The device requires a minimum number of external parts to implement a charging solution. The standard solution only requires the selection of three external resistors to configure the device to given charge specifications.

1. CHM_TMR – Resistor value to select battery chemistry and charge timer, see table 7-1 in Data sheet.
2. VSET – Resistor value to program charge voltage and enable optional intermittent charge (for NiMH only), see table 7-2 in Data sheet.
3. ISET – Resistor value to set fast charge current, see Table 7-1 in this application note.
4. Other connections:
   1. TS – NTC to monitor battery temperature, typically 10 kΩ.
   2. Cin and Cout – 1 µF effective capacitance connection to ground at both IN and OUT
   3. STAT1 and STAT2 – Open collector status outputs (optional connections)
   4. /CE – Charge enable input (optional connection)

Figure 3-1 provides a solution that will charge a Li-Ion battery at 700 mA or 70 mA with charge current selection controlled by a GPIO pin on a host device and termination at 10% of the selected charge current.

• R-ISET1 will set fast charge current to 70 mA, 4.2 kΩ.
• R-ISET2 will combine with R-ISET1 in parallel to set fast charge current to 700 mA, 480 Ω.
• The external NMOS transistor with gate voltage controlled by a host GPIO pin will set fast charge current to 700 mA when turned on.
• R-CHM_TMR will set chemistry to Li+ with a 10 hour safety timer, 82 kΩ.
• R-VSET will set the number of cells and regulation voltage to 1 cell Li-Ion 4.2 V, 18 kΩ.
• Charge enable (/CE) pin is connected to host MCU for external device control.
• R-TS is required, a 10 kΩ NTC in battery pack is connected to pin.
• STAT1/STAT2 LEDs are used together to indicate charge status and faults, they are optional.

This application supports enhanced charge current flexibility.

Figure 3-1 BQ25171-Q1 700-mA/70-mA Fast Charge