The APL5331 linear regulator is designed to provide a regulated voltage with bi-directional output current for DDR-SDRAM termination voltage. The APL5331 integrates two power transistors to source or sink current up to 3A. It also incorporates current-limit, thermal shutdown and shutdown control functions into a single chip. The current-limit circuit limits the short-circuit current. The on-chip thermal shutdown provides protection against any combination of overload that would create excessive junction temperature. The output voltage of APL5331 tracks the voltage at VREF pin. A resistor divider connected to VIN, GND and VREF pins is used to provide a half voltage of VIN to VREF pin. In addition, an external ceramic capacitor and an open-drain transistor connected to VREF pin provide soft-start and shutdown control respectively. Pulling and holding the VREF voltage to 0V shuts off the output. The output of the APL5331 will be high impedance in shutdown condition.