BQ24296: Design assistance with BQ24296 (LI ION CELL + 4G MODULE)

Hey Ignacio,

So just a few comments on things to consider when using this part.

The setting you want to look at is the Vsysmin setting (REG01[3:1]). The maximum settable Vsysmin is 3.7V on this part, which means that when the battery is below this threshold, the integrated buck converter will regulate the SYS voltage to that setting.

The other thing to note is that when the battery charges, it will rise above this regulation setting (3.7V) up to 4.2V (for typical Li-ion batteries).

So you need to make sure your LTE Modem can operate between 3.7 and 4.2V.

Lastly, one question I have is if you expect your battery to power the system by itself sometimes (i.e. no 5V system input)? If so, the SYS pin voltage will follow the battery voltage minus an IR drop from the Q4 FET. So be aware that a more discharged battery operating the system on its own could run down to 3.0V before it turns off.

Regards,
Joel H

Hey Ignacio,

Firstly, C29 should be a larger value. I'd recommend 8-10uF, rated for 25V

I recommend also pulling /PG, STAT, INT, and /CE to a fixed rail through a resistor. /CE is an input to the charger, and needs to some fixed logic state even if you have some GPIO attached from your PMIC. 

Same with SDA and SCL. Make sure you have strong enough pull-up resistors for proper I2C communication.

If you are not using the TS pin with a thermistor, you want to change the resistor network consisting of R5 and R8 such that the voltage @ TS pin is anywhere between 50-70% of VCC_REGN. This will ensure the charger is in normal operation.

Lastly, the important thing to note here is that once you are only using battery power, the VSYS voltage could drop to <3.4V if your battery is discharged enough. There are two options there:

1) Add a buck/boost DC/DC converter to VSYS to ensure constant 3.7V or

2) Make sure a valid 5V source is applied once VBAT is ~ 3.5V 

Hope this helps,

Regards,

Joel H

You current have ILIM set to 330 ohms, which right now would give you ~1.38A of input current. The ILIM resistor setting you will want could be based on a few things.

You definitely want it above your max desired charging current, which you said was 800mA.

You want to also account for input power requirements (use the efficiency graphs, Figure 2 and Figure 3) to determine how much input current you will need for your worst case input voltage (lowest), max charging current, and average system current.

Consider too peak current of 3A that your system will experience. If this is something you want the charger to handle, and not supplemented by the battery, then I'd recommend the max settable ILIM. Tying the pin to GND will set max input current.

If you will be controlling the input current limit via I2C through REG00[2:0], then I suggest setting the ILIM higher. The charger uses the lower of the two settings (I2C vs ILIM pin setting).

Lastly, you have to determine what is the power capability of your input source (+5.0V_SYS in your schematic).


Regards,
Joel H

Limiting the system current draw is something our charger does not do. That is inherent to your system to design.

The input limit only limits the power supply current to the charger, and ultimately to the battery (charging current) and the system. For example, if you select R_ILIM such that ILIM is 1A, and you program 800mA (via I2C) of charge current via I2C, and you reach the input current limit because your system requires more current, the battery charging current will be reduced until it begins to supplement the system current.

Like I said, make sure you supply can handle the ILIM you set, and understand that you can also set IINDPM (the I2C equivalent of ILIM) by adjusting register settings.


Hope this helps!

Regards,
Joel H