TPS40210 External clock

The RT function inside the TPS40210 is not a PLL.  It is a simple timing system that uses an external R and C that charges from then input voltage.  There is an internal switch that discharges the capacitor.  The external clock function shown is just an open drain that is used to discharge the the timing capacitor externally.  It must be operated at a higher frequency.  it may be possible to construct some other direct drive scheme, but the datasheet operation looks to be the simplest.