User's Manual Part 1

ZSTAR Sensor Board hardware overview

Wireless Sensing Triple Axis Reference design, Rev. 0.9

Freescale Semiconductor 25

3.4.10 Power supply

The Sensor board is powered by a Lithium coin-sized battery. The primary choice was the popular

CR2032, with a PCB layout provision made for the CR2477 size. This bigger battery holds roughly 4 times

more charge (~1000mAh), but it is not as popular as CR2032 size.

A surface mounted SMTU series battery holder from Renata

is placed on the underside of the PCB.

The SMTU series holders provide (by mechanical construction) battery reverse protection, so no

additional circuitry is required. Slide switch SW1 disconnects the battery from the application when not in

use.

A large tantalum capacitor (C14, 470µF/4V) improves the response of the power supply to current peaks

caused by reception or transmission. Coin-sized Lithium CR2032 batteries are targetted at a maximum

continuous discharge current in the range of 3mA. Such a large capacitor helps to supply enough current

to the MC13191 during a receive/transmit without significant V

voltage drops.