Switching to supplement mode of bq24296

Hello Alexy,

On our power-path chargers, the system output is regulated slightly above the battery voltage with a limit that it cannot drop lower below the selected MINSYS threshold (on the bq24296, this threshold can be programmed to be between 3V-3.7V).

Once the input adapter reaches its limit, the charger will begin reducing charge current to prioritize the system power. If the system continues to draw power, the charge current has already been reduced to 0 and the system voltage drops below the battery voltage, the device will go into supplement mode to use the battery power to sustain the system. The voltage at system will then be VBAT minus the RDSon drop of the internal BATFET.

Ideally you want the adapter to be able to sustain the majority of the power demand for your system and to allow charging during the system operation, e.g. if the device is always in supplement mode, the battery will never get charged and the system will crash at some point.

The BATFET will be driven to decrease/increase the current to maintain the device operational without overloading the adapter. The VSYS_BAT comparator is used to determine the BATFET operation.

Hope this helps.

Hello Fernando,

Thank you for your answer. You made the IC operation more clear. Can you clarify the questions shown on the picture?

My load(tablet) requires voltage in 3.7-4.2V range. If voltage will drop less than 3.7V the tablet can turn off or stuck, so I'm worry for the following conditions:

• battery is being charged
• has low level (e.g. 3.8V)
• load requires more current, that causes input voltage drop and entering to supplement mode

What voltage will I have in the load? I expect that voltage should be 3.8-Vbatfet, so 30-50mV less than 3.8V. What transient may I expect?

Looking forward to hearing from you soon,

Hello Alexy,

One thing I would like to clarify is that supplement mode is only one of the modes of the device. Before entering supplement mode, the charge current is reduced until the input is no longer limited. Once the charge current has already been reduced to 0, the battery will begin discharging if the input is still limited and the system is demanding power.

Having said that, I have a couple questions/comments:

• For a 5V/1.5A adapter and charging 2A, the device is more likely in input current limit (assuming 90% charge efficiency at 2A and 5V input). I would expect the actual charge current would be being reduced to ~1.9A from what I can make out of the plot.
• When the system load turns on, it is possible the battery is going into supplement momentarily. Can you increase the resolution of the battery and system currents and measure their exact values? 
• Where is the system voltage being measured? Near the decoupling caps on the system pin or at the system point of load? 

It seems you have the MINSYS threshold set to 3.7V, so your system should see 3.7-VBATREG. Of course, depending on the load, the transients might make this voltage go slightly below MINSYS. How much capacitance do you have on SYS?

Hi Fernando,

Thank you for your response.

• I don't have the system working on the bq24296. I have a tablet that I'm going to use in the design. I have tested its min operating voltage. I connected the one to the laboratory power supply and set supply voltage to 3.8V. I connected oscilloscope to detect voltage drops and min operable level was 3.7V. If I had drop below 3.7V the tablet stuck in most cases. From that I know the minimum level, if level will drop to 3.6V for very short time the tablet may survive, but I can't rely on that.
• These values of system voltage were measured on the decoupling caps.
• As for the current consumption, usually it is not more than 1A, but during startup and some other processes some current spikes reach 2A.

My current 5V supply used in the design may not give fast response and voltage can drop. In this case the bq24296 will switch to supplement more (whether the battery is charged or being charged).

Vsys drop in this case depends on 2 factors: load capacitors and time of the entering into supplement mode.

So the question is how much time will it take the bq24296 to detect Vin drop and turn on the BATFET if it was off as battery was charged.

I guess if the Vin drops, but the battery is being charged, the entering into supplement mode will happen automatically as the BATFET is ON and Vsys will drop to Vbat-Vbatfet.

Looking forward to hearing from you soon.