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Need Li-ion battery charger IC.....

udhayakumar a
Prodigy 120 points
Other Parts Discussed in Thread: BQ24075, TPS54231, BQ24702

Hi,

       I am working in a "GSM based vehicle tracking system ". The circuits are powered by a build in Li-ion battery. Now i need a charger circuit for 3.7V Li-ion battery. The charger circuit will be connected to the vehicle battery (It may be 12V or 24V).

specifications and requirement:

3.7V/1400mAH Li-ion battery

Output load  -  2A continuous.

Charger input    -   9V to 25V

Cost     -    Below 2$ (For 10K volume)

Package     -    (T)QFN not preferred

The IC should have automatic charge ON/OFF control.

 

How to choose (or what should be) the output voltage of charger IC for 3.7V battery (may i use 4.2V output IC like BQ24075), because the datasheet provides only the output voltage and not the battery rated voltage.

what should be the current capacity of the device if the battery is 1400mAH.

 

Sorry i don't have any experience in battery charging. So please advice me to select a good charger IC.

 

Thanks&regards

Udhaya kumar.A

SPECTRUM Embedded Solutions

  • 0
    TI__Guru*** 148100 points

    Here is some training that discusses battery charging:

    http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=OLT410002

    Basically, for a simple lithium ion chemistry, the cell is charged in a CC/CV method.  It is charged at a constant current (usually 1 C, which is 1400 mA for your cell) until the pack voltage reaches 4.2V.  Then, it is charged at a constant 4.2V as the current tapers to 0.  The reason why your cell is called a 3.7V cell is that it spends most of its life around 3.7V.  You can look at the discharge curve for the cell (provided by the manufacturer) and will notice that it starts at 4.2V but goes quickly down to around 3.7V where it spends most of the discharge time.

    You can pick any charge rate that you want, but get a recommendation from the battery manufacturer.  Usually, cells are not charged faster than a 1C rate, as this can degrade the life of the cell.

    Yes, a 4.2V charger is the right charger, if your battery is the typical lithium ion chemistry.  Special flavors, such as lithium phosphate, require a different charge voltage and a different charger.

    You have a couple of options for charging.  The bq2407x family is a linear charger.  These chargers do not switch and do not charge at input voltages above their OVP (6.6V for the 075).  They basically function as LDOs and would not be very efficient even if they did charge at 24V in.  24V to 4.2V at 1A is about 24W of dissipation!

    So, you must use a switching charger, or a buck converter that is followed by a linear charger, such as the bq24075.

    Switching chargers fall into 2 categories--host controlled and stand alone.  The bq246xx family are new stand alone chargers that do not require an MCU.  The bq247xx are controlled by an MCU, usually over SMbus.  These families will charge straight from the 24V bus to your cell.

    The second option is to step down the 24V input to around 5V for a linear charger.  This would require a buck converter, such as the TPS54231, followed by a linear charger, such as the bq24075.

    What voltage does your GSM circuit run off of?  If it requires a lower voltage around 5V, then the switching converter followed by the linear charger may be the best option, as you will need a buck anyway to step down your input voltage to power the GSM.

    Most of these chargers only come in a QFN, as this is the industry standard package topology.  The bq24702 comes in a TSSOP.