BQ24780S: Max duty

Hi Anthony,

The only thing that limits the duty cycle is the refresh pulse, although the performance does begin to degrade as you get close to 100% duty cycle from the standpoint of input ripple due to pulse skipping. 

Because of the refresh pulse, you will never get to 100% duty cycle, but if you take an EVM and set it to charge with nothing connected at the battery output, you will see it operate at very close to 100% with just the refresh pulse.

But again, there can be larger ripple at the input as you get into this condition.  We don't define a limit because the IC doesn't limit the duty cycle and the increased ripple is a gradual process so there is no hard cut-of.  I think you are okay as long as there is 1.2 V difference between the lowest possible adapter voltage and the battery charge voltage.  It is possible to push further than this, even to a difference that is below 1.0V, but I think customer should evaluate the performance on EVM if they need to go below a 1.0V difference.

Note that the input voltage can be brought a little closer to the battery voltage when the switching frequency is lower.  This helps reduce the ripple and also improves the efficiency, both of which help.  So using the 600 kHz setting may get you ~100 mV extra margin versus 800 kHz default.    It doesn't sound like a lot, but it can make a difference if you are right on the edge.

Regards,

Steve