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BQ25504 Battery Charging and Schematics

Matthew Althar
Prodigy 10 points
Other Parts Discussed in Thread: BQ25504, BQ25505

Hello all,

I am currently working with the BQ25504EVM, an eval module that takes in a low-voltage DC signal (from an energy harvester) and boosts/regulates the output voltage. The BQ25504 device itself has an output voltage range from 2.5V-5.2V, however, the eval module is factory programmed (via voltage dividers) to output ~3V.


My design goal is to charge a battery from the eval module.

1. So, WHY did they choose to make the output of the eval module 3V? It is marketed as being compatible with multiple battery chemistries, but many of the common rechargeable batteries need at least 3.7V-4.2V to charge, which this evaluation module cannot provide.

2. Are there any specific rechargeable batteries (or chemistries) that can be charged off of 3V? I have looked around and cannot seem to find anything.

Since I would like to get a higher voltage out of the device, I intend to design my own PCB for it. The BQ25504 datasheet contains a few sample schematics. However, the voltage divider equations that govern many of the output voltage thresholds mention a "VBIAS" in their equation, which really isn't mentioned anywhere else in the datasheet, and also does not appear on any of the schematics.

3. What is VBIAS referring to? How can/should it be controlled?


Thanks,
Matt A.

  • 0
    TI__Mastermind 20180 points

    Matt,

    Please refer to figure 11 in the datasheet. The operation of the boost charger is based on the thresholds mentioned in figure 11. For bq25504, VBAT_UV, VBAT_OK, and VBAT_OV thresholds are programmed by external resistors.

    Regarding 1, please refer to page 18, typical applications section. Examples with different storage elements are shown in this section. The BAT regulation point, VBAT_OV, can be set by R4 and R3+R13 on the evaluation board. In your case, you can set it to 4.2V. You can use the Design Help excel sheet in tools and software section under the bq25505 product folder in the link below.

    http://www.ti.com/product/BQ25504/toolssoftware

    Regarding 2, the 3V threshold is set to work with super caps.

    Regarding 3, VBIAS can be found in the electrical characterization table on page 6. It is referenct voltage set internally so youdo not have the capability to change it.