7.3.1 Internal Current Limit

The internal current limit of the TPS709 helps protect the regulator during fault conditions. In current limit, the current of the output source is fixed, which is largely independent of the output voltage. In this case, the output voltage is not regulated and can be measured as (VOUT = ILIMIT × RLOAD). The PMOS pass transistor dissipates [(VIN - VOUT)×ILIMIT] until thermal shutdown is triggered and the device shuts down. When cooling, the device is turned on by an internal thermal shutdown circuit. If the fault condition continues, the device cycles between current limit and thermal shutdown; see the Thermal Protection section for details.

The TPS709 features an output current range up to 150mA over the recommended operating range. The internal current limit begins to limit the output current to at least 200mA of output current. The output current of the TPS709 is still between 150mA and 200mA, but some datasheet parameters may not meet the requirements.

7.3.2 Trip voltage TPS709 adopts PMOS pass transistor to achieve low trip voltage. When (VIN - VOUT) is less than the drain voltage (VDO), the PMOS pass device is in the linear operating region, and the input and output resistance is the RDS(ON) of the PMOS pass device. Since a PMOS device functions like a resistor for input and output current, VDO is roughly proportional to the output current.

The ground pin current of many linear regulators increases substantially during tripping operation. This increase in ground current while operating in dropout can be orders of magnitude greater than when the device is not in dropout. The TPS709 employs a special control loop that limits the increase in ground current during a tripping operation. This feature allows for the most efficient operation, while in dropout conditions, can greatly increase battery runtime