As with any power conversion device,
the LM6x4xx-Q1 dissipates internal power while
operating. The effect of this power dissipation is to raise the internal temperature
of the converter above ambient temperature. The internal die temperature
(TJ) is a function of the following:
Ambient temperature
Power loss
Effective thermal resistance,
RθJA of the device
PCB layout
The maximum internal die temperature for the LM6x4xx-Q1 must be limited to 150°C. This establishes a limit on the
maximum device power dissipation and, therefore, the load current. Equation 8 shows the
relationships between the important parameters. Larger ambient temperatures
(TA) and larger values of RθJA reduce the maximum
available output current. The converter efficiency can be estimated by using the
curves provided in the Application Curves section. If the desired operating conditions
cannot be found in one of the curves, then interpolation can be used to estimate the
efficiency. Alternatively, the EVM can be adjusted to match the desired application
requirements and the efficiency can be measured directly. The correct value of
RθJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics Application
Report, the value of RθJA given in the Section 7.4 is not
valid for design purposes and must not be used to estimate the thermal performance
of the device in a real application. The values reported in the Section 7.4 table
were measured under a specific set of conditions that are rarely obtained in an
actual application.
Equation 8.
where
η = efficiency
TA = ambient temperature
TJ = junction temperature
RθJA = the effective thermal resistance of the IC junction to the air, mainly through
the PCB
The effective RθJA is a
critical parameter and depends on many factors (just to mention a few of the most
critical parameters:
Power dissipation
Air temperature
Airflow
PCB area
Copper heat-sink area
Number of thermal vias under
or near the package
Adjacent component
placement
Due to the ultra-miniature size of the VQFN (RNX) package, a die-attach pad is
not available, requiring most of the heat to flow from the pins to the board. This
means that this package exhibits a somewhat large RθJA value when the
layout does not allow for heat to flow from the pins. A typical curve of maximum
output current versus ambient temperature is shown in Figure 9-3 and Figure 9-4 for a good
thermal layout. This data was taken on the LM61495RPHEVM evaluation board with a
device and PCB combination, giving an RθJA of about 21.6°C/W. It must be
remembered that the data given in these graphs are for illustration purposes only,
and the actual performance in any given application depends on all of the previously
mentioned factors.
VIN = 13.5
V
VOUT = 5
V
ƒSW = 400
kHz
RθJA =
22°C/W
Figure 9-3 Maximum Output Current versus Ambient Temperature
VIN = 13.5
V
VOUT = 5
V
ƒSW = 2.2
MHz
RθJA =
22°C/W
Figure 9-4 Maximum Output Current versus Ambient Temperature
Use the following resources as a guide
to optimal thermal PCB design and estimating RθJA for a given application
environment: