4.1 Reduce the System Level Thermal Resistance

A low system thermal resistance ensures that the heat is transferred through the material much faster. Thermal resistance is directly proportional to the length of the thermal path and inversely proportional to the cross-sectional area and thermal conductivity of the thermal path.

Thermal resistance could be reduced by:

• Adding multiple thermal vias which transfers heat in vertical direction (preferably directly under the heating elements such as mmWave sensor and PMIC)
• Thicker Copper foils along with thicker traces helps in spreading heat in horizontal direction
• Component that has potential to dissipate heat (PMIC) placed away from the mmWave sensor to avoid hotspot.

Duty cycle of the Radar operation directly impacts the power dissipation and heat dissipation. Experiments are done to understand the effect of duty cycle on the chip temperature. Some of the below demo examples illustrate various duty-cycle options that are available for various applications.

Another major area of thermal management is in designing the low power supply distribution network. PMIC solution such as LP87524J without an LDO helps this cause.

For more details on the power management optimization using LC filter, see XWR1xxx Power Management Optimizations – Low Cost LC Filter Solution

mmWave sensor has multiple on-chip temperature sensors distributed across the die, which are used in measuring the die temperature.

There are various types of heat mitigating techniques that exists. In this application report, the following methods are explored:

• Board size scaling up to reduce the system thermal resistance
• Various Heat-sink techniques
• Application of Thermal Interface Material to spread the heat on to the larger PCB surface area
• PCB design techniques to effectively dissipate the heat.
• Lowering the power dissipation of the mmWave sensor itself depending upon application needs

For lowering the power dissipation, one of the easiest knobs is to reduce the frame rate in effect this would reduce the duty cycle, hence, there would be a reduction in the power dissipation. Active duty cycle is defined as time duration at which mmWave sensor is active (active duration of the chirps as compared to frame periodicity). Reduction of frame-rate has system level performance implications that must be examined.

NOTE

If the Tx start + idle time is greater than 10 µsec and the inter-chirp dynamic power save option is not disabled, then this duration would be excluded from the active duration of the chirp. Otherwise, it would be included in the active duration of the chirp in the duty-cycle calculation. For the exact duty-cycle calculation based on the chirp configuration, see the mmWave studio in sensor configuration tab.