The ISL9237 and ISL9238 monitor the input and output voltage to determine the correct mode of operation, and switch between each mode to maintain a stable output voltage. Shown here the adapter voltage (Vadp) starts from 5 V with a 2-cell battery, and the buck-boost charger operates in forward boost mode to boost the voltage to 8.4 V. Once the adapter input voltage is close to the battery voltage, the device operates in forward buck-boost mode. Finally, once the adapter voltage input is higher than battery voltage, the buck-boost charger operates at forward buck mode. The devices also have built-in hysteresis to avoid mode chatter.