Question: The schematic of the HX2LP™ Reference Design Kit has pin 57 (TAB) connected to ground. The HX2LP chip has only 56 pins. Where can pin 57 be found? What is its purpose?
The E-PAD on the bottom of the HX2LP chip is pin 57. The datasheet does not show this because it is just a thermal path. The HX2LP Reference Design Kit wants the E-PAD to be connected to ground. The schematic programs need a pin number, so the number is set to pin 57. This thermal E-PAD is a copper fill, which will be designed into the PCB and under the QFN package, in order to assist thermal transfer.