LTM4623
17
4623fc
For more information www.linear.com/LTM4623
applicaTions inForMaTion
respect to total package power loss. To reconcile this
complication without sacrificing modeling simplicity—but
also, not ignoring practical realities—an approach has been
taken using FEA software modeling along with laboratory
testing in a controlled environment chamber to reason
-
ably define and correlate the thermal resistance values
supplied in this data sheet
:
(1) Initially, FEA software is
used to accurately build the mechanical geometry of the
LTM4623 and the specified PCB with all of the correct
material coefficients along with accurate power loss source
definitions; (2) this model simulates a software-defined
JEDEC environment consistent with JESD 51-12 to predict
power loss heat flow and temperature readings at different
interfaces that enable the calculation of the JEDEC-defined
thermal resistance values; (3) the model and FEA software
is used to evaluate the LTM4623 with heat sink and airflow;
(4) having solved for and analyzed these thermal resistance
values and simulated various operating conditions in the
software model, a thorough laboratory evaluation replicates
the simulated conditions with thermocouples within a
controlled environment chamber while operating the device
at the same power loss as that which was simulated. An
outcome of this process and due diligence yields the set
of derating curves shown in this data sheet. After these
laboratory tests have been performed and correlated to
the LTM4623 model, then the θ
JB
and θ
BA
are summed
together to provide a value that should closely equal the
θ
JA
value because approximately 100% of power loss
flows from the junction through the board into ambient
with no airflow or top mounted heat sink.
The 1.0V, 1.5V, 3.3V and 5V loss curves in Figures8 to 11
can be used in coordination with the load current derating
curves in Figures 12 to 22 for calculating an approximate
θ
JA
thermal resistance for the LTM4623 with various air-
flow conditions. The power loss curves are taken at room
temperature, and are increased with a multiplicative factor
according to the ambient temperature. This approximate
factor is:
1.3 for 120°C at junction temperature. Maximum
load current is achievable while increasing ambient tem
-
perature as long as the junction temperature is less than
120
°C
, which is a 5°C guard band from maximum junction
temperature of 125°C. When the ambient temperature
reaches a point where the junction temperature is 120°C,
then the load current is lowered to maintain the junction at
120°C while increasing ambient temperature up to 120°C.
The derating curves are plotted with the output current
starting at 3A and the ambient temperature at 30°C. The
output voltages are 1.0V, 1.5V, 3.3V and 5V. These 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 air
flow
. 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 with 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
16 the load current is derated to 2.5A at ~95°C with no air
flow or heat sink and the power loss for the 12V to 1.5V
at 2.5A output is about 1.0W. The 1.0W loss is calculated
with the ~0.8W room temperature loss from the 12V to
1.5V power loss curve at 2.5A in Figure 9, and the 1.3
multiplying factor at 120°C junction temperature. If the
95°C ambient temperature is subtracted from the 120°C
junction temperature, then the difference of 25°C divided
by 1.0W equals a 25°C/W θ
JA
thermal resistance. Table 4
specifies a 25°C/W value which is very close. Table 3 to
Table 6 provide equivalent thermal resistances for
1.0V to
5V outputs with and without airflow. The derived thermal
resistances in Table 3 to Table6 for the various condi
-
tions 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 ef
-
ficiency curves in the Typical Performance Characteristics
section and adjusted with the above ambient temperature
multiplicative factors. The printed cir
cuit board is a
1.6mm
thick 4-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.
