2 Implementation

At a basic level, ship mode is a way to cut off battery supply to all parts of the circuit while still having a way to exit this low power state on demand. In the BQ24075, BQ24076, BQ24078, and BQ24079 devices, the SYSOFF pin controls the FET that connects the battery to the system or OUT pin. SYSOFF can be used to turn on the BATFET by pulling the pin low or turn off the BATFET by pulling the pin high. By default, the BATFET is turned off due to the 5-MΩ resistor pulled up internally to VBAT. However, this pin cannot be left floating and needs to either be pulled low or high externally. Normally, the SYSOFF pin needs to be held low to enable charging as well as supply power to the system when in battery-only mode. This ability to control when the battery is connected to the system allows the choice to enter and exit ship mode.

As Figure 2-1 shows, the implementation of ship mode consists of a few resistors, a push-button, and a FET that is controlled through an MCU General Purpose Input/Output (GPIO). With the push-button, the SYSOFF pin can be pulled low when pressed or the SYSOFF will be pulled high when the push-button is not pressed which puts the device into the ship-mode state. This feature can also be used as a battery undervoltage protection. If the microcontroller in the application has an ADC to monitor the battery voltage, this feature can allow the device to go into ship mode before the battery is deeply discharged or the battery protection IC trips through a General Purpose Input/Output (GPIO) to pulldown the SYSOFF pin through a FET. This can help increase battery life. When VIN is plugged in, the system is powered and SYSOFF can be pulled low using the GPIO to keep the battery connected to the output for charging.

Figure 2-1 Implementation