This detection is designed to give the charger advance notice that an adaptor or USB port is connect for the cases where the battery is discharged and device transceiver is not able to communicate with a USB host or there is not a device transceiver. If an adaptor is detected, then the charger can immediately start charging at the programmed ISET level. Without this early detection, the charger would have to default to the 100-mA input current level to make sure it was not over-loading a low power USB port. The detection method monitors the D+, D– communication lines looking for a short between the lines (Adaptor source connected) or pulldown resistors on D+, D– (USB source connected) to determine what source is connected (no USB communication takes place). If an adaptor source is detected then the charger will transition from the 100 mA startup level to the ISET programmed current level. If a USB port is detected, the input current limit will stay at the 100 mA level. If a different charge level is desired, than the one detected, the host has to change the state of the ISET2 pin (signals the internal logic to start using the ISET2 as the program pin) and then set to the desired state.

The D+ and D– pin connections inside the charger are disconnected within 100 ms of the D+ or D– lines being pulled high (start of detection), to minimize any interaction between the charger detection pins and the USB normal communications. If the device transceiver is able to communicate with the USB host, communication typically starts after 100 ms after the device has pulled the D+ or D– line high indicating it is “on line”, and by then the IC detection is complete and has been disconnected. The device host then may change the ISET2 level or disable the IC by pulling the TS pin low.