Schematic and PCB Design

The design for this board was based off of the typical application provided in the datasheet. This IC did not have any simulation resources availiable, with the exception of an EVM - photo shown below.

The typical application was designed specifically for using a 21V solar panel, MPPT at 18V, and charing a 2-cell li-ion battery at 2A and therefore a precharge current of 0.2A and an active charging temperature range from 0 to 45 degrees C. Since my design did not have these same specifications, I needed to change parts of the schematic. The general typical application schematic is shown here for reference, and can be found in the BQ24650 datasheet.

• Omitting the Thermistor: I omitted the thermistor since it wasn’t a necessary component of the circuit. This required that I set the resistor divider to mirror that and read as if the circuit is in a continual charging state in reference to the TS pin. Since the Vref pin output 3.3V, and the active charging qualifications required by the TS pin were between 70% of Vref (VLTF) and 45% of Vref (VHTF), I concluded that I wanted my TS pin set around 2V to ensure a little wiggle room on either side. I solved for the resistor values using a basic circuit analysis voltage divider. I arrived at R9 = 5.23k and R10 = 8.22k
• Charging current set as 0.5A (sense resistor and inductor selection): I wanted a charging current of 0.5A to the lipo battery as per reccommended. In order to set the charge current to 0.5A, I needed to select the apprpriate inductor and current sense resistor. Icharge = 40mV/Rsr => Rsr = 80mohms, and L = 22uH. This also implies that the precharge current will be 0.05A.
• Battery charge set point with resistors: I set the Battery charge at 8.4V with the resistor dividers– which should be the maximum charge capacity of the 2s lipo battery. This resistor ratio was calculated from the given formula in the datasheet. VFB = 2.1(1+R2/R1) => R1 = 100K and R2 = 300k
• MPPT set with resistor divider: I set the MPPSET point at 9V, which I thought was a good estimate (as I was unable to actually test for the MPPT given I didn’t have any resistors to use as the load). Like the VFB pin, the MPPSET pin is set with a similar voltage divider. This time, given by MPPSET = 1.2(1+R3/R4) => R3 = 499k and R4 = 76k
• Connector selection: The solar panel I had availiable used a 3.5mm coax cable, which I had an adapter for to just stray wires. For simplicity, I chose to use spade connectors on the circuit board, then I was able to use the coax cable adapter for the solar panel, and a deens connector for the lipo battery.

With these changes, I arrived at the schematic design drawn up in Kicad looking like this:

There are still some needed edits to this schematic for clarity, as well as some datasheets associated with parts that need to be updated. From this kicad schematic, I did my best to follow the layout instructions as mentioned in the datasheet. The PCB board layout, GND layer and 3D view are shown in the following pictures. Please also refer to the Code tab where I have the immediate kicad files stored.