STM32H747A in External SMPS supply mode not working.

[Paul_V]

I have a design were I am trying to use the SMPS regulator in "#4 External SMPS supply" mode.  Figure 22 in the RM0399.  When I power up the board, the processor was in reset and I couldn't connect. 

When scoping the output of the inductor (VFBSMPS) the output is not the +1.2V that is supposed to be the default out.  It bounces between +1.2 to +1.8. 

1) Looking carefully, I did miss the 220pF cap from VLX_SMPS to VSS_SMPS(GND).  I added that in and still the same problem.  There are some discussions that say the regulator can be permanently damaged without this cap. Is that true?  Even so, I took a brand new board, installed the cap before ever powering it up and tried it.  Same thing.  Bouncing VFB_SMPS.

2) If I provided +1.8V externally to the output side of the inductor (VFBSMPS) then I can connect and the processor is recognized by StLink. 

How much output capacitance can the SMPS output handle? 

What else should I try?

Paul

[CYANG.1 · Accepted solution]

Hi,

It is recommended in DS12930 page 110 Table 26, Cfilt is 220pF, it is better to follow this requirement. I think you may double check whether you have Cout (2x10uF) on the board and other components are in right value according to Table26. And you may check whether the setting of registers are right according to Table33 in RM0399.