JAJSR90 データシート

JAJSR90 September 2023 LMQ644A2-Q1

PRODMIX

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）

RXA|24
- MPQF597C

サーマルパッド・メカニカル・データ

発注情報

9.2.2.11 Typical Thermal Performance

As with any power conversion device, the LMQ644xx 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 (T_J) is a function of the following:

Ambient temperature
Power loss
Effective thermal resistance, R_θJA of the device
PCB layout

The maximum internal die temperature for the LMQ644xx must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 9 shows the relationships between the important parameters. Larger ambient temperatures (T_A) and larger values of R_θJA reduce the maximum available output current. For low ambient temperature designs 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 the junction temperature can be roughly estimated using the EVM thermal performance as a starting point. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly. The correct value of R_θJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics application report, the JEDEC value of R_θJA given in the electrical characteristics table is not always 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 electrical characteristics table were measured under a specific set of conditions that are rarely obtained in an actual application.

Equation 9.

I_{O U T_M A X} = (\frac{T_{J} - T_{A}}{R_{θ J A}} \times \frac{η}{(1 - η)} \times \frac{1}{V_{O U T}})

where

η = efficiency
T_A = ambient temperature
T_J = junction temperature
R_θJA = the effective thermal resistance of the IC junction to the air, mainly through the PCB

The effective R_θJA 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

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 is for the LMQ644A2 evaluation board . 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.

Figure 9-3 Typical Output Current versus Ambient Temperature, V_IN = 12 V F_SW = 400 kHz, Dual Output

Figure 9-5 Typical Output Current versus Ambient Temperature, V_IN = 12 V F_SW = 1 MHz, Dual Output

Figure 9-7 Typical Output Current versus Ambient Temperature, F_SW = 400 kHz, Single Output

Figure 9-4 Typical Output Current versus Ambient Temperature, V_IN = 24 V F_SW = 400 kHz, Dual Output

Figure 9-6 Typical Output Current versus Ambient Temperature, V_IN = 24 V F_SW = 1 MHz, Dual Output

Figure 9-8 Typical Output Current versus Ambient Temperature, F_SW = 1 MHz, Single Output

Use the following resources as a guide to optimal thermal PCB design and estimating R_θJA for a given application environment: