source current, sink current and drive current

This is definitely the wrong forum for DC/DC converter questions. I think they may start a forum in the near future.

So where should I ask this question?

I am studying TI TPS51116 and TPS51117 for notebook solution.

Drive current refers to the current required to charge the gate of the switching FET. So, the average value of the current is a function of the gate-source capacitance and the switching frequency. The gate resembles a large capacitor betweeen gate and the source, on the order of 1000-2000 pF, but requires only nanoamperes once the FET has reached the cutoff or saturated state. The gate drive circuit should be capable of driving a pure capacitance, which means high charging and discharging peak currents but little dc current.

In the TPS51117, gate drive is specified as a resistance (5ohms for turn on, 1.5 ohms for turn off. This is convenient as it lets you estimate the rise and fall time at the gate.

Thanks a lot.

Can I sense the drive current by current probe if I lift up the gate pin of MOSFET?

Sure. Just be careful to keep the measurement loop as short as possible to minimize the inductance.