When designing a wearable product, the first thing that comes into mind is probably its size. The available space for wearables is very limited, yet the battery occupies a higher percentage of the total size due to the long operating time between charges and various functions of wearable devices. The rest of the solution has to be more compact to be able to integrate more functions while also saving extra space for the possibility of a bigger battery.
There are a few options to make the solution size smaller. First of all, the size of the integrated circuit (IC) itself can be greatly reduced by choosing a different package. Compared to the quad flat no-lead (QFN) package, the wafer chip-scale package (WCSP) is on average more than 50% smaller and is almost the same as the true die size. However, because the output current is normally below 300mA for wearables, the power dissipation is also not as large as in the high current applications. Therefore thermal is no longer a big issue in low-power wearable applications.
Consider TI’s PicoStar™ IC packages and MicroSiP™ modules to shrink your solution further. SiP stands for system in package and combines common functions to reduce board space. PicoStar embeds the IC in the package substrate and stacks the other passive components on top of it. Reducing the space needed in the device by as much as half. Figure 1 shows the main concept: the caps and inductors are placed on top of the IC. Because of the 150µm thickness of the PicoStar package, the overall thickness of the module is not much different from regular packaged solutions.
Figure 1: MicroSiP™ Module with IC and Passive Components
Not only can the passive components be stacked, but also the ICs on top of the PCB. In wearable applications such as smart watches and other activity-monitoring devices, with PicoStar, it may make sense to eventually put the charger and gas gauge or the charger and DC/DC converter in a MicroSiP module, as they are always needed.
The TPS82740A is an ultra-low-power DC/DC converter that utilizes the TI MicroSiP package and integrates all of the needed components with a load switch. The total solution size is only 2.3mm x 2.9mm, smaller than many QFN packaged ICs.
In addition, be sure to choose the passive components with the smallest footprint but make sure to verify the voltage and temperature de-rating meet the application requirements.