Horrible load regulation with MPQ8623

Hi,

I am using MPQ8623 for 12V → 5V conversion. Everything works great except for the fact that the output voltage changes significantly with load, as measured at the output capacitor:

0 A - 5.14 V
1 A - 4.82 V
2 A - 4.64 V
3 A - 4.54 V
4 A - 4.46 V

My layout is a close copy of the example from the datasheet (p. 19), the only difference being that the inductor is rotated 90 degrees clockwise. The inductor is 2.2 uH, 9.5 A (FXL0630-2R2-M cjiang(Changjiang Microelectronics Tech) | C167218 - LCSC Electronics) and the resistor divider for the FB pin has been suitably adjusted (20.5/4.42 kOhm). MODE is set to Forced CCM 1.1 MHz. Imax is set to 6.3 A with a 4.7 kOhm resistor. Softstart is ~9 ms (100 nF). PGOOD is floating. PCB is 4-layer, with both internal planes being ground.

The multimeter shows 0.9 V on the FB pin, although on the scope the value is pretty noisy. Vin is stable 12 V from a linear bench PS.

What could be the reason for the observed behavior? This obviously is not acceptable, so either I am able to correct it, or I need to find another buck. :confused:

UPDATE: I thought that perhaps induction was affecting the feedback signal on its way to the resistor divider, but comparing its both ends shows 1-2 mV difference (which should result in approx. 5-10 mV of difference in Vout). So probably it’s not that.

Thanks,
Milek

I removed the Cff capacitor from the feedback circuit and… the problem vanished: the Vo is very stable no matter the load.

I would however not like to lose the ability to react to fast transients. What could be the reason for Cff capacitor causing issues like that?