TPS61178: External FET use case

Hi Satoshi:

Thanks for reaching out.

For your questions:

1. The TPS611781 is the Force PWM version, while TPS61178 is Auto PFM version.It means "the TPS611781 keeps the switching frequency being constant for the whole load range. When the load current decreases, the output of the internal error amplifier decreases as well to lower the inductor peak current and delivers less power from input to output. The high-side FET is not turned off even if the current through the FET goes negative to keep the switching frequency being the same as that of the heavy load."

It's suitable for the case care about the switch frequency, or the accuracy of output voltage for whole load.(As the PFM will cause 1% higher than target typically.) But the efficiency will decrease especially for the light load.

2. Actually, this P-FET will allow the current from Load to Vout_Pin through the parasitic diode no matter the FET is on or off. But the high side switch inside the converter will block the revise current during the shutdown.

May you help to share details information? Is the converter enable when you add 16V in output2.