TPS56428: Bode plot measurement issue

Muhsiu,

Your FF capacitor is too large.  Please try 12 pF.

Hi John,

The bode plot result still fail after change the FF to 12pF.

I also try to change the voltage divider resistors value. it's doesn't work. 

Muhsiu.

Muhsiu,

There is something unusual about your loop response plot.  You have a zero / pole pair at approx. 60 kHz / 100 kHz.  With Cff = 12 pF the zero / pole pair should be at 266 kHz / 530 kHz.  Please check it with no Cff.  If you still have that same response, then there is something with you load or PCB layout that is causing your issue.  Try measuring with the load disconnected.

Muhsiu,

Any update?

Hi John,

The illustration is remove Cff @1A load bode result. 

TPS56428 will operate at PSM when output current under 0.5A. 

The bode plot wo load result as show below. Actually, i can't read bode plot when converter operating at PSM.

Muhsiu.

Muhsiu,

You need to ensure that the TPS56428 is operating in CCM mode when you measure the loop.  If it is in eco-mode you will get an invalid measurement.

Hi John,

I'm pretty sure that the TPS56428 is operating in CCM mode when loading 1A.  Actually, the PCB layout looks ok. Could you help to review it? Thanks.

Muhsiu.

Hi John,

Sorry for reply late, we just back to work from national holidays. 

Due to i still can not optimize the bode plot measurement result of TPS56428. 

I try to verify this design through load transient test. As you can see, the voltage regulation under 5% when load step.

Do you have any suggestio for this? Thanks.  

Muhsiu.

Muhsiu,

What additional assistance do you require?