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BQ24250: Buck converter for BQ24250 to 3.3V for powering wireless sensor node

ram krishnan
Intellectual 820 points
Part Number: BQ24250
Other Parts Discussed in Thread: CC1120, TPS63000, TPS61175

We are working on the design for a solar powered outdoor wireless sensor node. We are using a BQ24250 to charge a Li-Ion battery. The micro (msp430), radio (cc1120) and the rest of the electronics will run a voltage less than 3.3V (to be decided - anywhere between 2.5V to 3.3V). The radio will operate at 433 MHz. Should we go for a switching regulator or an LDO to convert the BQ24250 SYS voltage output to the voltage range (2.5V to 3.3V) ? How do we ensure that the power supply noise has minimal effect on the radio and other sensors on board ? If switching noise will be an issue, we can live with the power wasted in the LDO otherwise we prefer a buck converter. Any suggestions on part numbers. Max current consumption will be less than 500 mA.

  • 0
    TI__Guru**** 167196 points
    Ram,

    The drop from 4.2V to 2.5V with 500mA load through a linear regulator gives 850mW. TI has several linear regulator options. I would suggest a package in QFN with powerpad that can be soldered to a ground plane for the heat dissipation. In fact you may want to get a regulator that is rated for higher current so that its package is bigger and will provide more heat sinking. Unless you plan to stop battery discharge below ~3.35V, you will have to use a buck-boost converter like TPS63000 family or a boost converter like TPS61175 turned into a SEPIC to provide 3.3V. Regardless, the BQ24250 is a switching converter and will produce switching EMI noise. For any switching converter, you can minimize EMI by doing the following:
    1. Tight layout of the input capacitor for buck and the output capacitor for a boost with a parallel 0.01-0.1uF capacitor
    2. Properly size an RC snubber from the SW node to ground
    A snubber almost always reduces EMI to acceptable levels but at the expense of converter efficiency. The appnote at the link below explains how to optimize a snubber for your specific board. www.ti.com/.../slva255.pdf