125°C ambient. If the 80°C ambient temperature
is subtracted from the 120°C junction temperature, then
the difference of 40°C divided 5.1W equals a 7.8°C/W Θ
JA
thermal resistance.
Table 2 specifies a 6.5 to 7°C/W value
which is pretty close. The airflow graphs are more accurate
due to the fact that the ambient temperature environment is
controlled better with airflow. As an example in Figure 16,
the load current is derated to ~22A at ~90°C with 200LFM
of airflow and the power loss for the 12V to 1.0V at 22A
output is a ~5.94W loss.
The 5.94W loss is calculated with the ~4.4W room tem
-
perature loss
from the 12V to 1.0V power loss curve at
22A, and the 1.35 multiplying factor at 125°C ambient. If
the 90°C ambient temperature is subtracted from the 120°C
junction temperature, then the difference of 30°C divided
5.94W equals a 5.1°C/W Θ
JA
thermal resistance. Table 2
specifies a 5.5°C/W value which is pretty close. Tables 2-4
provide equivalent
thermal resistances for 1.0V, 2.5V and
5V outputs with and without airflow and heat sinking.
The derived thermal resistances in Tables 2-4 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 and adjusted with
the 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 101mm × 114mm.
The BGA heat sinks are listed below Table 4.
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