BQ25713: High current VSYS application

Hey David,

Normally, I would recommend you move to using a 5mOhm RSR (I found some 5W and 10W ones on Mouser) so your peak power loss through the resistor is 4.5W. 

In this scenario, you can still use the discharge ADC measurement of the battery current, but you will have to scale the ADC values from the datasheet as it assumes a 10mOhm resistor. The same goes for the charging current setting values. 

Because of your scenario, you will likely need to forgo using the SYS node as your system rail, and instead tie directly to the battery as you mentioned. This means your system rail relies solely on the battery voltage, so if your system UVLO > battery depleted voltage, you will have a scenario where the system will brown out. 

If you want to remove the BATFET, you need to follow paragraph 3 of section 9.6.5.1 System Voltage Regulation in the datasheet. One caveat that is not listed in this section (although to be added in a revision to the D/S) is disabling charge. When the charger is operating without the BATFET, you can only disable charge by placing the converter in HIZ mode. 

Now to use the discharge ADC reading, you would actually connect your system load to the SRP node instead of the SRN node, such that discharge current still flows through the RSR resistor. 

And as a last comment concerns the slew rate of your 15-30A system load. If the slew rate is <50us, I would suggest utilizing the IL_AVG bits which program the cycle-by-cycle peak inductor current. By default, this value is 15A. For loads faster that 50us, the converter in this configuration will attempt to the provide the full 15-30A until the cycle-by-cylce current limit is reached. At slower loads, the IINDPM loop will kick in and forcibly limit the input power to VBUS x IIN_HOST setting. 

Regards,

Joel H

Hey David,

You can certainly leave the BATFET populated, and the converter will provide most of the current through the BATFET. 

Using the figure below, I can clarify some things, and reference some of the highlighted points:

The Red current path is the power coming from the converter from the input through the converter. 

The Blue current path is the battery current (either charging or discharging).

The Green current path is the system load discharge path. This is where you would connect your system.

While charging, the converter is trying to regulate area (1) to give you 3A of current. However, due to the system load connection, the 3A would be diverted to the system rather than the battery.

This is where area (2) comes in at the input. In the scenario above, the converter will continue to output current until it can provide 3A through area (1). If the system load is very high at that point, most likely the charger will hit the IINDPM threshold which is monitored and regulated through the area (2). This is an input current regulation loop which regulates the DC input power. 

However, because it is a regulation loop intended more for DC current regulation, my previous post comment about the load slew rate applies. Here, we focus on section (3) and the IL_AVG register setting. This control can take over in the time between a fast/large load step and the IINDPM regulation loop responding in order to regulate the peak inductor current. 

Regards,

Joel H

Hey David,

That is fine by me. This was to allow you to monitor the discharge current.

David Trotter said:

I still don't understand the point in connecting VSYS to the left of the PMOSFET as opposed to the right as you have drawn it in green... perhaps you could explain the purpose to me?

I am still connecting this to the right side of the PMOS; I'm just connecting to the left side of the sense resistor. This was only to allow you to utilize either the internal ADC or the IBAT pin to monitor the discharge current.

Regards,

Joel H