Need 19.5V DC charger / fuel gauge circuit for 12V 3800mAh Litium Ion Battery

Hello Carl,

We have two device architectures that you should consider for this application. 

1.  Turbo Boost architecture -- BQ24780s

2.  NVDC Architecture -- BQ2477x (Buck only) or BQ2571x (Buck-Boost)

Your first step should be to choose which architecture fits your application the best.  Both of these architectures will power laptop and charge the battery simultaneously when adapter is plugged in, will power from the battery when the adapter is not plugged in, and will supplement the adapter with the battery if the system draws more power than the adapter can provide.

The difference between the two architectures is that the Turbo Boost architecture passes the adapter voltage directly through to the system and then uses a separate buck switching regulator to charge the battery, whereas the NVDC passes the adapter voltage through the switching regulator and the output of the switching regulator (which is just nominally above the current battery voltage while charging) is passed through to the system and used to charge the battery.

In your example, if using the BQ24780s, you will pass 19.5V directly through to the system when the adapter is present, even while simultaneously charging the battery.  If the battery supplements the adapter (Turbo Boost mode), then the switching regulator will boost the battery voltage up to the adapter voltage.  When the adapter is removed, you will instead power off the battery and the system will see the ~12V battery voltage instead.

If using the NVDC architecture, the system will always see a voltage that is close to the current battery voltage whether the adapter is plugged in or not.  There are advantages to both architectures.  With Turbo Boost architecture, the adapter power is passed directly to the system so there is no power loss through a switching regulator when running off of the adapter.  But the negative is that the system has to be able to support a wider voltage range, i.e. it needs to be able to run off of either the 20V adapter voltage or the 12V battery voltage.

If you decide to use the NVDC architecture, you should look at both the BQ2477x and the BQ2571x.  For your application, you only need the buck operation to drop the 19.5V adapter down to battery charging voltage (i.e. you don't need the additional boost capability of the BQ2571x), so BQ2477x is likely the best fit, but you still might want to look at the BQ2571x which has some additional features such as an ADC that can be used to read battery voltage.

Regards,

Steve

Steve, thanks.  I'm leaning toward the NVDC architecture with the BQ2571x, because the max voltage of my system is 14VDC.  I'm also going with bigger batteries to increase capacity and cut down on charge time.

My current working view is to use a 19.5VDC laptop power supply and three Honcell HCP688196ZC 7500mAh batteries in a 3S series stack to get a nominal [10, 12.6]V power source for my system (V_sys). Thus, I need a charger circuit that will run from the 19.5V laptop supply to charge the three batteries at a max charge current of 3.75A. I also need the charger circuit to maintain charge balance on the three series cells while charging. I also need a “fuel gauge” circuit to monitor the charge in the 3S battery stack. I’m already using the TPS259827L eFuse with a 10V UVLO to cut out the battery if its voltage falls too low to power the system. Finally, while the laptop charger is plugged in, I want the system to run off the laptop supply (while the battery is charging, typical laptop use-case).

Can the BQ2571x deliver the charge-balancing requirement and the 3.75A charge current?  Please let me know what other TI parts and circuit suggestions you have to help realize this concept.

Thanks,

Carl Olen