BQ25703A: Capabilities Question.

Hi Matt,

1) When there is no power adapter or USB plugged in, can the BQ25703A use its' buck/boost to regulate the battery to 12V to supply the load?

Yes and no.  The BQ25703A has a feature called OTG (on-the go) where you can use the buck/boost to regulate the battery voltage.  The only caveat is that the voltage is regulated onto VBUS (the adapter connection) and not onto the SYS output.  The feature is intended for USB OTG and USB PD applications where a USB port can either source or sink power. 

In order to use this regulated voltage to provide SYS, you would need to design external power mux circuitry to select between SYS and VBUS to power your system.

2) When USB is plugged in, can the BQ25703A use the 5V-20V USB input and it's buck/boost to supply 12V to the load AND supply the variable voltage to charge the battery?

You can do this, but with one slight change -- the system voltage will be a minimum of 12V, not an always regulated 12V.  BQ25703A will regulate the system voltage slightly above the battery voltage, so as the battery voltage approaches 12V (and usually for 3S you would charge to at least 12.6V for Li-Ion) the system voltage will need to rise to a value nominally above the battery voltage.  But under this condition, it can supply the system power and charge the battery simultaneously, all from 5V-20V input.

3) If the USB source is just a normal USB 2.0 low current port or a USB 3.0 port, dedicated charging port, USB C port, or USB C PD supply, does it know, or can I tell it what the USB source is capable of? If the USB port can not supply enough current for the load and charge the battery, I would like to turn off the load and have it just charge the battery.

BQ25703A does not have any detection circuitry that would allow it to determine this information.  We do have devices with BC1.2 detection built in, but not on the buck-boost charger, and none of our devices currently have built-in USB C or USB PD detection, so you would need an external solution to perform that selection.  You might want to look at TI's TPS65987 (PD controller) or TUSB322 (type-C controller.) 

There is a minimum input voltage regulation loop on the BQ25703A in addition to a maximum input current regulation loop.  Ideally you program the maximum input current (which would be detected via PD or CC controller) and the BQ25703 will automatically limit its current draw to this amount.  If that information is not available, however, you can program a minimum adapter voltage as well.  If BQ25703A is drawing more power than the adapter can provide, then the voltage will begin to sag. Once it sags to the minimum input voltage BQ25703A will not draw any more power, so it will keep from browning out the adapter.  This may not be sufficient for a USB application, however, because by the time the voltage sags you may have already violated the power contract and the source may reset the connection as a result, so for robust behavior, I think you will really need a CC/PD controller or other external solution to determine the power supply's advertised capabilities.

Regards,

Steve

Hi Matt,

1) When you say the SYS will have a minimum of 12V when USB is plugged in and charging the battery, what happens when the 3S battery is low and only say 10V? During the constant current charge cycle for the battery, putting 12V on it may go over the maximum charge current allowed for the battery. Or is the SYS voltage regulated to 12V and the battery charging voltage further regulated as it charges?

When VSysMin is forcing the converter to regulate SYS above the battery, the BATFET is operated in LDO mode. This isn't really the preferred mode of operation, but it will allow you to have 12V SYS while charging 10V battery at regulated current level. The negative is that you will loose power in the BATFET LDO in addition to loosing power as it is converted by the buck/boost regulator, so efficiency will suffer and extra heat will be generated, which needs to be accounted for.

2) When USB is plugged in, in order to charge the battery and supply the SYS load, does that mean the USB power source MUST be capable of supplying the charge current + load? Or does this do some hybrid that allows the battery to supply the SYS load during times where the SYS draws more than the USB source can handle and goes back to charging the battery when the load is less than the USB supply?

It is the latter. When you set the input current limit, the BQ25703 will automatically scale back the battery charging current and, if necessary, allow the battery to discharge in order to supplement the adapter current so that the adapter current will stay within the set value while supplying the current that is pulled by the system.

3) Does the SYS load have priority over charging? So if the USB supply can supply the full load current and only a little bit left for the battery charging, does the BQ25703A always supply the SYS current and just reduce charging current?

Yes, this is the way that it works. The BQ25703A does not have control over how much load the system will pull on SYS, but it does have control over the charging current, and so the way that it maintains the input current limit is by scaling back the charging current if the SYS pulls a heavy load (or even discharging the battery if necessary as per #2 above.)

Regards,
Steve