Please explain the concept of permissible loss for motor drivers.

The permissible power dissipation is the maximum allowed power dissipation when the junction temperature Tj reaches its maximum rating of 150 ℃, under the condition that the ambient temperature Ta is 25 ℃. Consequently, this value changes as the temperature of Ta increases, requiring a derating calculation.

Fig. 1 TB67S569FTG PD-Ta graph
Fig. 1 TB67S569FTG PD-Ta graph

As an example, Fig. 1 below shows the relationship between power dissipation and ambient temperature for TB67S569FTG. The permissible power dissipation decreases as the ambient temperature exceeds 25 ℃. This reduction is due to the increased difficulty of heat dissipating into the surrounding air at higher temperature. Note that the allowed power consumption of the device is limited as the ambient temperature increase.

In addition, mounting the IC on a board reduces thermal resistance, resulting in a higher permissible power dissipation than when IC is used alone.
The permissible power dissipation for an IC when used alone, assumeno heat sink or PCB heat dissipation. The IC generates heat when a large current is applied. Without adequate heat dissipation, when the temperature reaches the detection threshold of the thermal shutdown function (TSD), the TSD will activate, causing the IC to shut down. If this condition persists, there is a high possibility that the IC will breakdown due to exceeding the absolute maximum rated temperature.
Therefore, please refer to the application note of each product and design the heat dissipation to ensure that the junction temperature does not exceed 150 °C, including a margin.

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