The TPS61299x/xA features down mode and pass-through operation when input voltage is close to or higher than output voltage.

During down mode operation, the device regulates the output voltage to the target voltage even when the input voltage is higher than the output voltage. The control circuit changes the behavior of the rectifying P-channel MOSFET by pulling its gate to input voltage instead of to ground. In this way, the voltage drop across the P-channel MOSFET is increasing as high as to regulate the output voltage.

In pass through mode, the TPS61299x/xA stops switching and turns on the high-side P-channel MOSFET. The output voltage is the input voltage minus the voltage drop across the DCR of the inductor and the on-resistane (R_DS(on)) of the P-channel MOSFET. During pass though operation, the device disables the input current limit function, reverse current protection, and thermal shutdown.

For the input current limit is equal or higher than 250mA version, TPS61299, TPS612995, TPS612996 and TPS612997. With input voltage ramping up, the device goes into down mode when Vin >Vout-35mV. The device stays in down mode until Vin >Vout+100mV and then goes automatically into pass through operation. In the pass through operation, output voltage follows input voltage. The TPS61299 exits pass though operation and goes back to boost mode when the output voltage drops below the setting target voltage minus 75mV.

For the input current limit equal or lower than 100mA version, TPS612991, TPS612992, TPS612993 and TPS612994. With input voltage ramping up, the device goes into down mode when Vin >Vout-35mV(Vboost_down). It stays in down mode until Vin >Vout+38mV(Vdown_pass) and then goes automatically into pass through operation. In the pass through operation, output voltage follows input voltage. The TPS61299X exits pass though operation and goes back to boost mode when the output voltage drops below the setting target voltage minus 67mV(Vpass_boost)device.