BQ25910: Why the efficiency of charge pump(BQ25970)>3- level buck(BQ25910)>2-level buck(BQ25890H)

Hi Miles,

First, I suggest reading the ADJ article here: http://www.ti.com/lit/an/slyt769/slyt769.pdf and/or watching the training videos on each topology at https://training.ti.com/node/1139543. 

There are PhD dissertations written on this subject but I will try to summarize here:

BQ25970 is a direct/flash charger.  The flash charger employs two shorting FETs between VBUS and VBAT to achieve charging.  In a direct charger without switched capacitor converter, the voltage at VBUS is set just high enough by a smart input adapter to allow the IBUS=ICHRG= (VBUS-VBAT)/RDSon to charge the battery with minimal losses across the FETs.  The charger has an ADC to continuously provide the VBAT info to the smart adapter. The BQ25970's switched cap converter is an unregulated switching converter that simply doubles the input current and halves the input voltage. Therefore, the BQ25970 requires a smart adapter that can be regulated to twice the battery voltage.  Because IBAT is now double IBUS, thanks to the SC converter, the losses from the adapter to the charger are much less.  The only loss elements in the charger are the FETs' RDSon

BQ25910 uses a 3-level converter. With the addition of a flying capacitor, CFLY, to a typical inductor-only buck converter, the three-level reduces voltage stress on switching FETs by half, doubles the effective switching frequency, and the inductor has one-fourth of the peak ripple current.  This results in fewer losses across the power FETs.  These gains translate into both high efficiency and high power density, which are usually at odds with each other in a buck-converter design. The switch node of a typical buck converter (like the BQ25890H) alternates from VBUS to GND at all times. In the three-level architecture, assuming that CFLY remains balanced at VBUS/2, the switch node alternates from VBUS to VBUS/2 or VBUS/2 to GND, depending on the conversion ratio. This results in a smaller inductance requirement.  Smaller valued inductors have lower DCR values and related losses which results in higher efficiency.

Regards,

Jeff

Thank you very much!