Other Parts Discussed in Thread: BQ40Z50, TPS2121, BQ24780S
I have a hand held tablet system currently designed with 2 field swappable 2S1P Li-ion (18650) protected battery modules (BQ40Z50). The current implementation is very complicated to charge and discharge the batteries and allow on the fly battery swapping without crashing the system. The battery to VDC internal bus switch system is controlled by the EC in the system, but to allow instant low VDC bus droop swapping (no brown out), there is also an analog logic side channel to override the EC control of the switchs until the EC can catch up. The present switches are a classic tablet/laptop design (not NVDC) with 3 PFETS in series (2 back to back and one to disconnect batteries from the AC adapter direct supply DC bus) which can create quite a bit of system loss.
I'd like to investigate a design using NVDC-1 chargers with the single NFET BATFET control to significantly reduce the system power loss and extend battery only run time.
The batteries must use 2 independent chargers and the system must also be able to run from the AC adapter without any batteries, plus all the normal dead battery recovery and in system charging you would expect.
The AC adapter comes in through a barrel jack at 19VDC.
Is there an elegant low loss way to connect the output of the 2 charger and battery systems into the system VDC bus to the regulator sub system without adding more FETs or worse blocking diodes? I was thinking that one charger could be the 'main', supplying AC adapter power to the system as well as managing one battery, and the second charger might be connected to the system VDC bus with an ideal diode, but this would add another NFET in the second battery path.
Another option might be to add some sort of 'bridge' supply, battery or super cap, to the VDC system bus, so that the switching from one to the other removable batteries could be handled entirely by the EC. This appears to add additional undesirable complexity.
What are your recommendations?
thanks,
Paul
