I need a battery charger that can take energy from a solar panel (3-5V, 10-30mA) and charge a Li-ion battery (3.7V) that supply a wireless sensor that consumes 50mA for a few seconds once a minute. Till now I found the bq25504 component. Could be another one more suitable for my application?
Thank you very much!
The bq25504 can charge a single cell Li-Ion battery from a solar cell, but the solar cell maximum voltage must be less than the battery's maximum rated voltage. The bq25504 is a boost charger. If Vin goes above the charger's set output voltage, the input voltage passes straight through to the output voltage through the boost converter's synchronous FET's parasitic body diode. This is common for almost all boost converters.
Howerver, you can still use the bq25504 and your solar cell if you place an LDO between your solar cell and the bq25504. The LDO can be used to keep the voltage to the charger at less than 3V. You might consider the TPS78001.
First of all thank you very much for your answer!
I also looked on BQ24210 but I read that it doesn't work for small currents. Do you recommend another component for this application?
At this time, these two ICs are the only solar chargers that come close to meeting your needs. Can you elaborate on what you found that says the bq24210 won't work well at low currents?
I found a reply on this forum: "The bq24210 will not work with an input source that is only capable of 4mA. Although the bq24210 quiescent current is only 800uA when charging, the minimum recommended charge current is 50mA. Please consider the bq25504. It is considered an "energy harvesting" IC that is designed to operate from very low input power sources." My solar cell provides a current around 20-30mA in full sun (and 5-6V). It is also possible to use a cell with maximum 0.5V and 3-4A. I have to find the best solution combining a IC with a cell.
You can use the bq24210 with a lower charge current than 50mA, but the charge current tolerance gets much larger at low charge currents. If you do not need extremely accurate charge currents, you can use the bq24210 to charge with less than 50mA. The datasheet provides the charge current tolerance across several different charge current ranges. You will need to choose a solar cell with with a voltage range greater than the battery voltage if you use the bq24210. You will need to choose a solar cell with with a voltage range less than the battery voltage if you use the bq25504.
I have a final version of my circuit (attached) and I want to ask you if you can take a short look. At the input I have a solar cell with a range of 0-7V (the charger circuit will work from 4.2V) and aprox 20-30 mA. After bq24210 the voltage could be between 0.9V (if the solar cell will not charge and the battery is discharged) and 4.2V. I need to obtain 3.8V and I choose TPS61200 because I also need at least 500mA for some spikes. This could be a reliable solution?
A lithium ion battery can not be used with a battery voltage of 0.9V. You can review battery basics at www.batteryuniversity.com Its voltage should stay between 3V and 4.2V (check battery datasheet for specifics). It is damaged outside of those ranges. You will need a buck boost (TPS63000, TPS63020, TPS63030, etc) after the battery if you need a 3.8V output voltage. I did not check specific component values. You will be able to calculate those from the datasheets and from lab testing. I would like to suggest that you work with your local distributor who can help you architect your system and choose suitable components.
The bq25504 has an VSTOR (output) overvoltage protection circuitry that will short the input if VSTOR gets too high, thus protecting the battery.
The part can only take a maximum of 400mW of input power so the harvestor does go above OV, its impedance has to be high enough so no more that 400mW is dissipated. Typically the impedance is plenty high enough and in only shorts the input to the point where it pulls down to the OV setting.
No LDO up front is needed.
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