Q:

3.3Vin to 1.2Vout, but application cannot tolerate Power-Save mode frequency change. 

What are the limits on the inductor value?  Can they change the inductor value to affect the PFM mode threshold?

Note 1: Question pertains to ADJUSTABLE version (TPS62220), but the the INDUCTOR chart in the datasheet is for FIXED versions.


A:

I would not recommend using this IC with inductances that are not recommended in the datasheet.  The internal control loop requires the double pole at the LC corner frequency to be in a narrow range.  By increasing the inductor, you exceed this range.

The PFM threshold is not changeable, per page 10 of the datasheet. 

Due to this limitation, I must recommend other parts with newer PFM modes.  The TPS62230 has a better PFM mode and a mode pin.  You could even use the TPS62150 with FSW = high to get around a ~1.25 MHz frequency.  With that family of ICs, you can change the inductance to alter the PFM mode entry current.  If the TPS62120 meets your load needs, then it is available at around 800 kHz.

The inductor table directly applies to the fixed version but also applies to the adjustable.  There is more flexibility with the adjustable since you have the feedback network that you can modify to tweak the loop response.  This allows you somewhat more accommodation of different filters but not that much.