BQ24296: Output voltage not supported when battery is removed and power cycled

Hi Phil,

At power up, the buck converter implements a soft start by limiting the input current in order to prevent overshoot.  If the load on SYS is too large and V(SYS) doesn't rise above a certain threshold (~2V) then the charger thinks that SYS is shorted.  If you remove the 2.35ohm load or increase it, the charger will start up.

Regards,

Jeff

That sounds great but as I am incorporating this chip in my system to allow the system to run on main power or battery and I'd like to make sure the final product runs even if the battery is dead or taken out it's not really a solution.

And the spec even says it should work as quoted

Hi Phil,

Datasheet section 9.3.1.4 Converter Power-Up gives more detail about soft start.  This applies to all of our NVDC power path chargers. 

If your system can run at input adapter voltage = 5V, can you use a charger like BQ24133 below?  Q1 is for soft start which you may not need and Q2 could be replaced by a diode for reverse blocking.  

So the spec is actually wrong or at least misleading then?            

And section 9.3.1.4 isn't really explicit on it's implications, might have been more useful if it stated that the output would be limited by the input current limit.

What would happen if a severely depleted battery was inserted for charging, ie. with less than 2.2V?

Suggesting an alternative part is not really solving the problem.

Hi Phil,

Most linear regulators, dc/dc converters and chargers based on them have some kind of soft start at power up.  The soft start can be implemented by either limiting the input current or by ramping a reference voltage/input current or some combo. Most electronics based load profiles are more capacitance than resistive.  If capacitive, having a soft start typically doesn't cause an issue for the dc/dc converter.  But as you have seen, if the load is mostly resistive and the soft start has a digital threshold, the converter ouptut can get stuck. 

With a battery charger, we make the assumption that either a partially charged battery or the adapter will be present during heavy system load.  If you insert a depleted battery and V(SYS) is below the threshold and then attach the input adapter, the charger will have the same power-up issue into heavy load. 

If you want to use the BQ24296, you may be able to add an external PFET between SYS and the 2.35 ohm load.  Then you can add external resistors and capacitors to soft start the PFET similar to what is shown in figure 2 of the app note below:

Regards,

Jeff